[GRASS-dev] PROJ6 support in GRASS

[Markus Metz]

On Thu, Nov 7, 2019 at 8:04 PM Even Rouault <even.rouault at spatialys.com>
wrote:
>
> On jeudi 7 novembre 2019 19:50:33 CET Markus Metz wrote:
> > On Thu, Nov 7, 2019 at 6:47 PM Martin Landa <landa.martin at gmail.com>
wrote:
> > > Hi,
> > >
> > > čt 7. 11. 2019 v 15:17 odesílatel Markus Metz
> > >
> > > <markus.metz.giswork at gmail.com> napsal:
> > > > PROJ 6 support is not yet finished in GRASS. Currently it sort of
works
> >
> > in master and relbr78, but I expect troubles because GRASS is heavily
> > relying on deprecated proj strings as CRS definitions. This is becoming
> > problematic e.g. when creating locations from GDAL/OGR datasets because
> > GRASS goes from OGR spatialreference through proj string to GRASS
> > definition.
> >
> > > OSGeo4W packages are planned to be switched to GDAL3/PROJ6 soon, are
> > > you aware of any known bugs?
> >
> > yes, EPSG:3857 is causing trouble. This is the WGS 84 / Pseudo-Mercator
> > projection on a sphere with radius 6378137 that needs special treatment
> > which no longer works with GRASS + PROJ 6 + GDAL 3.
> >
> > g.proj epsg=3857 -p gives me
> > -PROJ_INFO-------------------------------------------------
> > name       : WGS 84 / Pseudo-Mercator
> > datum      : wgs84
> > ellps      : wgs84
> > proj       : merc
> > lat_ts     : 0
> > lon_0      : 0
> > x_0        : 0
> > y_0        : 0
> > k          : 1
> > wktext     : defined
> > no_defs    : defined
> > -PROJ_EPSG-------------------------------------------------
> > epsg       : 3857
> > -PROJ_UNITS------------------------------------------------
> > unit       : meter
> > units      : meters
> > meters     : 1
> >
> > which is wrong because it should be a Mercator projection on a sphere,
not
> > an ellipsoid
>
> Well, this is a messy topic. EPSG:3857 uses the WebMercator projection,
which
> is not the standard Mercator projection since it uses spherical formulas
even
> if the datum and ellipsoid is WGS84

Indeed messy. Look at the WKT2 representation of projinfo -o WKT2_2018
EPSG:3857: projection on the ellipsoid "WGS 84", not on a sphere.
>
> So neither  proj=merc + ellps=wgs84 or proj=merc + ellps=sphere are
> technically correct
>
> Nothing new with PROJ 6 for that case
>
> > projinfo -o PROJ EPSG:3857
> > gives me
> > PROJ.4 string:
> > +proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1
> > +units=m +nadgrids=@null +wktext +no_defs +type=crs
>
> Yes, this is a hack of PROJ.4 still supported by PROJ 6 to more or less
get
> the correct result of WebMercator using the standard Mercator, by lying
on the
> ellipsoid definition (using the Sphere) and applying a nadgrids=null, so
that
> no datum transformation between WGS84 and that Sphere is attempted !

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

> >
> > that is correct, but unfortunately not returned by GDAL's function
> > OSRExportToProj4() which is now deprecated.
>
> Hum, I do ot agree with this. This *is* still what is returned by GDAL
> OSRExportToProj4() with GDAL + PROJ master :
>
> $ gdalsrsinfo EPSG:3857
>
> PROJ.4 : +proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0
> +k=1 +units=m +nadgrids=@null +wktext +no_defs

OK, I will figure out what goes wrong in GRASS regarding the representation
of EPSG:3857.

Thanks a lot for your very fast response, Even!

Markus M
>
> And you also actually use (PROJ 6 required here)
>
> $ gdalsrsinfo "+proj=webmerc +datum=WGS84"
> (or $ projinfo "+proj=webmerc +datum=WGS84 +type=crs" -o PROJ )
>
> PROJ.4 : +proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0
> +k=1 +units=m +nadgrids=@null +wktext +no_defs
>
>
> > That means GRASS needs to avoid
> > OSRExportToProj4() with GDAL 3+:
>
> In the general case, I agree with your conclusion, but not for the above
one
> :-)
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
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