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<book>
-<title>PostGIS Manual</title>
-<bookinfo>
-<editor><firstname>Paul</firstname><surname>Ramsey</surname><affiliation><orgname><ulink url="http://www.refractions.net">Refractions Research Inc</ulink></orgname><address><street>209 - 560 Johnson Street</street><city>Victoria</city><state>British Columbia</state><country>Canada</country><email>pramsey@refractions.net</email></address></affiliation></editor><abstract>
-<para>PostGIS is an extension to the PostgreSQL object-relational database
-system which allows GIS (Geographic Information Systems) objects to be stored
-in the database. PostGIS includes support for GiST indexes, and functions for
-basic analysis of GIS objects.</para>
-</abstract>
-</bookinfo>
-<chapter>
-<title>Introduction</title>
-<para>PostGIS is developed by Refractions Research Inc, as a research
-project into spatial database technology. Refractions is a GIS and database
-consulting company in Victoria, British Columbia, specializing in data
-integration and custom software development. We plan on supporting and
-developing PostGIS to support a range of important GIS functionality, including
-advanced topological constructs (coverages, surfaces, networks), desktop user
-interface tools for viewing and editing GIS data, and web-based access
-tools.</para>
-<sect1>
-<title>Credits</title>
-<variablelist>
-<varlistentry>
-<term>Dave Blasby <dblasby@refractions.net></term>
-<listitem><para>The principal developer of PostGIS. Dave maintains the
-server side objects and index support, as well as the server side analytical
-functions.</para></listitem>
-</varlistentry>
-<varlistentry>
-<term>Paul Ramsey <pramsey@refractions.net></term>
-<listitem><para>Maintains the JDBC objects and keeps track of the documentation and packaging.</para></listitem>
-</varlistentry>
-<varlistentry>
-<term>Jeff Lounsbury <jeffloun@refractions.net></term>
-<listitem><para>Maintains the Shape loader/dumper.</para></listitem>
-</varlistentry>
-</variablelist>
-</sect1>
-<sect1>
-<title>More Information</title>
-<itemizedlist>
-<listitem><para>The latest software, documentation and news items are available at the PostGIS web site, <ulink url="http://postgis.refractions.net">http://postgis.refractions.net</ulink>.</para></listitem>
-<listitem><para>More information about the PostgreSQL database server is available at the PostgreSQL main site <ulink url="http://www.postgreql.org">http://www.postgreql.org</ulink>.</para></listitem>
-<listitem><para>More information about GiST indexing is available at the GiST development site, <ulink url="http://www.sai.msu.su/~megera/postgres/gist">http://www.sai.msu.su/~megera/postgres/gist</ulink>.</para>
-</listitem>
-</itemizedlist>
-</sect1>
-</chapter>
-
-<chapter>
-<title>Installation</title>
-<sect1 id="PGInstall">
-<title>PostGIS</title>
-<para>The PostGIS module is a extension to the PostgreSQL backend server.
-As such, PostGIS <emphasis>requires</emphasis> a full copy of the PostgreSQL
-source tree in order to compile. The PostgreSQL source code is available at<ulink url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>
-<para>PostGIS 0.5 has been built and tested against PostgreSQL 7.1.2. It
-will probably work with any of the 7.1.x versions. It will
-<emphasis>not</emphasis> work with any version prior to 7.1.</para>
-<orderedlist>
-<listitem><para>Before you can compile the postgis server modules, you must
-compile and install the PostgreSQL package. </para></listitem>
-<listitem><para>Retrieve the PostGIS source archive from http://postgis.refractions.net/postgis-0.5.tar.gz. Uncompress and untar the
-archive in the "contrib" directory of the PostgreSQL source tree.</para><literallayout>cd [postgresql source tree]/contrib
-zcat postgis-0.5.tar.gz | tar xvf -</literallayout>
+ <title>PostGIS Manual</title>
+ <bookinfo>
+ <editor>
+ <firstname>Paul</firstname>
+ <surname>Ramsey</surname>
+ <affiliation>
+ <orgname><ulink url="http://www.refractions.net">Refractions Research
+ Inc</ulink></orgname>
+ <address><street>209 - 560 Johnson
+ Street</street><city>Victoria</city><state>British
+ Columbia</state><country>Canada</country><email>pramsey@refractions.net</email></address>
+
+ </affiliation>
+ </editor>
+ <abstract>
+ <para>PostGIS is an extension to the PostgreSQL object-relational
+ database system which allows GIS (Geographic Information Systems) objects to be
+ stored in the database. PostGIS includes support for GiST indexes, and
+ functions for basic analysis of GIS objects.</para>
+ </abstract>
+ </bookinfo>
+ <chapter>
+ <title>Introduction</title>
+ <para>PostGIS is developed by Refractions Research Inc, as a research
+ project into spatial database technology. Refractions is a GIS and database
+ consulting company in Victoria, British Columbia, specializing in data
+ integration and custom software development. We plan on supporting and
+ developing PostGIS to support a range of important GIS functionality, including
+ full OpenGIS support, advanced topological constructs (coverages, surfaces, networks), desktop user
+ interface tools for viewing and editing GIS data, and web-based access
+ tools.</para>
+ <sect1>
+ <title>Credits</title>
+ <variablelist>
+ <varlistentry>
+ <term>Dave Blasby <dblasby@refractions.net></term>
+ <listitem>
+ <para>The principal developer of PostGIS. Dave maintains the server
+ side objects and index support, as well as the server side analytical
+ functions.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Paul Ramsey <pramsey@refractions.net></term>
+ <listitem>
+ <para>Maintains the JDBC objects and keeps track of the
+ documentation and packaging.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Jeff Lounsbury <jeffloun@refractions.net></term>
+ <listitem>
+ <para>Maintains the Shape loader/dumper.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect1>
+ <sect1>
+ <title>More Information</title>
+ <itemizedlist>
+ <listitem>
+ <para>The latest software, documentation and news items are available
+ at the PostGIS web site,
+ <ulink url="http://postgis.refractions.net">http://postgis.refractions.net</ulink>.</para>
+
</listitem>
- <listitem><para>Once your PostgreSQL installation is up-to-date, enter the
- "postgis" directory, and run the compile and install commands. </para><literallayout>cd ./postgis-0.5
-make
-make install</literallayout>
+ <listitem>
+ <para>More information about the PostgreSQL database server is
+ available at the PostgreSQL main site
+ <ulink url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>
</listitem>
- <listitem><para>Before you can use the PostGIS objects in your database, you
- must load the object and function definitions. </para><literallayout>psql -d [yourdatabase] < postgis.sql</literallayout>
- <para>The PostGIS server extensions are now loaded and ready to
+ <listitem>
+ <para>More information about GiST indexing is available at the GiST
+ development site,
+ <ulink url="http://www.sai.msu.su/~megera/postgres/gist">http://www.sai.msu.su/~megera/postgres/gist</ulink>.</para>
+
+ </listitem><listitem><para>More information about Mapserver internet map server is available at <ulink url="http://mapserver.gis.umn.edu/">http://mapserver.gis.umn.edu</ulink>.</para></listitem><listitem><para>The "<ulink url="http://www.opengis.org/techno/specs/99-049.pdf">Simple Features for Specification for SQL</ulink>" is available at the OpenGIS Consortium web site: <ulink url="http://www.opengis.org">http://www.opengis.org</ulink>.</para></listitem>
+ </itemizedlist>
+ </sect1>
+ </chapter>
+ <chapter>
+ <title>Installation</title>
+ <sect1 id="PGInstall">
+ <title>PostGIS</title>
+ <para>The PostGIS module is a extension to the PostgreSQL backend server.
+ As such, PostGIS 0.6 <emphasis>requires</emphasis> a full copy of the PostgreSQL
+ 7.1.x source tree in order to compile. The PostgreSQL source code is available at
+ <ulink url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>
+ <para>PostGIS 0.6 has been built and tested against PostgreSQL 7.1.2. It
+ will probably work with any of the 7.1.x versions. It will
+ <emphasis>not</emphasis> work with any version prior to 7.1, or with version 7.2. The <emphasis>next</emphasis> version of PostGIS will work with version 7.2.</para>
+ <orderedlist>
+ <listitem>
+ <para>Before you can compile the postgis server modules, you must
+ compile and install the PostgreSQL package.</para>
+ </listitem>
+ <listitem>
+ <para>Retrieve the PostGIS source archive from
+ <ulink url="http://postgis.refractions.net/postgis-0.6.tar.gz">http://postgis.refractions.net/postgis-0.6.tar.gz</ulink>. Uncompress and untar the
+ archive in the "contrib" directory of the PostgreSQL source tree.</para>
+
+ <programlisting># cd [postgresql source tree]/contrib zcat
+# postgis-0.6.tar.gz | tar xvf -</programlisting></listitem>
+ <listitem>
+ <para>Once your PostgreSQL installation is up-to-date, enter the
+ "postgis" directory, and run the compile and install commands. </para>
+ <programlisting># cd ./postgis-0.6
+# make
+# make install</programlisting></listitem><listitem><para>As of version 0.6, PostGIS requires the PL/pgSQL procedural language extension. Before loading the postgis.sql file, you must first enable PL/pgSQL. The code snippet below gives an example, substitude the correct location of your <filename>plpgsql.so</filename> library.</para><programlisting>CREATE FUNCTION plpgsql_call_handler () RETURNS OPAQUE AS
+ '/usr/local/pgsql/lib/plpgsql.so' LANGUAGE 'C';
+CREATE TRUSTED PROCEDURAL LANGUAGE 'plpgsql'
+ HANDLER plpgsql_call_handler
+ LANCOMPILER 'PL/pgSQL';
+</programlisting></listitem>
+ <listitem>
+ <para>Finally, you
+ must load the PostGIS object and function definitions into your database. </para>
+ <programlisting># psql -d [yourdatabase] < postgis.sql</programlisting><para>The PostGIS server extensions are now loaded and ready to
use.</para>
</listitem>
</orderedlist>
- </sect1>
+ <sect2><title>Upgrading</title><para>Upgrading PostGIS can be tricky, because the underlying C libraries which
+support the object types and geometries may have changed between versions.
+To avoid problems when upgrading, you will have to dump all the tables
+in your database, destroy the database, create a new one, upload the
+new <filename>postgis.sql</filename> file, then upload your database dump:</para><programlisting># pg_dump -t "*" -f dumpfile.sql yourdatabase
+# dropdb yourdatabase
+# createdb yourdatabase
+# psql -f postgis.sql -d yourdatabase
+# psql -f dumpfile.sql -d yourdatabase
+# vacuumdb -z yourdatabase
+</programlisting><note><para>When upgrading to 0.6, all your geometries will be created with an SRID
+of -1. To create valid OpenGIS geometries, you will have to create a
+valid SRID in the SPATIAL_REF_SYS table, and then update your geometries
+to reference the SRID with the following SQL (with the appropriate
+substitutions:</para><programlisting>UPDATE TABLE <table> SET <geocolumn> = SetSRID(<geocolumn>,<SRID>);</programlisting></note></sect2></sect1>
<sect1>
- <title>JDBC</title>
+ <title>JDBC</title>
<para>The JDBC extensions provide Java objects corresponding to the
internal PostGIS types. These objects can be used to write Java clients which
query the PostGIS database and draw or do calculations on the GIS data in
PostGIS.</para>
<orderedlist>
- <listitem><para>Enter the "jdbc" sub-directory of the PostGIS distribution.
- </para></listitem>
- <listitem><para>Edit the "Makefile" to provide the correct paths of your java
- compiler (JAVAC) and interpreter (JAVA).
- </para></listitem>
- <listitem><para>Run the "make" command. Copy the "postgis.jar" file to
- wherever you keep your java libraries.</para>
+ <listitem>
+ <para>Enter the "jdbc" sub-directory of the PostGIS distribution.
