This algorithm is similar in spirit to the IGNORE_DEM_USE_ELLIPSOID, but
with a user-specified elevation instead of always using 0.0 elevation.

Implements feature #187.

Thanks to Espen Bjørntvedt for the translations.

Thanks to Lucian Barbulescu for the translation.

This should fix some problems in the vicinity of ±π longitude.

Thanks to Silvia Ríos Bergantiños and Beatriz Salazar for the
translations.