Measuring distances
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.
Getting ready
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 -lco GEOMETRY_NAME=the_geom -nln chp03.cities PG:"dbname='postgis_cookbook' user='me' password='mypassword'" citiesx020.shp
How to do it...
The steps you need to perform to complete this recipe are as follows:
First, use the
ST_Distancefunction to calculate...