+ </para>
+ </listitem>
+ <listitem>
+ <para>Edit the <filename>Makefile</filename> to provide the correct paths of your java
+ compiler (JAVAC) and interpreter (JAVA). </para>
+ </listitem>
+ <listitem>
+ <para>Run the "make" command. Copy the <filename>postgis.jar</filename> file to wherever
+ you keep your java libraries.</para>
</listitem>
</orderedlist>
</sect1>
<sect1>
- <title>Loader/Dumper</title>
- <para>The data loader should compile very simple.</para><literallayout>cd postgis-0.5/loader
-make</literallayout><para>The loader is called "shp2pgsql" and converts ESRI Shape files into SQL suitable for loading in PostGIS/PostgreSQL.</para>
+ <title>Loader/Dumper</title>
+ <para>The data loader should compile very easily.</para>
+ <programlisting># cd postgis-0.6/loader
+# make</programlisting><para>The loader is called <filename>shp2pgsql</filename> and converts ESRI Shape files into
+ SQL suitable for loading in PostGIS/PostgreSQL.</para>
</sect1>
</chapter>
<chapter>
- <title>Frequently Asked Questions</title>
- <qandaset>
- <qandaentry>
- <question><para>What kind of geometric objects can I store?</para></question>
- <answer><para>You can store point, line, polygon, multipoint, multiline,
- multipolygon, and geometrycollections. These are specified in the Open GIS Well
- Known Text Format (with 3d extentions).</para></answer>
- </qandaentry>
- <qandaentry>
- <question><para>How do I insert a GIS object into the database?</para></question>
- <answer><para>First, you need to create a table with a column of type "geometry"
- to hold your GIS data. Connect to your database with "psql" and try the
- following SQL: <emphasis>CREATE TABLE gtest ( ID int4, NAME varchar(20), GEOM
- geometry)</emphasis>. If the table creation fails, you probably have not loaded
- the PostGIS functions and objects into this database. See the
- <link linkend="PGInstall">installation instructions</link>.</para>
- <para>Then, you can insert a geometry into the table using a SQL insert
- statement. The GIS object itself is formatted using the OpenGIS Consortium
- "well-known text" format: <emphasis>INSERT INTO gtest (ID, NAME, GEOM) VALUES
- (1, 'First Geometry', 'LINESTRING(2 3,4 5,6 5,7 8)')</emphasis>. For more
- information about other GIS objects, see the <link linkend="RefObject">object
- reference</link>.</para>
- <para>To view your GIS data in the table: <emphasis>SELECT * FROM
- gtest</emphasis>. The return value should look something like this:</para> <literallayout> id | name | geom
+ <title>Frequently Asked Questions</title>
+ <qandaset>
+ <qandaentry>
+ <question>
+ <para>What kind of geometric objects can I store?</para>
+ </question>
+ <answer>
+ <para>You can store point, line, polygon, multipoint, multiline,
+ multipolygon, and geometrycollections. These are specified in the Open GIS Well
+ Known Text Format (with 3d extentions).</para>
+ </answer>
+ </qandaentry>
+ <qandaentry>
+ <question>
+ <para>How do I insert a GIS object into the database?</para>
+ </question>
+ <answer>
+ <para>First, you need to create a table with a column of type
+ "geometry" to hold your GIS data. Connect to your database with "psql" and try
+ the following SQL: </para><programlisting>CREATE TABLE gtest ( ID int4, NAME varchar(20) );
+SELECT AddGeometryColumn('dbname','gtest','geom',-1,'LINESTRING',2);</programlisting><para>If the table creation fails, you probably have not
+ loaded the PostGIS functions and objects into this database. See the
+ <link linkend="PGInstall">installation instructions</link>.</para>
+ <para>Then, you can insert a geometry into the table using a SQL
+ insert statement. The GIS object itself is formatted using the OpenGIS
+ Consortium "well-known text" format: </para><programlisting>INSERT INTO gtest (ID, NAME, GEOM)
+ VALUES (1, 'First Geometry', GeometryFromText('LINESTRING(2 3,4 5,6 5,7 8)', -1));
+SELECT id, name, AsText(geom) AS geom FROM gtest;</programlisting><para>For more information about other GIS objects, see the
+ <link linkend="RefObject">object reference</link>.</para>
+ <para>To view your GIS data in the table: <emphasis></emphasis>. The return value should look
+ something like this:</para>
+ <literallayout> id | name | geom
----+----------------+-----------------------------
- 1 | First Geometry | LINESTRING(2 3,4 5,6 5,7 8)
-(1 row)
-</literallayout>
- </answer>
- </qandaentry>
- <qandaentry>
- <question><para>How do I construct a spatial query?</para></question>
- <answer><para>There are a number of spatial operators available to PostgreSQL,
- and several of them have been implemented by PostGIS in order to provide
- indexing support. However, all the operators have been implemented with the
- following important simplifying assumption: <emphasis>all features shall be
- represented by their bounding boxes</emphasis>.</para>
- <para>We recognize that using bounding boxes to proxy for features is a
- limiting assumption, but it was an important one in moving from the conception
- of a PostgreSQL spatial database to the implementation. Using bounding boxes
- makes queries faster, indexes smaller, and operators simpler.</para>
- <para>The most important spatial operator from a user's perspective is
- the "&&" operator, which tests whether one feature's bounding box
- overlaps that of another. An example of a spatial query using && is:
- <emphasis>SELECT * FROM GTEST WHERE GEOM && 'BOX3D(3 4,4
- 5)'::box3d</emphasis>. Note that the bounding box used for querying must be
- explicitly declared as a "box3d" using the "::box3d" casting operation.</para>
- </answer>
- </qandaentry>
- <qandaentry>
- <question><para>How do I speed up spatial queries on large tables?</para></question>
- <answer><para>Fast queries on large tables is the <emphasis>raison
- d'etre</emphasis> of spatial databases (along with transaction support) so
- having a good index in important.</para>
- <para>To build a spatial index on a table with a "geometry" column, use
- the "CREATE INDEX" function as follows: <emphasis>CREATE INDEX [indexname] ON
- [tablename] USING GIST ( [geometrycolumn] gist_geometry_ops) WITH (
- islossy)</emphasis>.</para>
- <para>The "USING GIST" option tells the server to use a GiST (Generalized
- Search Tree) index. The reference to "gist_geometry_ops" tells the server to
- use a particular set of comparison operators for building the index: the
- "gist_geometry_ops" are part of the PostGIS extension. Finally, the "islossy"
- option tells the server that the features being indexed can be proxied by a
- smaller data structure -- in the case of geometries, the features are
- represented in the index by their bounding boxes.</para>
- </answer>
- </qandaentry>
- <qandaentry>
- <question><para>How can I get my search to return things that really are inside
- the search box, not just overlapping bounding boxes?</para></question>
- <answer><para>The '&&' operator only checks bounding box overlaps, but
- you can use the "truly_inside()" function to get only those feature which
- <emphasis>actually</emphasis> intersect the search box. For example, by
- combining the use of "&&" for a fast index search and truly_inside()
- for an accurate final check of the result set, you can get only those features
- inside the search box (note that this <emphasis>only</emphasis> works for
- search boxes right now, not any arbitrary geometry):</para>
- <para><emphasis>SELECT * FROM [TABLE] WHERE [GEOM_COLUMN] &&
- [BOX3d] AND truly_inside([GEOM_COLUMN],[BOX3d]);</emphasis></para>
-</answer>
- </qandaentry> <qandaentry>
- <question><para>Why aren't R-Tree's recommended?</para></question>
- <answer><para>R-Trees have two limitations which make them undesirable for use
- with GIS feature in the PostgreSQL database (note that these limitations are
- due to the PostgreSQL implementations, not the R-Tree concept in
- general):</para>
- <itemizedlist>
- <listitem><para>Building an R-Tree index on a large table of geometries can
- take over twice as long as a GiST index on the same table.</para>
- </listitem>
- <listitem><para>R-Tree indexes in PostgreSQL cannot handle features which are
- larger than 8K in size. GiST indexes can, using the "lossy" trick of
- substituting the bounding box for the feature itself.</para>
- </listitem>
- </itemizedlist>
- </answer>
- </qandaentry>
-</qandaset>
-</chapter>
+ 1 | First Geometry | LINESTRING(2 3,4 5,6 5,7 8)
+(1 row)</literallayout>
+ </answer>
+ </qandaentry>
+ <qandaentry>
+ <question>
+ <para>How do I construct a spatial query?</para>
+ </question>
+ <answer>
+ <para>There are a number of spatial operators available to
+ PostgreSQL, and several of them have been implemented by PostGIS in order to
+ provide indexing support. However, all the operators have been implemented with
+ the following important simplifying assumption: <emphasis>all features shall be
+ represented by their bounding boxes</emphasis>.</para>
+ <para>We recognize that using bounding boxes to proxy for features is
+ a limiting assumption, but it was an important one in moving from the
+ conception of a PostgreSQL spatial database to the implementation. Using
+ bounding boxes makes queries faster, indexes smaller, and operators
+ simpler. Commercial spatial databases use the same assumption -- bounding boxes are important to most indexing schemes.</para>
+ <para>The most important spatial operator from a user's perspective
+ is the "&&" operator, which tests whether one feature's bounding box
+ overlaps that of another. An example of a spatial query using && is:</para><programlisting>SELECT id,name FROM GTEST WHERE GEOM && 'BOX3D(3 4,4 5)'::box3d</programlisting><para>Note that the bounding box used for querying must be
+ explicitly declared as a <varname>box3d</varname> using the "::box3d" casting operation.</para>
+ </answer>
+ </qandaentry>
+ <qandaentry>
+ <question>
+ <para>How do I speed up spatial queries on large tables?</para>
+ </question>
+ <answer>
+ <para>Fast queries on large tables is the <emphasis>raison
+ d'etre</emphasis> of spatial databases (along with transaction support) so
+ having a good index in important.</para>
+ <para>To build a spatial index on a table with a <varname>geometry</varname> column,
+ use the "CREATE INDEX" function as follows:</para>
+ <programlisting>CREATE INDEX [indexname] ON [tablename]
+ USING GIST ( [geometrycolumn] gist_geometry_ops) WITH (islossy);</programlisting><para>The "USING GIST" option tells the server to use a GiST
+ (Generalized Search Tree) index. The reference to "gist_geometry_ops" tells the
+ server to use a particular set of comparison operators for building the index:
+ the "gist_geometry_ops" are part of the PostGIS extension. Finally, the
+ "islossy" option tells the server that the features being indexed can be
+ proxied by a smaller data structure -- in the case of geometries, the features
+ are represented in the index by their bounding boxes.</para>
+ </answer>
+ </qandaentry>
+ <qandaentry>
+ <question>
+ <para>How can I get my search to return things that really are inside
+ the search box, not just overlapping bounding boxes?</para>
+ </question>
+ <answer>
+ <para>The '&&' operator only checks bounding box overlaps,
+ but you can use the "truly_inside()" function to get only those feature which
+ <emphasis>actually</emphasis> intersect the search box. For example, by
+ combining the use of "&&" for a fast index search and truly_inside()
+ for an accurate final check of the result set, you can get only those features
+ inside the search box (note that this <emphasis>only</emphasis> works for
+ search boxes right now, not any arbitrary geometry):</para>
+ <programlisting>SELECT [COLUMN1],[COLUMN2],AsText([GEOMETRYCOLUMN])
+ FROM [TABLE] WHERE [GEOM_COLUMN] && [BOX3d]
+ AND truly_inside([GEOM_COLUMN],[BOX3d]);</programlisting>
+ </answer>
+ </qandaentry>
+ <qandaentry>
+ <question>
+ <para>Why aren't R-Tree's supported?</para>
+ </question>
+ <answer>
+ <para>Early versions of PostGIS support R-Tree indexing. R-Trees have been completely discarded since version 0.6. Our tests have shown search speed for R-Tree and GiST to be comparable. R-Trees have two limitations which make them undesirable for
+ use with GIS feature in the PostgreSQL database (note that these limitations
+ are due to the PostgreSQL implementations, not the R-Tree concept in
+ general):</para>
+ <itemizedlist>
+ <listitem>
+ <para>Building an R-Tree index on a large table of geometries can
+ take over twice as long as a GiST index on the same table.</para>
+ </listitem>
+ <listitem>
+ <para>R-Tree indexes in PostgreSQL cannot handle features which
+ are larger than 8K in size. GiST indexes can, using the "lossy" trick of
+ substituting the bounding box for the feature itself.</para>
+ </listitem>
+ </itemizedlist>
+ </answer>
+ </qandaentry><qandaentry><question><para>Why should I use the AddGeometryColumn() function (and all the other OpenGIS crap)?</para></question><answer><para>If you do not want to use the OpenGIS support functions, you do not have to. Simply create tables as in older versions, defining your geometry columns in the CREATE statement. All your geometries will have SRIDs of -1, and the OpenGIS meta-data tables will not be filled in properly. For most current applications, this will not matter.</para><para>However, in the future it is likely that client software will use the meta-data tables to interrogate the database about available layers and projections before rendering data. An obvious early example is the Mapserver internet mapping software, which could be altered to interrogate the SPATIAL_REF_SYS table for projection information on the layers it is rendering.</para></answer></qandaentry>
+ </qandaset>
+ </chapter>
<chapter>
- <title>Using PostGIS</title>
+ <title>Using PostGIS</title>
<sect1 id="RefObject">
- <title>GIS Objects</title>
+ <title>GIS Objects</title>
<para>The GIS objects supported by PostGIS are the "Simple Features"
defined by the OpenGIS Consortium (OGC). Note that PostGIS currently support
the features and the representation APIs, but the various comparison and
<para>Examples of the text representations of the features are as
follows:</para>
<itemizedlist>
- <listitem><para>POINT(0 0 0)</para>
+ <listitem>
+ <para>POINT(0 0 0)</para>
</listitem>
- <listitem><para>LINESTRING(0 0,1 1,1 2)</para>
+ <listitem>
+ <para>LINESTRING(0 0,1 1,1 2)</para>
</listitem>
- <listitem><para>POLYGON((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2 0,1
- 2 0,1 1 0))</para>
+ <listitem>
+ <para>POLYGON((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2 0,1 2
+ 0,1 1 0))</para>
</listitem>
- <listitem><para>MULTIPOINT(0 0 0,1 2 1)</para>
+ <listitem>
+ <para>MULTIPOINT(0 0 0,1 2 1)</para>
</listitem>
- <listitem><para>MULTILINESTRING((0 0 0,1 1 0,1 2 1),(2 3 1,3 2 1,5 4 1))</para>
+ <listitem>
+ <para>MULTILINESTRING((0 0 0,1 1 0,1 2 1),(2 3 1,3 2 1,5 4 1))</para>
+
</listitem>
- <listitem><para>MULTIPOLYGON(((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2
