In this recipe, we will check out the PostGIS functions needed for distance measurements (
ST_Distance and its variants) and find out how considering the earth's curvature makes a big difference when measuring distances between distant points.
You should import the shapefile representing the cities from the USA that we generated in a previous recipe (the PostGIS table named
chp03.cities). In case you haven't already done so, download that shapefile from the nationalatlas.gov website at http://dds.cr.usgs.gov/pub/data/nationalatlas/citiesx020_nt00007.tar.gz (this archive is also included in the code bundle available with this book) and import it to PostGIS:
$ ogr2ogr -f PostgreSQL -s_srs EPSG:4269 -t_srs EPSG:4326 ...