[GRASS-dev] PROJ6 support in GRASS

Even Rouault even.rouault at spatialys.com
Thu Nov 7 12:47:41 PST 2019

> Indeed messy. Look at the WKT2 representation of projinfo -o WKT2_2018
> EPSG:3857: projection on the ellipsoid "WGS 84", not on a sphere.

as does the authority says:

> I don't understand "lying on the ellipsoid definition (using the Sphere)",
> it should be a sphere with radius 6378137, no?

What I mean is that in the following
"+proj=merc +a=6378137 +b=6378137 +nadgrids=@null"
we use a clever hack to use the standard Mercator projection to simulate the 
""Popular Visualisation Pseudo Mercator" by speciyfing the sphere as an 
ellipsoid with the a & b parameters, and using +nadgrids=@null which is 
interpreted by PROJ as "this sphere is equivalent to WGS84 for the purpose of 
datum transformations". Otherwise if you transformed from another datum than 
WGS84, you might get a incorrect datum transformation...

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