- 2 0,1 2 0,1 1 0)),((-1 -1 0,-1 -2 0,-2 -2 0,-2 -1 0,-1 -1 0)))</para>
+ <listitem>
+ <para>MULTIPOLYGON(((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2
+ 0,1 2 0,1 1 0)),((-1 -1 0,-1 -2 0,-2 -2 0,-2 -1 0,-1 -1 0)))</para>
</listitem>
- <listitem><para>GEOMETRYCOLLECTION(POINT(2 3 9),LINESTRING((2 3 4,3 4 5)))</para>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(POINT(2 3 9),LINESTRING((2 3 4,3 4
+ 5)))</para>
</listitem>
</itemizedlist>
<para>Note that in the examples above there are features with both
extract it. See the sections on the 2d() and 3d() functions for information on
converting features to a particular coordinate dimension representation.</para>
+ <sect2>
+ <title>Standard versus Canonical Forms</title>
+ <para>The OpenGIS specification defines two standard ways of expressing
+ spatial objects: the Well-Known Text (WKT) form (shown in the previous section)
+ and the Well-Known Binary (WKB) form. Both WKT and WKB include information
+ about the type of the object and the coordinates which form the object.</para>
+ <para>However, the OpenGIS specification also requires that the
+ internal storage format of spatial objects include a spatial referencing system
+ identifier (SRID). The SRID is required when creating spatial objects for
+ insertion into the database. For example, a valid insert statement to create
+ and insert a spatial object would be:</para>
+ <programlisting>INSERT INTO SPATIALTABLE ( THE_GEOM, THE_NAME )
+ VALUES (
+ GeometryFromText('POINT(-126.4 45.32)', 312),
+ 'A Place'
+ ) </programlisting>
+ <para>Note that the "GeometryFromText" function requires an SRID
+ number.</para>
+ <para>The "canonical form" of the spatial objects in PostgreSQL is a
+ text representation which includes all the information necessary to construct
+ the object. Unlike the OpenGIS standard forms, it includes the type,
+ coordinate, and SRID information. The canonical form is the default form
+ returned from a SELECT query. The example below shows the difference between the OGC standard and PostGIS canonical forms:</para><programlisting>db=> SELECT AsText(geom) AS OGCGeom FROM thetable;
+OGCGeom
+-------------------------------------------------
+LINESTRING(-123.741378393049 48.9124018962261,-123.741587115639 48.9123981907507)
+(1 row)
+
+db=> SELECT geom AS PostGISGeom FROM thetable;
+PostGISGeom
+-------------------------------------------------
+SRID=123;LINESTRING(-123.741378393049 48.9124018962261,-123.741587115639 48.9123981907507)
+(1 row)
+</programlisting></sect2>
</sect1>
<sect1>
- <title>Creating a Table with Geometry</title>
- <para>Creating a table with spatial data is done using the "CREATE TABLE"
- SQL command as normal, declaring the spatial columns as type "geometry". Note
- that:</para>
- <itemizedlist>
- <listitem><para>There is no distinction between points, lines and polygons at
- the SQL datatype level. All GIS objects are declared as "geometry", no matter
- when their underlying topology. This means that you can have points, lines, and
- polygons all sharing the same spatial column of a database table. If you are
- making assumptions about feature type homogeneity, you will have to enforce
- them at the time of data loading or retrieval, the database will not do it for
- you.</para>
- </listitem>
- <listitem><para>Unlike some commercial spatial databases, there is no
- limitation on the number of spatial columns you can put in a table.</para>
- </listitem>
- <listitem><para>"Geometry" is a reserved word now (it is the GIS datatype) so
- you cannot name your spatial column "geometry". Try "geom" or "parks" or
- "frank".</para>
- </listitem>
- </itemizedlist>
- <para>Here are some examples of "CREATE TABLE" commands using the
- "geometry" type:</para>
- <itemizedlist>
- <listitem><para>CREATE TABLE bc_parks ( PID int4, PIN int4, PARK_NAME
- varchar(80), PARK_DATE date, PARK_GEOM geometry)</para>
- </listitem>
- <listitem><para>CREATE TABLE road_segments ( GID int4, GEOM geometry, SURFACE
- varchar(20), NUM_LANES int2)</para>
- </listitem>
- </itemizedlist>
+ <title>Using OpenGIS Standards</title>
+ <para>The OpenGIS "Simple Features Specification for SQL" defines
+ standard GIS object types, the functions required to manipulate them, and a set
+ of meta-data tables. In order to ensure that meta-data remain consistent,
+ operations such as creating and removing a spatial column are carried out
+ through special procedures defined by OpenGIS.</para>
+ <para>There are two OpenGIS meta-data tables: SPATIAL_REF_SYS and
+ GEOMETRY_COLUMNS. The SPATIAL_REF_SYS table holds the numeric IDs and textual
+ descriptions of coordinate systems used in the spatial database.</para>
+ <sect2>
+ <title>The SPATIAL_REF_SYS Table</title>
+ <para>The SPATIAL_REF_SYS table definition is as follows:</para>
+ <programlisting>CREATE TABLE SPATIAL_REF_SYS (
+ SRID INTEGER NOT NULL PRIMARY KEY,
+ AUTH_NAME VARCHAR(256),
+ AUTH_SRID INTEGER,
+ SRTEXT VARCHAR(2048)
+)
+ </programlisting>
+ <para>The SPATIAL_REF_SYS columns are as follows:</para>
+ <variablelist>
+ <varlistentry>
+ <term>SRID</term>
+ <listitem>
+ <para>An integer value that uniquely identifies the Spatial
+ Referencing System within the database.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>AUTH_NAME</term>
+ <listitem>
+ <para>The name of the standard or standards body that is being
+ cited for this reference system. For example, "EPSG" would be a valid
+ AUTH_NAME.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>AUTH_SRID</term>
+ <listitem>
+ <para>The ID of the Spatial Reference System as defined by the
+ Authority cited in the AUTH_NAME. In the case of EPSG, this is where the EPSG
+ projection code would go.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>SRTEXT</term>
+ <listitem>
+ <para>The Well-Known Text representation of the Spatial Reference
+ System. An example of a WKT SRS representation is:</para>
+ <programlisting>PROJCS["NAD83 / UTM Zone 10N",
+ GEOGCS["NAD83",
+ DATUM["North_American_Datum_1983",
+ SPHEROID["GRS 1980",6378137,298.257222101]
+ ],
+ PRIMEM["Greenwich",0],
+ UNIT["degree",0.0174532925199433]
+ ],
+ PROJECTION["Transverse_Mercator"],
+ PARAMETER["latitude_of_origin",0],
+ PARAMETER["central_meridian",-123],
+ PARAMETER["scale_factor",0.9996],
+ PARAMETER["false_easting",500000],
+ PARAMETER["false_northing",0],
+ UNIT["metre",1]
+]</programlisting>
+ <para>For a listing of EPSG projection codes and their
+ corresponding WKT representations, see
+ <ulink url="http://www.opengis.org/techno/interop/EPSG2WKT.TXT">http://www.opengis.org/techno/interop/EPSG2WKT.TXT</ulink>.
+ For a discussion of WKT in general, see the OpenGIS "Coordinate Transformation
+ Services Implementation Specification" at
+ <ulink url="http://www.opengis.org/techno/specs.htm">http://www.opengis.org/techno/specs.htm</ulink>.</para>
+
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+ <sect2>
+ <title>The GEOMETRY_COLUMNS Table</title>
+ <para>The GEOMETRY_COLUMNS table definition is as follows:</para>
+ <programlisting>CREATE TABLE GEOMETRY_COLUMNS (
+ F_TABLE_CATALOG VARCHAR(256) NOT NULL,
+ F_TABLE_SCHEMA VARCHAR(256) NOT NULL,
+ F_TABLE_NAME VARCHAR(256) NOT NULL,
+ F_GEOMETRY_COLUMN VARCHAR(256) NOT NULL,
+ COORD_DIMENSION INTEGER NOT NULL,
+ SRID INTEGER NOT NULL,
+ TYPE VARCHAR(30) NOT NULL
+)
+ </programlisting>
+ <para>The columns are as follows:</para>
+ <variablelist>
+ <varlistentry>
+ <term>F_TABLE_CATALOG, F_TABLE_SCHEMA, F_TABLE_NAME</term>
+ <listitem>
+ <para>The fully qualified name of the feature table containing
+ the geometry column. Note that the terms "catalog" and "schema" are Oracle-ish.
+ There is not PostgreSQL analogue of "catalog" so that column is left blank --
+ for "schema" the database name is used.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>F_GEOMETRY_COLUMN</term>
+ <listitem>
+ <para>The name of the geometry column in the feature
+ table.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>COORD_DIMENSION</term>
+ <listitem>
+ <para>The spatial dimension (2 or 3 dimensional) of the
+ column.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>SRID</term>
+ <listitem>
+ <para>The ID of the spatial reference system used for the
+ coordinate geometry in this table. It is a foreign key reference to the
+ SPATIAL_REF_SYS.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>TYPE</term>
+ <listitem>
+ <para>The type of the spatial object. To restrict the spatial
+ column to a single type, use one of: POINT, LINESTRING, POLYGON, MULTPOINT,
+ MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION. For heterogeneous
+ (mixed-type) collections, you can use "GEOMETRY" as the type. </para>
+ <note>
+ <para>This attribute is (probably) not part of the OpenGIS
+ specification, but is required for ensuring type homogeneity.</para>
+ </note>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+ <sect2>
+ <title>Creating a Spatial Table</title>
+ <para>Creating a table with spatial data is done in two stages:</para>
+ <itemizedlist>
+ <listitem>
+ <para>Create a normal non-spatial table.</para>
+ <para>For example: CREATE TABLE ROADS_GEOM (ID int4, NAME
+ varchar(25))</para>
+ </listitem>
+ <listitem>
+ <para>Add a spatial column to the table using the OpenGIS
+ "AddGeometryColumn" function. The syntax is: AddGeometryColumn(<db_name>,
+ <table_name>, <column_name>, <srid>, <type>,
+ <dimension>).</para>
+ <para>For example: SELECT AddGeometryColumn('roads_db',
+ 'roads_geom', 'geom', 423, 'LINESTRING', 2)</para>
+ </listitem>
+ </itemizedlist>
+ <para>Here is an example of SQL used to create a table and add a
+ spatial column (assuming the db is 'parks_db' and that an SRID of 128 exists
+ already):</para>
+ <programlisting>CREATE TABLE PARKS ( PARK_ID int4, PARK_NAME varchar(128), PARK_DATE date, PARK_TYPE varchar(2) );
+SELECT AddGeometryColumn('parks_db', 'parks', 'park_geom', 128, MULTIPOLYGON, 2 );
+</programlisting>
+ <para>Here is another example, using the generic "geometry" type and
+ the undefined SRID value of -1:</para>
+ <programlisting>CREATE TABLE ROADS ( ROAD_ID int4, ROAD_NAME varchar(128) );
+SELECT AddGeometryColumn( 'roads_db', 'roads', 'roads_geom', -1, GEOMETRY, 3 );
+</programlisting>
+ </sect2>
</sect1>
<sect1>
- <title>Loading GIS Data</title>
+ <title>Loading GIS Data</title>
<para>Once you have created a spatial table, you are ready to upload GIS
data to the database. Currently, there are two ways to get data into a
PostGIS/PostgreSQL database: using formatted SQL statements or using the Shape
file loader/dumper.</para>
<sect2>
- <title>Using SQL</title>
+ <title>Using SQL</title>
<para>If you can convert your data to a text representation, then using
formatted SQL might be the easiest way to get your data into PostGIS. As with
Oracle and other SQL databases, data can be bulk loaded by piping a large text
file full of SQL "INSERT" statements into the SQL terminal monitor.</para>
- <para>A data upload file ("roads.sql" for example) might look like
- this:</para> <literallayout>INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (1,'LINESTRING(191232 243118,191108 243242)','Jeff Rd');
-INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (2,'LINESTRING(189141 244158,189265 244817)','Geordie Rd');
-INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (3,'LINESTRING(192783 228138,192612 229814)','Paul St');
-INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (4,'LINESTRING(189412 252431,189631 259122)','Graeme Ave');
-INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (5,'LINESTRING(190131 224148,190871 228134)','Phil Tce');
-INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (6,'LINESTRING(198231 263418,198213 268322)','Dave Cres');
-</literallayout>
+ <para>A data upload file (<filename>roads.sql</filename> for example) might look like
+ this:</para>
+ <literallayout>INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (1,GeometryFromText('LINESTRING(191232 243118,191108 243242)',-1),'Jeff Rd');
+INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (2,GeometryFromText('LINESTRING(189141 244158,189265 244817)',-1),'Geordie Rd');
+INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (3,GeometryFromText('LINESTRING(192783 228138,192612 229814)',-1),'Paul St');
+INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (4,GeometryFromText('LINESTRING(189412 252431,189631 259122)',-1),'Graeme Ave');
+INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (5,GeometryFromText('LINESTRING(190131 224148,190871 228134)',-1),'Phil Tce');
+INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (6,GeometryFromText('LINESTRING(198231 263418,198213 268322)',-1),'Dave Cres');</literallayout>
<para>The data file can be piped into PostgreSQL very easily using the
- "psql" SQL terminal monitor:</para><literallayout>psql -d [database] -f roads.sql</literallayout>
+ "psql" SQL terminal monitor:</para>
+ <literallayout>psql -d [database] -f roads.sql</literallayout>
</sect2>
<sect2>
- <title>Using the Loader</title>
- <para>The data loader converts ESRI Shape files into SQL suitable for insertion into a PostGIS/PostgreSQL database. The loader has several operating modes distinguished by command line flags:</para><variablelist><varlistentry><term>-d</term><listitem><para>Drops the database table before creating a new table with the data in the Shape file.</para></listitem></varlistentry><varlistentry><term>-a</term><listitem><para>Appends data from the Shape file into the database table. Note that to use this option to load multiple files, the files must have the same attributes and same data types.</para></listitem></varlistentry><varlistentry><term>-c</term><listitem><para>Creates a new table and populates it from the Shape file. <emphasis>This is the default mode.</emphasis></para></listitem></varlistentry><varlistentry><term>-dump</term><listitem><para>Creates a new table and populates it from the Shape file. This uses the PostgreSQL "dump" format for the output data and is much faster to load than the default "insert" SQL format. Use this for very large data sets.</para></listitem></varlistentry></variablelist><para>An example session using the loader to create an input file and uploading it might look like this:</para><literallayout>shp2pgsql shaperoads roadstable > roads.sql
-psql -d roadsdb -f roads.sql</literallayout><para>A conversion and upload can be done all in one step using UNIX pipes:</para><literallayout>shp2pgsql shaperoads roadstable | psql -d roadsdb</literallayout></sect2>
+ <title>Using the Loader</title>
+ <para>The data loader converts ESRI Shape files into SQL suitable for
+ insertion into a PostGIS/PostgreSQL database. The loader has several operating
+ modes distinguished by command line flags:</para>
+ <variablelist>
+ <varlistentry>
+ <term>-d</term>
+ <listitem>
+ <para>Drops the database table before creating a new table with
+ the data in the Shape file.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>-a</term>
+ <listitem>
+ <para>Appends data from the Shape file into the database table.
