Historical geodetic datums: a latitude/longitude point defines a geographic location on or above/below the earth’s surface, measured in degrees from the equator & the International Reference Meridian and metres above the ellipsoid, and based on a given datum. The datum is based on a reference ellipsoid and tied to geodetic survey reference points. Modern geodesy is generally based on the WGS84 datum (as used for instance by GPS systems), but previously various reference ellipsoids and datum references were used. This module extends the core latlon-ellipsoidal module to include ellipsoid parameters and datum transformation parameters, and methods for converting between different (generally historical) datums. It can be used for UK Ordnance Survey mapping (OS National Grid References are still based on the otherwise historical OSGB36 datum), as well as for historical purposes. q.v. Ordnance Survey ‘A guide to coordinate systems in Great Britain’ Section 6, www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf, and also www.ordnancesurvey.co.uk/blog/2014/12/2.