[Proj] Proj-5.0.0 cct path to gridfiles

[stevejking]

Kristian,
Definitely the ellipsoid the grids use is important. You won’t be able to
replicate the transformations otherwise. The official transformation tool is
here

https://www.ordnancesurvey.co.uk/gps/transformation/

Cs2cs gets the correct result using these grids, which are latitude
longitude grids of shifts realised on the Airy ellipsoid. The GTX comprises
shifts in metres to the GRS80 ellipsoid. Whilst the GSB file comprises sets
of 4 values for each grid point. These are latitude shift, longitude shift
both in arc seconds, and two values indicating error. The previous versions
of proj applied these shifts after first transforming the projection to
latitude longitude on its own ellipsoid, and the resulting latitude and
longitudes were then by definition wgs84 (which etrs is standing in for in
this case).

Steve





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