+ Note that to use this option to load multiple files, the files must have the
+ same attributes and same data types.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>-c</term>
+ <listitem>
+ <para>Creates a new table and populates it from the Shape file.
+ <emphasis>This is the default mode.</emphasis></para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>-dump</term>
+ <listitem>
+ <para>Creates a new table and populates it from the Shape file.
+ This uses the PostgreSQL "dump" format for the output data and is much faster
+ to load than the default "insert" SQL format. Use this for very large data
+ sets.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>An example session using the loader to create an input file and
+ uploading it might look like this:</para>
+ <literallayout>shp2pgsql shaperoads roadstable > roads.sql psql -d
+ roadsdb -f roads.sql</literallayout>
+ <para>A conversion and upload can be done all in one step using UNIX
+ pipes:</para>
+ <literallayout>shp2pgsql shaperoads roadstable | psql -d
+ roadsdb</literallayout>
+ </sect2>
</sect1>
<sect1>
- <title>Retrieving GIS Data</title>
+ <title>Retrieving GIS Data</title>
<para>Data can be extracted from the database using either SQL or the
Shape file loader/dumper. In the section on SQL we will discuss some of the
operators available to do comparisons and queries on spatial tables.</para>
<sect2>
- <title>Using SQL</title>
+ <title>Using SQL</title>
<para>The most straightforward means of pulling data out of the
database is to use a SQL select query and dump the resulting columns into a
- parsable text file:</para> <literallayout>db=# SELECT * FROM ROADS_GEOM;
- id | geom | name
- 1 | LINESTRING(191232 243118,191108 243242) | Jeff Rd
- 2 | LINESTRING(189141 244158,189265 244817 | Geordie Rd
- 3 | LINESTRING(192783 228138,192612 229814) | Paul St
- 4 | LINESTRING(189412 252431,189631 259122) | Graeme Ave
- 5 | LINESTRING(190131 224148,190871 228134) | Phil Tce
- 6 | LINESTRING(198231 263418,198213 268322) | Dave Cres
+ parsable text file:</para>
+ <literallayout>db=# SELECT id, AsText(geom) AS geom, name FROM ROADS_GEOM;
+id | geom | name
+---+-----------------------------------------+-----------
+ 1 | LINESTRING(191232 243118,191108 243242) | Jeff Rd
+ 2 | LINESTRING(189141 244158,189265 244817) | Geordie Rd
+ 3 | LINESTRING(192783 228138,192612 229814) | Paul St
+ 4 | LINESTRING(189412 252431,189631 259122) | Graeme Ave
+ 5 | LINESTRING(190131 224148,190871 228134) | Phil Tce
+ 6 | LINESTRING(198231 263418,198213 268322) | Dave Cres
(6 rows)
-
-db=#
</literallayout>
<para>However, there will be times when some kind of restriction is
necessary to cut down the number of fields returned. In the case of
attribute-based restrictions, just use the same SQL syntax as normal with a
non-spatial table. In the case of spatial restrictions, the following operators
are available/useful:</para>
- <variablelist><varlistentry><term>&&</term><listitem><para>This operator tells whether the bounding box of one geometry
- overlaps the bounding box of another.</para>
- </listitem></varlistentry><varlistentry><term>~=</term><listitem><para>This operators tests whether two geometries are geometrically
- identical. For example, if "POLYGON((0 0,1 1,1 0,0 0))" is the same as
- "POLYGON((0 0,1 0,1 1,0 0))" (it is).</para>
- </listitem></varlistentry><varlistentry><term>=</term><listitem><para>This operator is a little more naive, it only tests whether
- the bounding boxes of to geometries are the same.</para>
- </listitem></varlistentry></variablelist><para>Next, you can use these operators in queries. Note that when
- specifying geometries and boxes on the SQL command line, you should explicitly
- cast the string representations to their correct type using the "::" casting
- operator. So, for example:</para><literallayout>SELECT * FROM ROADS_GEOM WHERE GEOM ~= 'LINESTRING(191232 243118,191108 243242)'::geometry;</literallayout>
+ <variablelist>
+ <varlistentry>
+ <term>&&</term>
+ <listitem>
+ <para>This operator tells whether the bounding box of one
+ geometry overlaps the bounding box of another.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>~=</term>
+ <listitem>
+ <para>This operators tests whether two geometries are
+ geometrically identical. For example, if "POLYGON((0 0,1 1,1 0,0 0))" is the
+ same as "POLYGON((0 0,1 0,1 1,0 0))" (it is).</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>=</term>
+ <listitem>
+ <para>This operator is a little more naive, it only tests whether
+ the bounding boxes of to geometries are the same.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Next, you can use these operators in queries. Note that when
+ specifying geometries and boxes on the SQL command line, you must explicitly
+ turn the string representations into geometries by using the "GeometryFromText()" function. So, for example:</para>
+ <literallayout>SELECT ID, NAME FROM ROADS_GEOM
+ WHERE GEOM ~= GeometryFromText('LINESTRING(191232 243118,191108 243242)',-1);</literallayout>
<para>The above query would return the single record from the
"ROADS_GEOM" table in which the geometry was equal to that value.</para>
<para>When using the "&&" operator, you can specify either a
BOX3D as the comparison feature or a GEOMETRY. When you specify a GEOMETRY,
- however, the bounding box will be used for the comparison.</para><literallayout>SELECT * FROM ROADS_GEOM WHERE GEOM && 'POLYGON((191232 243117,191232 243119,191234 243117,191232 243117))'::geometry;</literallayout>
+ however, the bounding box will be used for the comparison.</para>
+ <literallayout>SELECT ID, NAME FROM ROADS_GEOM
+ WHERE GEOM && GeometryFromText('POLYGON((191232 243117,191232 243119,191234 243117,191232 243117))',-1);</literallayout>
<para>The above query will use the bounding box of the polygon for
- comparison purposes.</para>
+ comparison purposes.</para>
<para>The most common spatial query will probably be a "frame-based"
query, used by client software, like data browsers and web mappers, to grab a
"map frame" worth of data for display. Using a "BOX3D" object for the frame,
- such a query looks like this:</para> <literallayout>SELECT GEOM FROM ROADS_GEOM WHERE GEOM && 'BOX3D(191232 243117,191232 243119)'::box3d;</literallayout>
- <para>Note the use of the "::box3d" casting opertator at the end of the
- SQL statement to ensure that the BOX3D string representation is properly cast
- into a BOX3D object before the query executes.</para>
+ such a query looks like this:</para>
+ <literallayout>SELECT AsText(GEOM) AS GEOM FROM ROADS_GEOM
+ WHERE GEOM && GeometryFromText('BOX3D(191232 243117,191232 243119)',-1);</literallayout>
+ <para>Note the use of the SRID, to specify the projection of the BOX3D. The -1 is used to indicate no specified SRID.</para>
</sect2>
<sect2>
- <title>Using the Dumper</title>
+ <title>Using the Dumper</title>
<para>This section to be written.</para>
</sect2>
- </sect1>
+ <sect2><title>Using Minnesota Mapserver</title><para>The Minnesota Mapserver is an internet web-mapping server. The latest versions conform to the OpenGIS Web Map Specification.</para><itemizedlist><listitem> <para>The Mapserver homepage is at <ulink url="http://mapserver.gis.umn.edu">http://mapserver.gis.umn.edu</ulink>.</para></listitem><listitem> <para>The OpenGIS Web Map Specification is at <ulink url="http://www.opengis.org/techno/specs/01-047r2.pdf">http://www.opengis.org/techno/specs/01-047r2.pdf</ulink>.</para></listitem></itemizedlist><para>To use PostGIS with Mapserver, you will need to know about how to configure Mapserver, which is beyond the scope of this documentation. This section will cover specific PostGIS issues and configuration details.</para><para>To use PostGIS with Mapserver, you will need:</para><itemizedlist><listitem><para>The latest version of PostGIS.</para></listitem><listitem><para>Version 3.5 of Mapserver. At the date of writing, version 3.5 has not been released, but the CVS version is fully functional with PostGIS.</para></listitem></itemizedlist><para>Mapserver accesses PostGIS/PostgreSQL data like any other PostgreSQL client -- using <filename>libpq</filename>. This means that Mapserver can be installed on any machine with network access to hte PostGIS server, as long as the system has the <filename>libpq</filename> PostgreSQL client libraries.</para><orderedlist><listitem><para>Compile and install Mapserver, with whatever options you desire, including the "--with-postgis" configuration option.</para></listitem><listitem><para>In your Mapserver map file, add a PostGIS layer. For example:</para><programlisting>LAYER
+ CONNECTIONTYPE postgis
+ NAME "widehighways"
+ # Connect to a remote spatial database
+ CONNECTION "user=dbuser dbname=gisdatabase host=bigserver"
+ # Get the lines from the 'geom' column of the 'roads' table
+ DATA "geom from roads"
+ STATUS ON
+ TYPE LINE
+ # Of the lines in the extents, only render the wide highways
+ FILTER "type = 'highway' and numlanes >= 4"
+ CLASS
+ # Make the superhighways brighter and 2 pixels wide
+ EXPRESSION ([numlanes] >= 6)
+ COLOR 255 22 22
+ SYMBOL "solid"
+ SIZE 2
+ END
+ CLASS
+ # All the rest are darker and only 1 pixel wide
+ EXPRESSION ([numlanes] < 6)
+ COLOR 205 92 82
+ END
+END</programlisting><para>In the example above, the PostGIS-specific directives are as follows:</para><variablelist><varlistentry><term>CONNECTIONTYPE</term><listitem><para>For PostGIS layers, this is always "postgis".</para></listitem></varlistentry><varlistentry><term>CONNECTION</term><listitem><para>The database connection is governed by the a 'connection string' which
+is a standard set of keys and values like this (with the default values
+in <>):</para><para>user=<username> password=<password> dbname=<username> hostname=<server> port=<5432></para><para>An empty connection string is still valid, and any of the key/value
+pairs can be omitted. At a minimum you will generally supply the database
+name and username to connect with.</para></listitem></varlistentry><varlistentry><term>DATA</term><listitem><para>The form of this parameter is "<column> from <tablename>" where the column is the spatial column to be rendered to the name.</para></listitem></varlistentry><varlistentry><term>FILTER</term><listitem><para>The filter must be a valid SQL string corresponding to the logic normally following the "WHERE" keyword in a SQL query. So, for example, to render only roads with 6 or more lanes, use a filter of "num_lanes >= 6".</para></listitem></varlistentry></variablelist></listitem><listitem><para>In your spatial database, ensure you have spatial (GiST) indexes built for any the layers you will be drawing.</para></listitem><listitem><para>If you will be querying your layers using Mapserver you will also require an "oid index".</para><para>Mapserver requires unique identifiers for each spatial record when doing queries, and the PostGIS module of Mapserver uses the PostgreSQL <varname>oid</varname> value to provide these unique identifiers. A side-effect of this is that in order to do fast random access of records during queries, an index on the <varname>oid</varname> is needed. </para><para>To build an "oid index", use the following SQL:</para><programlisting>CREATE INDEX <indexname> ON <tablename> ( oid );</programlisting></listitem></orderedlist></sect2></sect1>
<sect1>
- <title>Building Indexes</title>
+ <title>Building Indexes</title>
<para>Indexes are what make using a spatial database for large databases
possible. Without indexing, any search for a feature would require a
"sequential scan" of every record in the database. Indexing speeds up searching
by organizing the data into a search tree which can be quickly traversed to
find a particular record. PostgreSQL supports three kinds of indexes by
- default: B-Tree indexes, R-Tree indexes, and GiST indexes. B-Trees are used for
- data which can be sorted along one axis; for example, numbers, letters, dates.
