[GRASS-user] projection issue

[Moritz Lennert]

On 24/03/14 13:10, Vincent Bain wrote:
> 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 ;-(

You're not wrong. The difference comes from the GRASS file 
/etc/ogr_csv/pcs.csv in which there is a specific rule set for 3857.

Don't know why, though.

Moritz