[GRASS-user] projection issue

[Vincent Bain]

Comment on my previous message :

if we have a closer look at parameters, we notice a difference between
the current (grass location) projection parameters and epsg:3857
parameters :

Current :
        a (semi-major axis) = 6378137
        rf (flattening factor inverse) =298.257223563
am I wrong or as a consequence 
        b (semi-minor axis) = 6356752 ?

epsg:3857 :
        a (semi-major axis) = 6378137
        b (semi-minor axis) = 6378137 (a sphere ?)
        
Now if I run cs2cs with these distinct params :

         echo "6.46311951 45.78037144" | cs2cs +init=epsg:4326 -f "%.8f"
        +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
        +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext
        +no_defs
returns
        719471.17278927	5745223.17292760 0.00000000

and
        echo "6.46311951 45.78037144" | cs2cs +init=epsg:4326 -f "%.8f"
        +to +proj=merc +a=6378137 +b=6356752 +lat_ts=0.0 +lon_0=0.0
        +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext
        +no_defs
returns
        719471.17278927	5714587.36406099 0.00000000
that is what m.proj initially said.

It looks as if grass was taking into account the flatten-shape of the
ellipsoid, where epsg registry considers it is a sphere.

Sorry for the noise, but I would actually enjoy understanding where I am
wrong ;-(

Bye,
Vincent