- GIS data cannot be rationally sorted along one axis (which is greater, (0,0) or
- (0,1) or (1,0)?) so B-Tree indexing is of no use for us. Both R-Tree and GiST
- indexing can be used to index GIS data, and PostGIS has implementations for
- both. Note however, that <emphasis>only GiST indexes are supported and R-Trees
- will likely be dropped in the future</emphasis>.</para>
+ default: B-Tree indexes, R-Tree indexes, and GiST indexes.</para>
+ <itemizedlist>
+ <listitem>
+ <para>B-Trees are used for data which can be sorted along one axis;
+ for example, numbers, letters, dates. GIS data cannot be rationally sorted
+ along one axis (which is greater, (0,0) or (0,1) or (1,0)?) so B-Tree indexing
+ is of no use for us. </para>
+ </listitem>
+ <listitem>
+ <para>R-Trees break up data into rectangles, and sub-rectangles, and
+ sub-sub rectanges, etc. R-Trees are used by some spatial databases to index GIS
+ data, but the PostgreSQL R-Tree implementation is not as robust as the GiST
+ implementation.</para>
+ </listitem>
+ <listitem>
+ <para> GiST (Generalized Search Trees) indexes break up data into
+ "things to one side", "things which overlap", "things which are inside" and can
+ be used on a wide range of datatypes, including GIS data. PostGIS uses GiST
+ indexing to index GIS data.</para>
+ </listitem>
+ </itemizedlist>
<sect2>
- <title>GiST Indexes</title>
+ <title>GiST Indexes</title>
<para>GiST stands for "Generalized Search Tree" and is a generalized
form of R-Tree indexing. In addition to GIS indexing, GiST is used to speed up
searches on all kinds of irregular data structures (integer arrays, spectral
- data, etc) which are not amenable to normal B-Tree indexing.</para>
+ data, etc) which are not amenable to normal B-Tree indexing.</para>
<para>Once a GIS data table exceeds a few thousand rows, you will want
to build an index to speed up spatial searches of the data (unless all your
searches are based on attributes, in which case you'll want to build a normal
- index on the attribute fields).</para>
+ index on the attribute fields).</para>
<para>The syntax for building a GiST index on a "geometry" column is as
- follows:</para><literallayout>CREATE INDEX [indexname] ON [tablename] USING GIST
- ( [geometryfield] GIST_GEOMETRY_OPS ) WITH ( ISLOSSY );</literallayout>
+ follows:</para>
+ <literallayout>CREATE INDEX [indexname] ON [tablename] USING GIST (
+ [geometryfield] GIST_GEOMETRY_OPS ) WITH ( ISLOSSY ); </literallayout>
<para>Building a spatial index is a computationally intensive exercise:
on tables of around 1 million rows, on a 300MHz Solaris machine, we have found
- building a GiST index takes about 1 hour.</para>
+ building a GiST index takes about 1 hour.</para>
<para>GiST indexes have two advantages over R-Tree indexes in
PostgreSQL. Firstly, GiST indexes build much faster than R-Trees; we have found
that it takes about 4 times more time to build an R-Tree than a GiST index on
phase.</para>
</sect2>
<sect2>
- <title>R-Tree Indexes</title>
- <para>R-Tree indexes use rectangles to break up a spatial plane into
- chunks for searching. Big rectangles are parents of smaller rectangles which
- are in turn parents of yet smaller rectangles, and so on. The algorithms are
- complex, but the principal is the same as that of the B-Tree: by traversing
- from parent to child down the structure of rectangles, you can locate
- particular spatial objects extremely quickly.</para>
- <para>R-Trees are not recommended for GIS objects: they take much
- longer to build, and do not support objects larger than 8K in size.</para>
- <para>The syntax for building an R-Tree index on a "geometry" column is
- as follows:</para> <literallayout>CREATE INDEX [indexname] ON [tablename] USING RTREE
- ( [geometryfield] RT_GEOMETRY_OPS );</literallayout>
- </sect2>
- <sect2>
- <title>Using Indexes</title>
+ <title>Using Indexes</title>
<para>Ordinarily, indexes invisibly speed up data access: once the
index is built, the query planner tranparently decides when to use index
information to speed up a query plan. Unfortunately, the PostgreSQL query
planner does not optimize the use of R-Tree and GiST indexes well, so very
often searches which should use a spatial index instead default to a sequence
- scan of the whole table.</para>
+ scan of the whole table.</para>
<para>If you find your spatial indexes are not being used (or your
attribute indexes, for that matter) there are a couple things you can
do:</para>
<itemizedlist>
- <listitem><para>Firstly, make sure you run the "VACUUM ANALYZE
- [tablename]" command on the tables you are having problems with. "VACUUM
- ANALYZE" gathers statistics about the number and distributions of values in a
- table, to provide the query planner with better information to make decisions
- around index usage.</para>
- </listitem>
- <listitem><para>Secondly, you can force the planner to use the index
- information by using the "SET ENABLE_SEQSCAN=OFF" command. You should only use
- this command sparingly, and only on spatially indexed queries: generally
- speaking, the planner knows better than you do about when to use normal B-Tree
- indexes. Once you have run your query, you should consider setting
- "ENABLE_SEQSCAN" back on, so that other queries will utilize the planner as
- normal.</para>
+ <listitem>
+ <para>Firstly, make sure you run the "VACUUM ANALYZE [tablename]"
+ command on the tables you are having problems with. "VACUUM ANALYZE" gathers
+ statistics about the number and distributions of values in a table, to provide
+ the query planner with better information to make decisions around index
+ usage.</para>
</listitem>
+ <listitem>
+ <para>Secondly, you can force the planner to use the index
+ information by using the "SET =OFF" command. You should only use
+ this command sparingly, and only on spatially indexed queries: generally
+ speaking, the planner knows better than you do about when to use normal B-Tree
+ indexes. Once you have run your query, you should consider setting
+ "ENABLE_SEQSCAN" back on, so that other queries will utilize the planner as
+ normal.</para>
+ <note><para>As of version 0.6, it should not be necessary to force the planner to use the index with "ENABLE_SEQSCAN".</para></note></listitem>
</itemizedlist>
- </sect2>
+ </sect2>
</sect1>
<sect1>
- <title>Java Clients (JDBC)</title>
+ <title>Java Clients (JDBC)</title>
<para>Java clients can access PostGIS "geometry" objects in the
PostgreSQL database either directly as text representations or using the JDBC
extension objects bundled with PostGIS. In order to use the extension objects,
the "postgis.jar" file must be in your CLASSPATH along with the
- "postgresql.jar" JDBC driver package.</para> <programlisting>import java.sql.*;
-import java.util.*;
-import java.lang.*;
-import org.postgis.*;
-
-public class JavaGIS
-{
- public static void main(String[] args)
- {
- java.sql.Connection conn;
-
- try {
-
- /*
- * Load the JDBC driver and establish a connection.
- */
- Class.forName("org.postgresql.Driver");
- String url = "jdbc:postgresql://localhost:5432/database";
- conn = DriverManager.getConnection(url, "postgres", "");
+ "postgresql.jar" JDBC driver package.</para>
+ <programlisting>import java.sql.*;
+import java.util.*;
+import java.lang.*;
+import org.postgis.*;
- /*
- * Add the geometry types to the connection. Note that you
- * must cast the connection to the pgsql-specific connection
- * implementation before calling the addDataType() method.
- */
- ((org.postgresql.Connection)conn).addDataType("geometry","org.postgis.PGgeometry");
- ((org.postgresql.Connection)conn).addDataType("box3d","org.postgis.PGbox3d");
-
- /*
- * Create a statement and execute a select query.
- */
- Statement s = conn.createStatement();
- ResultSet r = s.executeQuery("select geom,id from geomtable");
- while( r.next() )
- {
- /*
- * Retrieve the geometry as an object then cast it to the geometry type.
- * Print things out.
+public class JavaGIS {
+ public static void main(String[] args)
+ {
+ java.sql.Connection conn;
+ try
+ {
+ /*
+ * Load the JDBC driver and establish a connection.
+ */
+ Class.forName("org.postgresql.Driver");
+ String url = "jdbc:postgresql://localhost:5432/database";
+ conn = DriverManager.getConnection(url, "postgres", "");
+
+ /*
+ * Add the geometry types to the connection. Note that you
+ * must cast the connection to the pgsql-specific connection * implementation before calling the addDataType() method.
*/
- PGgeometry geom = (PGgeometry)r.getObject(1);
- int id = r.getInt(2);
- System.out.println("Row " + id + ":");
- System.out.println(geom.toString());
- }
- s.close();
- conn.close();
- } catch( Exception e ) {
- e.printStackTrace();
- }
+ ((org.postgresql.Connection)conn).addDataType("geometry","org.postgis.PGgeometry");
+ ((org.postgresql.Connection)conn).addDataType("box3d","org.postgis.PGbox3d");
+
+ /*
+ * Create a statement and execute a select query.
+ */
+ Statement s = conn.createStatement();
+ ResultSet r = s.executeQuery("select AsText(geom) as geom,id from geomtable");
+ while( r.next() )
+ {
+ /*
+ * Retrieve the geometry as an object then cast it to the geometry type.
+ * Print things out.
+ */
+ PGgeometry geom = (PGgeometry)r.getObject(1);
+ int id = r.getInt(2);
+ System.out.println("Row " + id + ":");
+ System.out.println(geom.toString());
+ }
+ s.close();
+ conn.close();
+ }
+ catch( Exception e )
+ {
+ e.printStackTrace();
+ }
}
-}</programlisting>
+}</programlisting>
<para>The "PGgeometry" object is a wrapper object which contains a
specific topological geometry object (subclasses of the abstract class
"Geometry") depending on the type: Point, LineString, Polygon, MultiPoint,
- MultiLineString, MultiPolygon.</para><programlisting>PGgeometry geom = (PGgeometry)r.getObject(1);
-if( geom.getType() = Geometry.POLYGON ) {
+ MultiLineString, MultiPolygon.</para>
+ <programlisting>PGgeometry geom = (PGgeometry)r.getObject(1);
+if( geom.getType() = Geometry.POLYGON )
+{
Polygon pl = (Polygon)geom.getGeometry();
- for( int r = 0; r < pl.numRings(); r++ ) {
+ for( int r = 0; r < pl.numRings(); r++ )
+ {
LinearRing rng = pl.getRing(r);
- System.out.println("Ring: " + r);
- for( int p = 0; p < rng.numPoints(); p++ ) {
- Point pt = rng.getPoint(p);
+ System.out.println("Ring: " + r);
+ for( int p = 0; p < rng.numPoints(); p++ )
+ {
+ Point pt = rng.getPoint(p);
System.out.println("Point: " + p);
- System.out.println(pt.toString());
- }
- }
-}</programlisting>
+ System.out.println(pt.toString());
+ }
+ }
+}</programlisting>
<para>The JavaDoc for the extension objects provides a reference for the
- various data acessor functions in the geometric objects.</para>
+ various data acessor functions in the geometric objects.</para>
</sect1>
<sect1>
- <title> C Clients (libpq)</title>
+ <title> C Clients (libpq)</title>
<para>...</para>
<sect2>
- <title>Text Cursors</title>
+ <title>Text Cursors</title>
<para>...</para>
</sect2>
<sect2>
- <title>Binary Cursors</title>
+ <title>Binary Cursors</title>
<para>...</para>
</sect2>
</sect1>
</chapter>
<chapter>
- <title>PostGIS Reference</title>
- <para>The functions given below are the ones which a user of PostGIS is likely to need. There are other functions which are required support functions to the PostGIS objects which are not of use to a general user.</para>
+ <title>PostGIS Reference</title>
+ <para>The functions given below are the ones which a user of PostGIS is
+ likely to need. There are other functions which are required support functions
+ to the PostGIS objects which are not of use to a general user.</para>
<sect1>
- <title>OpenGIS Functions</title>
+ <title>OpenGIS Functions</title>
<variablelist>
+ <varlistentry>
+ <term>AddGeometryColumn(varchar, varchar, varchar, integer, varchar, integer)</term>
+ <listitem>
+ <para>Syntax: AddGeometryColumn(<db_name>,
+ <table_name>, <column_name>, <srid>, <type>,
+ <dimension>). Adds a geometry column to an existing table of attributes. The <varname>srid</varname> must be an integer value reference to an entry in the SPATIAL_REF_SYS table. The <varname>type</varname> must be an uppercase string corresponding to the geometry type.</para>
+ </listitem>
+ </varlistentry><varlistentry><term>DropGeometryColumn(varchar, varchar, varchar)</term><listitem><para>Syntax: DropGeometryColumn(<db_name>,<table_name>,<column_name>). Remove a geometry column from a spatial table.</para></listitem></varlistentry>
<varlistentry>
<term>AsBinary(geometry)</term>
- <listitem><para>Returns the geometry in the OGC "well-known-binary" format,
- using the endian encoding of the server on which the database is running. This is useful in binary cursors to pull data out
- of the database without converting it to a string representation.</para>
+ <listitem>
+ <para>Returns the geometry in the OGC "well-known-binary" format,
+ using the endian encoding of the server on which the database is running. This
+ is useful in binary cursors to pull data out of the database without converting
+ it to a string representation.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>Dimension(geometry)</term>
- <listitem><para>Returns '2' if the geometry is two dimensional and '3' if the geometry is three dimensional.</para></listitem>
- </varlistentry>
-
+ <listitem>
+ <para>Returns '2' if the geometry is two dimensional and '3' if the
+ geometry is three dimensional.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>Envelope(geometry)</term>
- <listitem><para>Returns a POLYGON representing the bounding box of the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Returns a POLYGON representing the bounding box of the
+ geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>GeometryType(geometry)</term>
- <listitem><para>Returns the type of the geometry. Eg: LINESTRING, POLYGON, MULTIPOINT, etc.</para></listitem>
- </varlistentry><varlistentry>
+ <listitem>
+ <para>Returns the type of the geometry. Eg: LINESTRING, POLYGON,
+ MULTIPOINT, etc.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
<term>X(geometry)</term>
- <listitem><para>Find and return the X coordinate of the first point in the geometry. Return NULL if there is no point in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Find and return the X coordinate of the first point in the
+ geometry. Return NULL if there is no point in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>Y(geometry)</term>
- <listitem><para>Find and return the Y coordinate of the first point in the geometry. Return NULL if there is no point in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Find and return the Y coordinate of the first point in the
+ geometry. Return NULL if there is no point in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>Z(geometry)</term>
- <listitem><para>Find and return the Z coordinate of the first point in the geometry. Return NULL if there is no point in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Find and return the Z coordinate of the first point in the
+ geometry. Return NULL if there is no point in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>NumPoints(geometry)</term>
- <listitem><para>Find and return the number of points in the first linestring in the geometry. Return NULL if there is no linestring in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Find and return the number of points in the first linestring
+ in the geometry. Return NULL if there is no linestring in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>PointN(geometry,integer)</term>
