Changes between Initial Version and Version 1 of UsersWikiNewbieDistanceMeasurement

Timestamp:

Apr 15, 2009, 8:02:43 AM (15 years ago)

Author:

pracine

Comment:

—

UsersWikiNewbieDistanceMeasurement

@@ +1 @@
+= Newbie Question =
+
+
+'''What is the measurement returned by ST_Distance?'''
+
+
+ST_Distance always returns in the units of the projection.
+
+So if for example you chose SRID 4326 - then your measurement would be in degrees.
+
+It is best to use a meter based or feet based projection for measurement.
+
+Unfortunately you can not do that unless you are dealing with regional data such as country, state etc.
+
+To get meters when using 4326 or other degree projection - use ST_Distance_Sphere or ST_Distance_Spheroid.  These unfortunately only work with point geometries.
+
+Faster but less precise
+
+
+{{{
+SELECT ST_Distance_Sphere(
+    ST_GeomFromText('POINT(5.0 52.0)', 4326),
+    ST_GeomFromText('POINT(6.0 56.0)', 4326) );
+
+SELECT ST_Distance_Sphere(t1.the_geom, t2.the_geom)
+FROM t1 INNER JOIN t2 ON t1.somefield1 = t2.somefield2;
+
+}}}
+
+Slower but more precise
+
+{{{
+SELECT ST_Distance_Spheroid(
+       ST_GeomFromText('POINT(5.0 52.0)', 4326),
+       ST_GeomFromText('POINT(6.0 56.0)', 4326),
+         'SPHEROID[wiki:UsersWiki"WGS 84",6378137,298.257223563 "WGS 84",6378137,298.257223563]'
+     );
+
+SELECT ST_Distance_Spheroid(t1.the_geom, t2.the_geom,  'SPHEROID[wiki:UsersWiki"WGS 84",6378137,298.257223563 "WGS 84",6378137,298.257223563]')
+FROM t1 INNER JOIN t2 ON t1.somefield1 = t2.somefield2;
+
+}}}