</varlistentry>
<varlistentry>
- <term>-b </term>
+ <term>-b</term>
<listitem>
<para>Use a binary cursor. This will help reduce the likelihood
------------+-------------------+-------------------
gid | integer | Unique ID
name | character varying | Road Name
- type | character varying | Road Type Number</programlisting>
+ the_geom | geometry | Location Geometry (Linestring)</programlisting>
<para>The table definition for the <varname>bc_municipality</varname>
table is:</para>
<qandadiv>
<qandaentry>
<question>
- <para>Find all the roads within 1 kilometer of a point.</para>
+ <para>What is the total length of all roads, expressed in
+ kilometers?</para>
</question>
<answer>
<para>You can answer this question with a very simple piece of
SQL:</para>
- <programlisting>SELECT * FROM geom_table WHERE Distance( the_geom, </programlisting>
+ <programlisting>postgis=# SELECT sum(length(the_geom))/1000 AS km_roads FROM bc_roads;
+ km_roads
+------------------
+ 70842.1243039643
+(1 row)</programlisting>
</answer>
</qandaentry>
<qandaentry>
<question>
- <para>Find the length of all roads</para>
+ <para>How large is the city of Prince George, in
+ hectares?</para>
</question>
<answer>
- <para>You can answer this question with a very simple piece of
- SQL:</para>
+ <para>This query combines an attribute condition (on the
+ municipality name) with a spatial calculation (of the
+ area):</para>
+
+ <programlisting>postgis=# SELECT area(the_geom)/10000 AS hectares FROM bc_municipality
+ WHERE name = 'PRINCE GEORGE';
+ hectares
+------------------
+ 32657.9103824927
+(1 row) </programlisting>
+ </answer>
+ </qandaentry>
+
+ <qandaentry>
+ <question>
+ <para>What is the largest municipality in the province, by
+ area?</para>
+ </question>
+
+ <answer>
+ <para>This query brings a spatial measurement into the query
+ condition. There are several ways of approaching this problem,
+ but the most efficient is below:</para>
+
+ <programlisting>postgis=# SELECT name, area(the_geom)/10000 AS hectares
+ FROM bc_municipality
+ ORDER BY hectares DESC
+ LIMIT 1;
+ name | hectares
+---------------+-----------------
+ TUMBLER RIDGE | 155020.02556131
+(1 row)</programlisting>
- <programlisting>SELECT * FROM geom_table WHERE Distance( the_geom, </programlisting>
+ <para>Note that in order to answer this query we have to
+ calculate the area of every polygon. If we were doing this a
+ lot it would make sense to add an area column to the table
+ that we could separately index for performance. By ordering
+ the results in a descending direction, and them using the
+ PostgreSQL "LIMIT" command we can easily pick off the largest
+ value without using an aggregate function like max().</para>
+ </answer>
+ </qandaentry>
+
+ <qandaentry>
+ <question>
+ <para>What is the length of roads fully contained within each
+ municipality?</para>
+ </question>
+
+ <answer>
+ <para>This is an example of a "spatial join", because we are
+ bringing together data from two tables (doing a join) but
+ using a spatial interaction condition ("contained") as the
+ join condition rather than the usual relational approach of
+ joining on a common key:</para>
+
+ <programlisting>postgis=# SELECT m.name, sum(length(r.the_geom))/1000 as roads_km
+ FROM bc_roads AS r,bc_municipality AS m
+ WHERE r.the_geom && m.the_geom
+ AND contains(m.the_geom,r.the_geom)
+ GROUP BY m.name
+ ORDER BY roads_km;
+
+ name | roads_km
+----------------------------+------------------
+ SURREY | 1539.47553551242
+ VANCOUVER | 1450.33093486576
+ LANGLEY DISTRICT | 833.793392535662
+ BURNABY | 773.769091404338
+ PRINCE GEORGE | 694.37554369147
+ ...</programlisting>
+
+ <para>This query takes a while, because every road in the
+ table is summarized into the final result (about 250K roads
+ for our particular example table). For smaller overlays
+ (several thousand records on several hundred) the response can
+ be very fast.</para>
+ </answer>
+ </qandaentry>
+
+ <qandaentry>
+ <question>
+ <para>Create a new table with all the roads within the city of
+ Prince George.</para>
+ </question>
+
+ <answer>
+ <para>This is an example of an "overlay", which takes in two
+ tables and outputs a new table that consists of spatially
+ clipped or cut resultants. Unlike the "spatial join"
+ demonstrated above, this query actually creates new
+ geometries. An overlay is like a turbo-charged spatial join,
+ and is useful for more exact analysis work:</para>
+
+ <programlisting>postgis=# CREATE TABLE pg_roads as
+ SELECT intersection(r.the_geom, m.the_geom) AS intersection_geom,
+ length(r.the_geom) AS rd_orig_length,
+ r.*
+ FROM bc_roads AS r, bc_municipality AS m
+ WHERE r.the_geom && m.the_geom
+ AND intersects(r.the_geom, m.the_geom)
+ AND m.name = 'PRINCE GEORGE';</programlisting>
+ </answer>
+ </qandaentry>
+
+ <qandaentry>
+ <question>
+ <para>What is the length in kilometers of "Douglas St" in
+ Victoria?</para>
+ </question>
+
+ <answer>
+ <programlisting>postgis=# SELECT sum(length(r.the_geom))/1000 AS kilometers
+ FROM bc_roads r, bc_municipality m
+ WHERE r.the_geom && m.the_geom
+ AND r.name = 'Douglas St'
+ AND m.name = 'VICTORIA';
+ kilometers
+------------------
+ 4.89151904172838
+(1 row)</programlisting>
+ </answer>
+ </qandaentry>
+
+ <qandaentry>
+ <question>
+ <para>What is the largest municipality polygon that has a
+ hole?</para>
+ </question>
+
+ <answer>
+ <programlisting>postgis=# SELECT gid, name, area(the_geom) AS area
+ FROM bc_municipality
+ WHERE nrings(the_geom) > 1
+ ORDER BY area DESC LIMIT 1;
+ gid | name | area
+-----+--------------+------------------
+ 12 | SPALLUMCHEEN | 257374619.430216
+(1 row)</programlisting>
</answer>
</qandaentry>
</qandadiv>
<listitem>
<para>Return a modified [multi]polygon having no ring segment
- longer then the given distance. Interpolated points will
- have Z and M values (if needed) set to 0. Distance computation
- is performed in 2d only.
- </para>
+ longer then the given distance. Interpolated points will have Z
+ and M values (if needed) set to 0. Distance computation is
+ performed in 2d only.</para>
</listitem>
</varlistentry>
-
</variablelist>
</sect1>
</chapter>
-</book>
+</book>
\ No newline at end of file
projects / postgis / commitdiff