Searchable Coverage Checker Map

The right approach depends on how your coverage area is defined. Assuming it is in the form of GIS polygons, the underlying data is a series of latitude/longitude coordinates. A user’s location can be queried from their mobile device. But to get a lat/long location based on address entry, you’ll want to make use of a geocoding service. Google provides one, there are others.

There’s some algorithm to determine if a particular location lies within a polygon or not. I don’t know how it works though. A complex polygon that has “bays” and “peninsulas” can be particularly difficult to evaluate.

OTOH, determining the distance between any two geographic points is relatively simple, but it still involves at least spherical geometry. Spherical calculations are still an approximation, but generally close enough for practical use. For full accuracy, complex ellipsoidal geometry is called for.