Geo::WebService::Elevation::USGS executes elevation queries against the
United States Geological Survey's web server. You provide the latitude
and longitude in degrees, with south latitude and west longitude being
negative. The return is typically a hash containing the data you want.
Query errors are exceptions by default, though the object can be configured
to signal an error by an undef response, with the error retrievable from
the 'error' attribute.