Returns cluster number for each input geometry, based on a 2D implementation of the
<ulink url="https://en.wikipedia.org/wiki/DBSCAN">Density-based spatial clustering of applications with noise (DBSCAN)</ulink>
algorithm. Unlike <xref linkend="ST_ClusterKMeans" />, it does not require the number of clusters to be specified, but instead
- uses the desired distance (<varname>eps</varname>) and density(<varname>minpoints</varname>) parameters to construct each cluster.
+ uses the desired <link linkend="ST_Distance">distance</link> (<varname>eps</varname>) and density (<varname>minpoints</varname>) parameters to construct each cluster.
</para>
<para>
<itemizedlist>
<listitem>
<para>
- A "core" geometry, that is within <varname>eps</varname> distance (Cartesian) of at least <varname>minpoints</varname> input geometries (including itself) or
+ A "core" geometry, that is within <varname>eps</varname> <link linkend="ST_Distance">distance</link> of at least <varname>minpoints</varname> input geometries (including itself) or
</para>
</listitem>
<listitem>
<para>
- A "border" geometry, that is within <varname>eps</varname> distance of a core geometry.
+ A "border" geometry, that is within <varname>eps</varname> <link linkend="ST_Distance">distance</link> of a core geometry.
</para>
</listitem>
</itemizedlist>
<refsection>
<title>See Also</title>
- <para><xref linkend="ST_Intersects"/>ST_Intersects</para>
+ <para><xref linkend="ST_Intersects"/></para>
</refsection>
</refentry>
<refnamediv>
<refname>ST_Distance</refname>
- <refpurpose>For geometry type Returns the 2D Cartesian distance between two geometries in
- projected units (based on spatial ref). For geography type defaults to return minimum geodesic distance between two geographies in meters.</refpurpose>
+ <refpurpose>For geometry type returns the 2D Cartesian distance between two geometries in
+ projected units (based on spatial reference system).
+ For geography type defaults to return minimum geodesic distance between two geographies in meters.</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<refsection>
<title>Description</title>
- <para>For geometry type returns the minimum 2D Cartesian distance between two geometries in
- projected units (spatial ref units). For geography type defaults to return the minimum geodesic distance between two geographies in meters. If use_spheroid is
+ <para>For <xref linkend="geometry"/> type returns the minimum 2D Cartesian distance between two geometries in
+ projected units (spatial ref units). For <xref linkend="geography"/> type defaults to return the minimum geodesic distance between two geographies in meters. If use_spheroid is
false, a faster sphere calculation is used instead of a spheroid.</para>
<para>&sfs_compliant;</para>
<programlisting>
--Geometry example - units in planar degrees 4326 is WGS 84 long lat unit=degrees
SELECT ST_Distance(
- ST_GeomFromText('POINT(-72.1235 42.3521)',4326),
- ST_GeomFromText('LINESTRING(-72.1260 42.45, -72.123 42.1546)', 4326)
+ 'SRID=4326;POINT(-72.1235 42.3521)'::geometry,
+ 'SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geometry
);
st_distance
-----------------
0.00150567726382282
+-- Geometry example - units in meters (SRID: 3857, proportional to pixels on popular web maps)
+-- although the value is off, nearby ones can be compared correctly,
+-- which makes it a good choice for algorithms like KNN or KMeans.
+SELECT ST_Distance(
+ ST_Transform('SRID=4326;POINT(-72.1235 42.3521)'::geometry, 3857),
+ ST_Transform('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geometry, 3857)
+ );
+st_distance
+-----------------
+167.441410065196
+
+-- Geometry example - units in meters (SRID: 3857 as above, but corrected by cos(lat) to account for distortion)
+SELECT ST_Distance(
+ ST_Transform('SRID=4326;POINT(-72.1235 42.3521)'::geometry, 3857),
+ ST_Transform('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geometry, 3857)
+ ) * cosd(42.3521);
+st_distance
+-----------------
+123.742351254151
+
-- Geometry example - units in meters (SRID: 26986 Massachusetts state plane meters) (most accurate for Massachusetts)
SELECT ST_Distance(
- ST_Transform(ST_GeomFromText('POINT(-72.1235 42.3521)',4326),26986),
- ST_Transform(ST_GeomFromText('LINESTRING(-72.1260 42.45, -72.123 42.1546)', 4326),26986)
+ ST_Transform('SRID=4326;POINT(-72.1235 42.3521)'::geometry, 26986),
+ ST_Transform('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geometry, 26986)
);
st_distance
-----------------
-- Geometry example - units in meters (SRID: 2163 US National Atlas Equal area) (least accurate)
SELECT ST_Distance(
- ST_Transform(ST_GeomFromText('POINT(-72.1235 42.3521)',4326),2163),
- ST_Transform(ST_GeomFromText('LINESTRING(-72.1260 42.45, -72.123 42.1546)', 4326),2163)
+ ST_Transform('SRID=4326;POINT(-72.1235 42.3521)'::geometry, 2163),
+ ST_Transform('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geometry, 2163)
);
st_distance
<programlisting>-- same as geometry example but note units in meters - use sphere for slightly faster less accurate
SELECT ST_Distance(gg1, gg2) As spheroid_dist, ST_Distance(gg1, gg2, false) As sphere_dist
FROM (SELECT
- ST_GeogFromText('SRID=4326;POINT(-72.1235 42.3521)') As gg1,
- ST_GeogFromText('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)') As gg2
+ 'SRID=4326;POINT(-72.1235 42.3521)'::geography as gg1,
+ 'SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)'::geography as gg2
) As foo ;
spheroid_dist | sphere_dist
projects / postgis / commitdiff