- <listitem><para>Return the N'th point in the first linestring in the geometry. Return NULL if there is no linestring in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Return the N'th point in the first linestring in the
+ geometry. Return NULL if there is no linestring in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>ExteriorRing(geometry)</term>
- <listitem><para>Return the exterior ring of the first polygon in the geometry. Return NULL if there is no polygon in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Return the exterior ring of the first polygon in the
+ geometry. Return NULL if there is no polygon in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>NumInteriorRings(geometry)</term>
- <listitem><para>Return the number of interior rings of the first polygon in the geometry. Return NULL if there is no polygon in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Return the number of interior rings of the first polygon in
+ the geometry. Return NULL if there is no polygon in the geometry.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>InteriorRingN(geometry,integer)</term>
- <listitem><para>Return the N'th interior ring of the first polygon in the geometry. Return NULL if there is no polygon in the geometry.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>Return the N'th interior ring of the first polygon in the
+ geometry. Return NULL if there is no polygon in the geometry.</para>
+ </listitem>
+ </varlistentry><varlistentry><term>IsClosed(geometry)</term><listitem><para>Returns true of the geometry start and end points are coincident.</para></listitem></varlistentry>
<varlistentry>
<term>NumGeometries(geometry)</term>
- <listitem><para>If geometry is a GEOMETRYCOLLECTION return the number of geometries, otherwise return NULL.</para></listitem>
- </varlistentry>
+ <listitem>
+ <para>If geometry is a GEOMETRYCOLLECTION return the number of
+ geometries, otherwise return NULL.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term>GeometryN(geometry)</term>
- <listitem><para>Return the N'th geometry if the geometry is a GEOMETRYCOLLECTION. Otherwise, return NULL.</para></listitem>
- </varlistentry>
-
-
-
-
-
-
-
-
-
- </variablelist>
+ <listitem>
+ <para>Return the N'th geometry if the geometry is a
+ GEOMETRYCOLLECTION. Otherwise, return NULL.</para>
+ </listitem>
+ </varlistentry><varlistentry><term>Distance(geometry,geometry)</term><listitem><para>Return the cartesian distance between two geometries in projected units.</para></listitem></varlistentry><varlistentry><term>AsText(geometry)</term><listitem><para>Return the Well-Known Text representation of the geometry. For example: POLYGON(0 0,0 1,1 1,1 0,0 0)</para></listitem></varlistentry><varlistentry><term>SRID(geometry)</term><listitem><para>Returns the integer SRID number of the spatial reference system of the geometry.</para></listitem></varlistentry><varlistentry><term>GeometryFromText(varchar, integer)</term><listitem><para>Syntax: GeometryFromText(<geometry>,<SRID>) Convert a Well-Known Text representation of a geometry into a geometry object.</para></listitem></varlistentry><varlistentry><term>SetSRID(geometry)</term><listitem><para>Set the SRID on a geometry to a particular integer value. Useful in constructing geometry for queries.</para></listitem></varlistentry>
+ <varlistentry><term>EndPoint(geometry)</term><listitem><para>Returns the last point of the geometry as a point.</para></listitem></varlistentry><varlistentry><term>StartPoint(geometry)</term><listitem><para>Returns the first point of the geometry as a point.</para></listitem></varlistentry><varlistentry><term>Centroid(geometry)</term><listitem><para>Returns the centroid of the geometry as a point.</para></listitem></varlistentry>
+ </variablelist>
</sect1>
- <sect1><title>Other Functions</title><variablelist>
-
-
-
+ <sect1>
+ <title>Other Functions</title>
+ <variablelist>
<varlistentry>
<term>A <& B</term>
- <listitem><para>The "<&" operator returns true if A's bounding box
- overlaps or is to the right of B's bounding box.</para>
+ <listitem>
+ <para>The "<&" operator returns true if A's bounding box
+ overlaps or is to the right of B's bounding box.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A &> B</term>
- <listitem><para>The "&>" operator returns true if A's bounding box
- overlaps or is to the left of B's bounding box.</para>
+ <listitem>
+ <para>The "&>" operator returns true if A's bounding box
+ overlaps or is to the left of B's bounding box.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A << B</term>
- <listitem><para>The "<<" operator returns true if A's bounding box
- is strictly to the right of B's bounding box.</para>
+ <listitem>
+ <para>The "<<" operator returns true if A's bounding box is
+ strictly to the right of B's bounding box.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A >> B</term>
- <listitem><para>The ">>" operator returns true if A's bounding box
- is strictly to the left of B's bounding box.</para>
+ <listitem>
+ <para>The ">>" operator returns true if A's bounding box is
+ strictly to the left of B's bounding box.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A ~= B</term>
- <listitem><para>The "~=" operator is the "same as" operator. It tests
- actual geometric equality of two features. So if A and B are the same feature,
- vertex-by-vertex, the operator returns true.</para>
+ <listitem>
+ <para>The "~=" operator is the "same as" operator. It tests actual
+ geometric equality of two features. So if A and B are the same feature,
+ vertex-by-vertex, the operator returns true.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A ~ B</term>
- <listitem><para>The "~" operator returns true of A's bounding box is
- completely contained by B's bounding box.</para>
+ <listitem>
+ <para>The "~" operator returns true of A's bounding box is
+ completely contained by B's bounding box.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>A && B</term>
- <listitem><para>The "&&" operator is the "overlaps" operator. If
- A's bounding boux overlaps B's bounding box the operator returns true.</para>
+ <listitem>
+ <para>The "&&" operator is the "overlaps" operator. If A's
+ bounding boux overlaps B's bounding box the operator returns true.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>area2d(geometry)</term>
- <listitem><para>Returns the area of the geometry if it is a polygon or
- multi-polygon.</para>
+ <listitem>
+ <para>Returns the area of the geometry if it is a polygon or
+ multi-polygon.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>asbinary(geometry,'NDR')</term>
- <listitem><para>Returns the geometry in the OGC "well-known-binary" format,
- using little-endian encoding. This is useful in binary cursors to pull data out
- of the database without converting it to a string representation.</para>
+ <listitem>
+ <para>Returns the geometry in the OGC "well-known-binary" format,
+ using little-endian encoding. This is useful in binary cursors to pull data out
+ of the database without converting it to a string representation.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>asbinary(geometry,'XDR')</term>
- <listitem><para>Returns the geometry in the OGC "well-known-binary" format,
- using big-endian encoding. This is useful in binary cursors to pull data out
- of the database without converting it to a string representation.</para>
+ <listitem>
+ <para>Returns the geometry in the OGC "well-known-binary" format,
+ using big-endian encoding. This is useful in binary cursors to pull data out of
+ the database without converting it to a string representation.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>box3d(geometry)</term>
- <listitem><para>Returns a BOX3D representing the maximum extents of the geometry.</para>
+ <listitem>
+ <para>Returns a BOX3D representing the maximum extents of the
+ geometry.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>extent(geometry)</term>
- <listitem><para>The extent() function is an "aggregate" function in the
- terminology of PostgreSQL. That means that it operators on lists of data, in
- the same way the sum() and mean() functions do. For example, "SELECT
- EXTENT(GEOM) FROM GEOMTABLE" will return a BOX3D giving the maximum extend of
- all features in the table. Similarly, "SELECT EXTENT(GEOM) FROM GEOMTABLE GROUP
- BY CATEGORY" will return one extent result for each category.</para>
- </listitem>
- </varlistentry><varlistentry>
+ <listitem>
+ <para>The extent() function is an "aggregate" function in the
+ terminology of PostgreSQL. That means that it operators on lists of data, in
+ the same way the sum() and mean() functions do. For example, "SELECT
+ EXTENT(GEOM) FROM GEOMTABLE" will return a BOX3D giving the maximum extend of
+ all features in the table. Similarly, "SELECT EXTENT(GEOM) FROM GEOMTABLE GROUP
+ BY CATEGORY" will return one extent result for each category.</para>
+ </listitem>
+ </varlistentry><varlistentry><term>find_srid(varchar,varchar,varchar)</term><listitem><para>The syntax is find_srid(<db/schema>, <table>, <column>) and the function returns the integer SRID of the specified column.</para></listitem></varlistentry>
+ <varlistentry>
<term>force_collection(geometry)</term>
- <listitem><para>Converts the geometry into a GEOMETRYCOLLECTION. This is useful for simplifying the WKB representation.</para>
+ <listitem>
+ <para>Converts the geometry into a GEOMETRYCOLLECTION. This is
+ useful for simplifying the WKB representation.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>force_2d(geometry)</term>
- <listitem><para>Forces the geometries into a "2-dimensional mode" so that
- all output representations will only have the X and Y coordinates. This is
- useful for force OGC-compliant output (since OGC only specifies 2-D
- geometries).</para>
+ <listitem>
+ <para>Forces the geometries into a "2-dimensional mode" so that all
+ output representations will only have the X and Y coordinates. This is useful
+ for force OGC-compliant output (since OGC only specifies 2-D
+ geometries).</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>force_3d(geometry)</term>
- <listitem><para>Forces the geometries into a "3-dimensional mode" so that
- all output representations will have the X, Y and Z coordinates.</para>
+ <listitem>
+ <para>Forces the geometries into a "3-dimensional mode" so that all
+ output representations will have the X, Y and Z coordinates.</para>
</listitem>
- </varlistentry>
-
+ </varlistentry>
<varlistentry>
<term>length2d(geometry)</term>
- <listitem><para>Returns the 23-dimensional length of the geometry if it is
- a linestring or multi-linestring.</para>
+ <listitem>
+ <para>Returns the 23-dimensional length of the geometry if it is a
+ linestring or multi-linestring.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>length3d(geometry)</term>
- <listitem><para>Returns the 3-dimensional length of the geometry if it is a
- linestring or multi-linestring.</para>
+ <listitem>
+ <para>Returns the 3-dimensional length of the geometry if it is a
+ linestring or multi-linestring.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>length_spheroid(geometry,spheroid)</term>
- <listitem><para>Calculates the length of of a geometry on an elipsoid. This is useful if the coordinates of the geometry are in latitude/longitude and a length is desired without reprojection. The elipsoid is a separate database type and can be constructed as follows:<literallayout>SPHEROID[<NAME>,<SEMI-MAJOR AXIS>,<INVERSE FLATTENING>]
-Eg: SPHEROID["GRS_1980",6378137,298.257222101]</literallayout>An example calculation might look like this:<literallayout>SELECT length_spheroid(geometry_column,'SPHEROID["GRS_1980",6378137,298.257222101]') from geometry_table;</literallayout></para>
+ <listitem>
+ <para>Calculates the length of of a geometry on an elipsoid. This
+ is useful if the coordinates of the geometry are in latitude/longitude and a
+ length is desired without reprojection. The elipsoid is a separate database
+ type and can be constructed as follows:
+ <literallayout> SPHEROID[<NAME>,<SEMI-MAJOR
+ AXIS>,<INVERSE FLATTENING>] Eg:
+ SPHEROID["GRS_1980",6378137,298.257222101] </literallayout>An example
+ calculation might look like this:
+ <literallayout>SELECT
+ length_spheroid(geometry_column,'SPHEROID["GRS_1980",6378137,298.257222101]')
+ from geometry_table;</literallayout></para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>length3d_spheroid(geometry,spheroid)</term>
- <listitem><para>Calculates the length of of a geometry on an elipsoid, taking the elevation into account. This is just like length_spheroid except vertical coordinates (expressed in the same units as the spheroid axes) are used to calculate the extra distance vertical displacement adds.</para>
+ <listitem>
+ <para>Calculates the length of of a geometry on an elipsoid, taking
+ the elevation into account. This is just like length_spheroid except vertical
+ coordinates (expressed in the same units as the spheroid axes) are used to
+ calculate the extra distance vertical displacement adds.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>mem_size(geometry)</term>
- <listitem><para>Returns the amount of space (in bytes) the geometry takes.</para>
+ <listitem>
+ <para>Returns the amount of space (in bytes) the geometry
+ takes.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>npoints(geometry)</term>
- <listitem><para>Returns the number of points in the geometry.</para>
+ <listitem>
+ <para>Returns the number of points in the geometry.</para>
</listitem>
- </varlistentry>
+ </varlistentry>
<varlistentry>
<term>nrings(geometry)</term>
- <listitem><para>If the geometry is a polygon or multi-polygon returns the
- number of rings.</para>
+ <listitem>
+ <para>If the geometry is a polygon or multi-polygon returns the
+ number of rings.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>numb_sub_objects(geometry)</term>
- <listitem><para>Returns the number of objects stored in the geometry. This
- is useful for MULTI-geometries and GEOMETRYCOLLECTIONs.</para>
+ <listitem>
+ <para>Returns the number of objects stored in the geometry. This is
+ useful for MULTI-geometries and GEOMETRYCOLLECTIONs.</para>
</listitem>
- </varlistentry><varlistentry>
-
+ </varlistentry>
+ <varlistentry>
<term>perimeter2d(geometry)</term>
- <listitem><para>Returns the 2-dimensional perimeter of the geometry, if it
- is a polygon or multi-polygon.</para>
+ <listitem>
+ <para>Returns the 2-dimensional perimeter of the geometry, if it is
+ a polygon or multi-polygon.</para>
</listitem>
- </varlistentry><varlistentry><term>perimeter3d(geometry)</term>
- <listitem><para>Returns the 3-dimensional perimeter of the geometry, if it
- is a polygon or multi-polygon.</para>
+ </varlistentry>
+ <varlistentry>
+ <term>perimeter3d(geometry)</term>
+ <listitem>
+ <para>Returns the 3-dimensional perimeter of the geometry, if it is
+ a polygon or multi-polygon.</para>
</listitem>
- </varlistentry><varlistentry><term>point_inside_circle(geometry,float,float,float)</term>
- <listitem><para>The syntax for this functions is point_inside_circle(<geometry>,<circle_center_x>,<circle_center_y>,<radius>). Returns the true if the geometry is a point and is inside the circle. Returns false otherwise.</para>
+ </varlistentry>
+ <varlistentry>
+ <term>point_inside_circle(geometry,float,float,float)</term>
+ <listitem>
+ <para>The syntax for this functions is
+ point_inside_circle(<geometry>,<circle_center_x>,<circle_center_y>,<radius>).
