Number of kilometers around the equator of the earth. Defaults to 6371.64.
Number of units in a single decimal degree (lat or lon) at the equator. The default, 0.0174532925199433, is derived from:
$gis->distance( 10,0 => 11,0 )->km() / $gis->kilometer_rho()
This method is just a placeholder and throws an errors if called. The distance() method should be re-implemented by any class that extends this class.
