[PROJ] [EXTERNAL] Re: Proj 6 API questions

[Lesparre, Jochem]

Kristian is right that just using a modern ellipsoid for distances/areas in an old but still used national CRS would give errors.
However, I think computing distances/areas using the old ellipsoid of that CRS is also not correct for most users. Therefore, I suggest:

1. Back-project the projected coordinates to geodetic coordinates using the Ellipsoid parameters of the CRS datum.
2. Perform datum transformation to the most recent realisation of the currently accepted scientific CRS of that planet (currently ITRF2014 for Earth) with its ellipsoid (currently GRS80 for Earth).
3. Perform geodesic calculations using this ellipsoid parameters of this CRS datum.

Kind regards, Jochem



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