The steps you need to perform to complete this recipe are as follows:
- First, use the ST_Distance function to calculate the distances between cities in the USA that have more than 1 million inhabitants using the Spherical Mercator planar projection coordinate system (EPSG:900913, EPSG:3857, or EPSG:3785; all of these SRID representations are equivalent). Use the ST_Transform function as follows to convert the point coordinates from longitude latitude degrees (as the coordinates are originally in EPSG:4326) to a planar metric system if you want the results in meters:
postgis_cookbook=# SELECT c1.name, c2.name,
ST_Distance(ST_Transform(c1.the_geom, 900913), ST_Transform(c2.the_geom, 900913))/1000 AS distance_900913 FROM chp03.cities ...
