This is an object whos class inherits from GIS::Distance::Formula. This
object is used to calculate distance. The formula may be specified as either
a blessed object, or as a string, such as Haversine or any of the other formulas.
If you specify the formula as a string then a few different class names will be
searched for. So, if you did:
$gis->formula( Haversine );
Then this list of packages would automatically be looked for. The first one that
exists will be created and used:
If you are using your own custom formula class make sure it applies
the GIS::Distance::Formula role.
Note that a ::Fast version of the class will be looked for first. By default
the ::Fast versions of the formulas, written in C, are not available and the
pure perl ones will be used instead. If you would like the ::Fast formulas
then install GIS::Distance::Fast and they will be automatically used.