Transformations between coordinates in different datums

 It is usual to describe geographical points in terms of their polar coordinates
 (latitude, longitude and altitude) referenced to a "datum ellipsoid", which is
 used to approximate the Earth's geoid. The latitude, longitude and altitude of
 a given physical point vary depending on which datum ellipsoid is in use.
 Unfortunately, a number of ellipsoids are in everyday use, and so it is often
 necessary to transform geographical coordinates between different datum
 ellipsoids.
 .
 Two different datum ellipsoids may differ in the locations of their centers, or
 in their shape; and there may be an angle between their equatorial planes or
 the meridians relative to which longitude is measured. The Helmert Transform,
 which this module implements, is a linear transformation of coordinates between
 pairs of datum ellipsoids in the limit of small angles of deviation between
 them.