+ Returns the true if the geometry is a point and is inside the circle. Returns
+ false otherwise.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry><varlistentry><term>postgis_version()</term><listitem><para>Returns the version number of the PostGIS functions installed in this database.</para></listitem></varlistentry><varlistentry>
<term>summary(geometry)</term>
- <listitem><para>Returns a text summary of the contents of the geometry.</para>
+ <listitem>
+ <para>Returns a text summary of the contents of the
+ geometry.</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>translate(geometry,float8,float8,float8)</term>
- <listitem><para>Translates the geometry to a new location using the numeric
- parameters as offsets. Ie: translate(geom,X,Y,Z).</para>
+ <listitem>
+ <para>Translates the geometry to a new location using the numeric
+ parameters as offsets. Ie: translate(geom,X,Y,Z).</para>
</listitem>
- </varlistentry><varlistentry>
+ </varlistentry>
+ <varlistentry>
<term>truly_inside(geometryA,geometryB)</term>
- <listitem><para>Returns true if any part of A is within the bounding box
- of B.</para>
+ <listitem>
+ <para>Returns true if any part of A is within the bounding box of
+ B.</para>
</listitem>
- </varlistentry></variablelist></sect1></chapter>
+ </varlistentry>
+ </variablelist>
+ </sect1>
+ </chapter>
<chapter>
- <title>Programming Information</title>
- <para> The following sections provide information about the internal structure of the PostGIS spatial objects, and some information about certain design decisions.</para>
+ <title>Programming Information</title>
+ <para> The following sections provide information about the internal
+ structure of the PostGIS spatial objects, and some information about certain
+ design decisions.</para>
<sect1>
- <title>Internal Object Representation</title>
- <para>The internal representation information is useful to people who intend to write C functions to directly manipulate the backend objects at
-the server. Reading <filename>postgis.h</filename> will give the exact type definitions used, and reading <filename>postgis.c</filename> will give some examples of how to
-manipulate the objects.</para>
- <sect2><title>Basic Structure</title><para>In the most basic sense, a GEOMETRY type is a list of sub-objects and a bounding volume. In an ideal world, the type should be defined
-as: </para><programlisting>typedef struct
-{
-int32 size; // postgres variable-length type requirement
-int32 type; // this type of geometry
-bool is3d; // true if the points are 3d (only for output)
-BOX3D bvol; // bounding volume of all the geo objects
-int32 nobjs; // how many sub-objects in this object
-int32 objType[1]; // type of object
-int32 objOffset[1]; // offset (in bytes) into this structure where the object is located
-char objData[1]; // store for actual objects
-} GEOMETRY;</programlisting><para>However, because of the way PostgreSQL handles memory allocation for objects, the type is actually organized as:</para><programlisting>typedef struct
-{
-int32 size; // postgres variable-length type requirement
-int32 type; // this type of geometry
-bool is3d; // true if the points are 3d (only for output)
-BOX3D bvol; // bounding volume of all the geo objects
-int32 nobjs; // how many sub-objects in this object
-char data[?]; //info about the sub-objects
-} GEOMETRY;</programlisting><para>This is because a Postgres type is a just a chuck of memory - it cannot contain any pointers. In a normal program <varname>objType</varname> and
-objData would be pointers to separate chunks of memory for these two lists (in fact, <varname>objData</varname> would likely be a list of pointers to other memory locations). That would look something like:</para><para>Since we cannot have pointers, the geometry structure actually has pseudo-pointers to its data portion, and looks like:
-</para><para>Since we cannot have pointers, the geometry structure actually has pseudo-pointers to its data portion, and looks like:
-</para><itemizedlist><listitem><para>The structure is assumed to start off double-aligned (see the SQL define type).</para></listitem><listitem><para>Inside the structure, <varname>bvol</varname> and all the objects are double-aligned (there is unused space between <varname>is3d</varname> and <varname>bvol</varname>, there could be
- unused space between <varname>objOffset[n]</varname> and object 0, and there could be unused space between object i and object i+1.</para></listitem><listitem><para>The actual location of <varname>objType[0]</varname> is always <varname>geometry->objType[0]</varname>.</para></listitem><listitem><para>The actual location of <varname>objOffset[0]</varname> is <varname>sizeof(int32)*geometry->nobjs</varname> after objType[0]. </para></listitem><listitem><para>The actual location of object 0 is <varname>objOffset[0]</varname> bytes into the structure . </para></listitem><listitem><para>Object 0 is also <varname>sizeof(int32)*geometry->nobjs</varname> bytes after <varname>objOffset[0]</varname> (could be 4 more bytes along if this isn't
- double-aligned) . </para></listitem><listitem><para>The actual location of object 1 is <varname>objOffset[1]</varname> bytes into the structure.
-</para></listitem></itemizedlist></sect2><sect2><title>Usage</title><para>Normally, one will want to find the location of <varname>objOffset[0]</varname> so it is accessible:</para><programlisting>Int32 *offsets;
-offsets = (int32 *) ( ((char *) &(geom1->objType[0] ))+ sizeof(int32) * geom1->nobjs );</programlisting><para>Next, one might want to get object i:</para><programlisting>Char *obji;
-oi = (char *) geom1 +offsets[i] ;
-typei= geom1->objType[i];</programlisting><para>Then one can re-cast the object:</para><programlisting>POLYGON3D *poly;
-LINE3D *line;
-POINT3D *point;
-if (typei == POINTTYPE) //point
-{
-point = (LINE3D *) oi;
-...
-}
-if (type1 == LINETYPE) //line
-{
-line = (LINE3D *) oi;
-...
-}
-if (type1 == POLYGONTYPE) //POLYGON
-{
-poly = (POLYGON3D *) oi;
-...
-}</programlisting><para>What does everything mean? </para><variablelist><varlistentry><term>Size </term><listitem><para>The total size of the structure.</para></listitem></varlistentry><varlistentry><term>Type</term><listitem><para>Generic type for this Structure.</para><para>This is mostly used so output knows how the object was originally entered into the system. </para></listitem></varlistentry><varlistentry><term>Is3d</term><listitem><para>TRUE if and only if, when the object was created, any of the points were given in 3D.</para><para>For example,
-
- MULTIPOINT(1 1,2 2) -> is3d = FALSE
-
- MULTIPOINT(1 1, 2 2, 3 3 3) -> is3d = TRUE</para><para>Since POINT(1 1) is stored as a POINT(1 1 0), we use the is3d flag during output to give POINT( 1 1) or POINT(1 1 0). Its not
- used for anything else.</para></listitem></varlistentry><varlistentry><term>Bvol</term><listitem><para>Bounding Volume for all the points in this geometry </para></listitem></varlistentry><varlistentry><term>nobjs</term><listitem><para>The number of point/line/polygons there are in this geometry.</para></listitem></varlistentry><varlistentry><term>objType[i]</term><listitem><para>The type of each of the sub-objects.</para><programlisting>#define POINTTYPE 1
-#define LINETYPE 2
-#define POLYGONTYPE 3</programlisting></listitem></varlistentry><varlistentry><term>objOffset[i]</term><listitem><para>Offset, in bytes, into the structure where the object i is actually stored.</para></listitem></varlistentry><varlistentry><term>objdata</term><listitem><para>Data block with all the objects stored in it.</para></listitem></varlistentry></variablelist></sect2><sect2><title>Input / Output</title><para>The geometry type tries really hard to consistently reproduce objects. For example, if you insert a MULTIPOINT(), you will get a
-MULTIPOINT() when you select.
-
-This is more difficult than you might expect because, internally, all the objects listed below are stored the same:
-</para><itemizedlist><listitem><para>MULTIPOINT( 1 1, 2 2, 3 3)
-
-
-
-</para></listitem><listitem><para>GEOMETRYCOLLECTION(POINT(1 1), POINT(2 2), POINT(3 3) )</para></listitem><listitem><para>GEOMETRYCOLLECTION(MULTIPOINT( 1 1, 2 2), POINT( 3 3) ) </para></listitem><listitem><para>GEOMETRYCOLLECTION(MULTIPOINT( 1 1, 2 2, 3 3))
-
-</para></listitem></itemizedlist><para>Every GEOMETRY is a set (GEOMETRYCOLLECTION) of sub-objects (point, line, polygon). So, truthfully, the 2nd example is the best
-OGC WKT of the internal structure.
-
-</para><para>Therefore, if you start with a MULTI-geometry (MULTIPOINT, MULTILINE, MULTIPOLYGON), the type member will record that fact. On
-output, it does a simple conversion: </para><itemizedlist><listitem><para>GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2)) becomes MULTIPOINT(1 1 , 2 2)</para></listitem><listitem><para>GEOMETRYCOLLECTION(LINESTRING(1 1,2 2),LINESTRING(10 10, 20 20)) becomes MULTILINESTRING((1 1,2 2), (10
- 10, 20 20))</para></listitem><listitem><para>GEOMETRYCOLLECTION(POLYGON((0 0, 0 1, 1 1, 0 0)), POLYGON((0 0, 0 10, 10 10 , 0 0) ) becomes
- MULTIPOLYGON( ((0 0, 0 1, 1 1, 0 0)), ((0 0, 0 10, 10 10 , 0 0))) </para></listitem></itemizedlist><para>Single point, line, or polygon objects are returned as single point, line or polygon objects.
-
-Unfortunately, GEOMETRYCOLLECTIONS() are not always returned the same as they are entered. They are always returned as a set
-of base types.
-
-For example: </para><itemizedlist><listitem><para>GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2),POINT(3 3)) becomes GEOMETRYCOLLECTION(POINT(1 1),POINT(2
- 2),POINT(3 3)) </para></listitem><listitem><para>GEOMETRYCOLLECTION(MULTIPOINT(1 1,2 2,3 3)) becomes GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2),POINT(3 3)) </para></listitem></itemizedlist></sect2><sect2><title>Fundamental Geometry Types</title><para>Inside each GEOMETRY object there are only four different possible geomtry types, and the <varname>Type</varname> variable of the GEOMETRY controls how the collection will be presented to the outside world (as a GEOMETRYCOLLECTION or a MULTIPOLYGON, for example).</para><sect3><title>POINT3D</title><para>This is the base geometry used by all the other sub-points. Its simply the X,Y,Z location represented as a double-precision floating point
-number. 2D locations are represented with the Z location set to 0.0. See above for a discussion on how to tell the difference between a
-2D point, and 3D points with Z=0.0.</para><programlisting>typedef struct
-{
-double x,y,z;
-} POINT3D;</programlisting></sect3><sect3><title>LINE3D</title><para>A lines is a set of connected points. The line moves from <varname>points[0]->points[1]->points[2]</varname>
-</para><programlisting>typedef struct
-{
-int32 npoints; // how many points in the line
-int32 junk; // double-word alignment
-POINT3D points[1]; // array of actual points
-} LINE3D;</programlisting></sect3><sect3><title>POLYGON3D</title><para>A polygon is a set of rings. Each ring is a closed line.
