[GRASS5] French projections

[Markus Neteler]

On Thu, Mar 07, 2002 at 09:42:18AM +0000, Michel Wurtz wrote:
> Speaking about updating projection definitions, I have noticed that
> lcc (Lambert Conformal conic) doesn't have false easting or northing
> (at least when using g.setproj).

That is surprising: in geo_init.c is defined:

        TABLE[LCC][LAT0].ask = 1;
        TABLE[LCC][LAT0].def_exists = 1;
        TABLE[LCC][LAT0].deflt = 23.0;
 
        TABLE[LCC][LON0].ask = 1;
        TABLE[LCC][LON0].def_exists = 1;
        TABLE[LCC][LON0].deflt = -96.0;
 
        TABLE[LCC][LAT1].ask = 1;
        TABLE[LCC][LAT1].def_exists = 1;
        TABLE[LCC][LAT1].deflt = 33.0;
 
        TABLE[LCC][LAT2].ask = 1;
        TABLE[LCC][LAT2].def_exists = 1;
        TABLE[LCC][LAT2].deflt = 45.0;
 
        TABLE[LCC][X0].ask = 1;
        TABLE[LCC][X0].def_exists = 1;
        TABLE[LCC][X0].deflt = 0.0;
 
        TABLE[LCC][Y0].ask = 1;
        TABLE[LCC][Y0].def_exists = 1;
        TABLE[LCC][Y0].deflt = 0.0;

So it should ask for x_0 and y_0. Obviously there is another
test somewhere to suppress that?

> On the other hand, France will now
> use a new system (Lambert 93) based on GRS80 ellispoid, instead of
> the old one (a familly of lcc projections) based on Clarke 1880
> ellipsoid (IGN modified).  This has lead to a conversion grid for
> precise transformation (something like NAD27/NAD83).
> Since proj has the ability to use such a grid, it sounds relatively
> easy to incorporate french geodetic systems.  All the détails
> parameters, formulas and algorithms can be found (in french :-) at
> 
> http://www.ign.fr/fr/PI/activites/geodesie/rgf93/OUTILS/index.html

This sounds like a datum transform...

Markus