-</para><itemizedlist><listitem><para>The first ring is the outer ring, the others are holes.</para></listitem><listitem><para><varname>Npoints[i]</varname> is the number of points in ring i.</para></listitem><listitem><para><varname>&points[0]</varname> is at <varname>&polygon->npoints[0] + sizeof(int32)*polygon->nrings</varname>.</para></listitem><listitem><para>Once you've calculated <varname>&points[0]</varname>, ensure it is double-aligned.</para></listitem></itemizedlist><para>For example: </para><programlisting>POINT3D *pts;
-pts = (POINT3D *) ( (char *)&(poly->npoints[poly->nrings] ) );
-if ((int32) pts1 != (((int32)pts1>>3)<<3) )
-pts1 = (POINT3D *) ((char *) pts1 + 4);</programlisting><para>Therefore, </para><itemizedlist><listitem><para>Ring 0's points are <varname>points[0]</varname> to <varname>points[npoint[0]-1]</varname>.</para></listitem><listitem><para>Ring 1's points are from <varname>points[npoint[0]]</varname> to <varname>points[npoints[0] + npoints[1]-1]</varname>.</para></listitem></itemizedlist><programlisting>typedef struct
-{
-int32 nrings; // how many rings in this polygon
-int32 npoints[1]; //how many points in each ring
-/* could be 4 byes of filler here to make sure points[] is
-double-word aligned*/
-POINT3D points[1]; // array of actual points
-} POLYGON3D;</programlisting></sect3><sect3><title>BOX3D</title><para>Representation of a long diagonal of a cube, from the lower-left bottom (LLB) to the upper-right top (URT). </para><itemizedlist><listitem><para>Minimum X is box3d->LLB.x</para></listitem><listitem><para>Minimum Y is box3d->LLB.y</para></listitem><listitem><para>Minimum Z is box3d->LLB.z</para></listitem><listitem><para>Maximum X is box3d->URT.x</para></listitem><listitem><para>Maximum Y is box3d->URT.y</para></listitem><listitem><para>Maximum Y is box3d->URT.z</para></listitem></itemizedlist><programlisting>typedef struct
-{
-POINT3D LLB,URT; /* corner POINT3Ds on long diagonal */
-} BOX3D;</programlisting></sect3></sect2></sect1>
+ <title>Internal Object Representation</title>
+ <para>The internal representation information is useful to people who
+ intend to write C functions to directly manipulate the backend objects at the
+ server. Reading <filename>postgis.h</filename> will give the exact type
+ definitions used, and reading <filename>postgis.c</filename> will give some
+ examples of how to manipulate the objects.</para>
+ <sect2>
+ <title>Basic Structure</title>
+ <para>In the most basic sense, a GEOMETRY type is a list of sub-objects
+ and a bounding volume. In an ideal world, the type should be defined as:
+ </para>
+ <programlisting>typedef struct {
+ int32 size; // postgres variable-length type requirement
+ int32 type; // this type of geometry
+ bool is3d; // true if the points are 3d (only for output)
+ BOX3D bvol; // bounding volume of all the geo objects
+ int32 nobjs; // how many sub-objects in this object
+ int32 objType[1]; // type of object
+ int32 objOffset[1]; // offset (in bytes) into this structure where the object is located
+ char objData[1]; // store for actual objects
+} GEOMETRY;</programlisting>
+ <para>However, because of the way PostgreSQL handles memory allocation
+ for objects, the type is actually organized as:</para>
+ <programlisting>typedef struct {
+ int32 size; // postgres variable-length type requirement
+ int32 type; // this type of geometry bool
+ is3d; // true if the points are 3d (only for output)
+ BOX3D bvol; // bounding volume of all the geo objects
+ int32 nobjs; // how many sub-objects in this object
+ char data[?]; // info about the sub-objects
+} GEOMETRY;</programlisting>
+ <para>This is because a Postgres type is a just a chuck of memory - it
+ cannot contain any pointers. In a normal program
+ <varname>objType</varname> and objData would be pointers to separate
+ chunks of memory for these two lists (in fact,
+ <varname>objData</varname> would likely be a list of pointers to
+ other memory locations). That would look something like:</para>
+ <para>Since we cannot have pointers, the geometry structure actually
+ has pseudo-pointers to its data portion, and looks like: </para>
+ <para>Since we cannot have pointers, the geometry structure actually
+ has pseudo-pointers to its data portion, and looks like: </para>
+ <itemizedlist>
+ <listitem>
+ <para>The structure is assumed to start off double-aligned (see the
+ SQL define type).</para>
+ </listitem>
+ <listitem>
+ <para>Inside the structure,
+ <varname>bvol</varname> and all the objects are double-aligned
+ (there is unused space between
+ <varname>is3d</varname> and
+ <varname>bvol</varname>, there could be unused space between
+ <varname>objOffset[n]</varname> and object 0, and there could be
+ unused space between object i and object i+1.</para>
+ </listitem>
+ <listitem>
+ <para>The actual location of
+ <varname>objType[0]</varname> is always
+ <varname>geometry->objType[0]</varname>.</para>
+ </listitem>
+ <listitem>
+ <para>The actual location of
+ <varname>objOffset[0]</varname> is
+ <varname>sizeof(int32)*geometry->nobjs</varname> after
+ objType[0]. </para>
+ </listitem>
+ <listitem>
+ <para>The actual location of object 0 is
+ <varname>objOffset[0]</varname> bytes into the structure .
+ </para>
+ </listitem>
+ <listitem>
+ <para>Object 0 is also
+ <varname>sizeof(int32)*geometry->nobjs</varname> bytes after
+ <varname>objOffset[0]</varname> (could be 4 more bytes along if
+ this isn't double-aligned) . </para>
+ </listitem>
+ <listitem>
+ <para>The actual location of object 1 is
+ <varname>objOffset[1]</varname> bytes into the structure. </para>
+
+ </listitem>
+ </itemizedlist>
+ </sect2>
+ <sect2>
+ <title>Usage</title>
+ <para>Normally, one will want to find the location of
+ <varname>objOffset[0]</varname> so it is accessible:</para>
+ <programlisting>Int32 *offsets;
+offsets = (int32 *) ( ((char *) &(geom1->objType[0] ))+ sizeof(int32) * geom1->nobjs);</programlisting>
+ <para>Next, one might want to get object i:</para>
+ <programlisting>Char *obji;
+oi = (char *) geom1 +offsets[i] ;
+typei = geom1->objType[i];</programlisting>
+ <para>Then one can re-cast the object:</para>
+ <programlisting>POLYGON3D *poly;
+LINE3D *line;
+POINT3D *point;
+if (typei == POINTTYPE) // point {
+ point = (LINE3D *) oi;
+ ...
+}
+if (type1 == LINETYPE) // line {
+ line = (LINE3D *) oi;
+ ...
+}
+if (type1 == POLYGONTYPE) // polygon {
+ poly = (POLYGON3D *) oi;
+ ...
+}</programlisting>
+ <para>What does everything mean? </para>
+ <variablelist>
+ <varlistentry>
+ <term>Size </term>
+ <listitem>
+ <para>The total size of the structure.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Type</term>
+ <listitem>
+ <para>Generic type for this Structure.</para>
+ <para>This is mostly used so output knows how the object was
+ originally entered into the system. </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Is3d</term>
+ <listitem>
+ <para>TRUE if and only if, when the object was created, any of
+ the points were given in 3D.</para>
+ <para>For example, MULTIPOINT(1 1,2 2) -> is3d = FALSE
+ MULTIPOINT(1 1, 2 2, 3 3 3) -> is3d = TRUE</para>
+ <para>Since POINT(1 1) is stored as a POINT(1 1 0), we use the
+ is3d flag during output to give POINT( 1 1) or POINT(1 1 0). Its not used for
+ anything else.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Bvol</term>
+ <listitem>
+ <para>Bounding Volume for all the points in this geometry </para>
+
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>nobjs</term>
+ <listitem>
+ <para>The number of point/line/polygons there are in this
+ geometry.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>objType[i]</term>
+ <listitem>
+ <para>The type of each of the sub-objects.</para>
+ <programlisting>#define POINTTYPE 1
+#define LINETYPE 2
+#define POLYGONTYPE 3
+</programlisting>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>objOffset[i]</term>
+ <listitem>
+ <para>Offset, in bytes, into the structure where the object i is
+ actually stored.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>objdata</term>
+ <listitem>
+ <para>Data block with all the objects stored in it.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+ <sect2>
+ <title>Input / Output</title>
+ <para>The geometry type tries really hard to consistently reproduce
+ objects. For example, if you insert a MULTIPOINT(), you will get a MULTIPOINT()
+ when you select. This is more difficult than you might expect because,
+ internally, all the objects listed below are stored the same: </para>
+ <itemizedlist>
+ <listitem>
+ <para>MULTIPOINT( 1 1, 2 2, 3 3) </para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(POINT(1 1), POINT(2 2), POINT(3 3)
+ )</para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(MULTIPOINT( 1 1, 2 2), POINT( 3 3) )
+ </para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(MULTIPOINT( 1 1, 2 2, 3 3)) </para>
+ </listitem>
+ </itemizedlist>
+ <para>Every GEOMETRY is a set (GEOMETRYCOLLECTION) of sub-objects
+ (point, line, polygon). So, truthfully, the 2nd example is the best OGC WKT of
+ the internal structure. </para>
+ <para>Therefore, if you start with a MULTI-geometry (MULTIPOINT,
+ MULTILINE, MULTIPOLYGON), the type member will record that fact. On output, it
+ does a simple conversion: </para>
+ <itemizedlist>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2)) becomes
+ MULTIPOINT(1 1 , 2 2)</para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(LINESTRING(1 1,2 2),LINESTRING(10 10, 20
+ 20)) becomes MULTILINESTRING((1 1,2 2), (10 10, 20 20))</para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(POLYGON((0 0, 0 1, 1 1, 0 0)), POLYGON((0
+ 0, 0 10, 10 10 , 0 0) ) becomes MULTIPOLYGON( ((0 0, 0 1, 1 1, 0 0)), ((0 0, 0
+ 10, 10 10 , 0 0))) </para>
+ </listitem>
+ </itemizedlist>
+ <para>Single point, line, or polygon objects are returned as single
+ point, line or polygon objects. Unfortunately, GEOMETRYCOLLECTIONS() are not
+ always returned the same as they are entered. They are always returned as a set
+ of base types. For example: </para>
+ <itemizedlist>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2),POINT(3 3)) becomes
+ GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2),POINT(3 3)) </para>
+ </listitem>
+ <listitem>
+ <para>GEOMETRYCOLLECTION(MULTIPOINT(1 1,2 2,3 3)) becomes
+ GEOMETRYCOLLECTION(POINT(1 1),POINT(2 2),POINT(3 3)) </para>
+ </listitem>
+ </itemizedlist>
+ </sect2>
+ <sect2>
+ <title>Fundamental Geometry Types</title>
+ <para>Inside each GEOMETRY object there are only four different
+ possible geomtry types, and the
+ <varname>Type</varname> variable of the GEOMETRY controls how the
+ collection will be presented to the outside world (as a GEOMETRYCOLLECTION or a
+ MULTIPOLYGON, for example).</para>
+ <sect3>
+ <title>POINT3D</title>
+ <para>This is the base geometry used by all the other sub-points. Its
+ simply the X,Y,Z location represented as a double-precision floating point
+ number. 2D locations are represented with the Z location set to 0.0. See above
+ for a discussion on how to tell the difference between a 2D point, and 3D
+ points with Z=0.0.</para>
+ <programlisting>typedef struct {
+ double x,y,z;
+} POINT3D;</programlisting>
+ </sect3>
+ <sect3>
+ <title>LINE3D</title>
+ <para>A lines is a set of connected points. The line moves from
+ <varname>points[0]->points[1]->points[2]</varname> </para>
+ <programlisting>typedef struct {
+ int32 npoints; // how many points in the line
+ int32 junk; // double-word alignment
+ POINT3D points[1]; // array of actual points
+} LINE3D;</programlisting>
+ </sect3>
+ <sect3>
+ <title>POLYGON3D</title>
+ <para>A polygon is a set of rings. Each ring is a closed line.
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>The first ring is the outer ring, the others are
+ holes.</para>
+ </listitem>
+ <listitem>
+ <para><varname>Npoints[i]</varname> is the number of points in
+ ring i.</para>
+ </listitem>
+ <listitem>
+ <para><varname>&points[0]</varname> is at
+ <varname>&polygon->npoints[0] +
+ sizeof(int32)*polygon->nrings</varname>.</para>
+ </listitem>
+ <listitem>
+ <para>Once you've calculated
+ <varname>&points[0]</varname>, ensure it is
+ double-aligned.</para>
+ </listitem>
+ </itemizedlist>
+ <para>For example: </para>
+ <programlisting>POINT3D *pts;
+pts = (POINT3D *) ( (char *)&(poly->npoints[poly->nrings] ) );
+if ((int32) pts1 != (((int32)pts1>>3)<<3) ) pts1 = (POINT3D *) ((char *) pts1 + 4);</programlisting>
+ <para>Therefore, </para>
+ <itemizedlist>
+ <listitem>
+ <para>Ring 0's points are
+ <varname>points[0]</varname> to
+ <varname>points[npoint[0]-1]</varname>.</para>
+ </listitem>
+ <listitem>
+ <para>Ring 1's points are from
+ <varname>points[npoint[0]]</varname> to
+ <varname>points[npoints[0] + npoints[1]-1]</varname>.</para>
+ </listitem>
+ </itemizedlist>
+ <programlisting>typedef struct {
+ int32 nrings; // how many rings in this polygon
+ int32 npoints[1]; //how many points in each ring
+ /*
+ * could be 4 byes of filler here to make sure
+ * points[] is double-word aligned
+ */
+ POINT3D points[1]; // array of actual points
+} POLYGON3D;</programlisting>
+ </sect3>
+ <sect3>
+ <title>BOX3D</title>
+ <para>Representation of a long diagonal of a cube, from the
+ lower-left bottom (LLB) to the upper-right top (URT). </para>
+ <itemizedlist>
+ <listitem>
+ <para>Minimum X is box3d->LLB.x</para>
+ </listitem>
+ <listitem>
+ <para>Minimum Y is box3d->LLB.y</para>
+ </listitem>
+ <listitem>
+ <para>Minimum Z is box3d->LLB.z</para>
+ </listitem>
+ <listitem>
+ <para>Maximum X is box3d->URT.x</para>
+ </listitem>
+ <listitem>
+ <para>Maximum Y is box3d->URT.y</para>
+ </listitem>
+ <listitem>
+ <para>Maximum Y is box3d->URT.z</para>
+ </listitem>
+ </itemizedlist>
+ <programlisting>typedef struct {
+ POINT3D LLB,URT; /* corner POINT3Ds on long diagonal */
+} BOX3D;</programlisting>
+ </sect3>
+ </sect2>
+ </sect1>
<sect1>
- <title>Internal Functions</title>
+ <title>Internal Functions</title>
<para>To be done.</para>
</sect1>
</chapter>
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