[Proj] What projections arguments to specify a custom ellipsoid ?

[Frank Warmerdam]

Romain Cherchi wrote:
> Hello,
> 
> I'm trying to add a line in the PostGIS SPATIAL_REF_SYS Table.
> 
> My SRS WKT is:
> PROJCS["NTF_Lambert_II_étendu",
>     GEOGCS["GCS_NTF",
>         DATUM["Nouvelle_Triangulation_Francaise",
>             SPHEROID["Clarke_1880_IGN",6378249.2,293.46602]],
>         PRIMEM["Greenwich",0.0],
>         UNIT["Degree",0.0174532925199433]],
>     PROJECTION["Lambert_Conformal_Conic_1SP"],
>     PARAMETER["False_Easting",600000.0],
>     PARAMETER["False_Northing",2200000.0],
>     PARAMETER["Central_Meridian",2.3372291667],
>     PARAMETER["Standard_Parallel_1", 45.8989188889],
>     PARAMETER["Standard_Parallel_2",47.6960144444],
>     PARAMETER["Scale_Factor",1.0],
>     PARAMETER["Latitude_Of_Origin",46.8],
>     UNIT["Meter",1.0]]
> 
> And I'm trying to figure out what is the corresponding proj4text:
> +proj=lcc    +x_0=600000
>                   +y_0=2200000
>                   +lon_0=2.3372291667
>                   +lat_1=45.8989188889
>                   +lat_2= 47.6960144444
>                   +k_0=1
>                   +lat_0=46.8
>                   +units=m
>                   +a=6378249.2
>                   +towgs84=-168,-60,320,0,0,0,0
>                   +pm=greenwich
> 
> However, I've some problems finding information about projections 
> arguments, especially about spheroid information.
> 
> SPHEROID["Clarke_1880_IGN",6378249.2,293.46602]],
> 
> I guess I can use "+a" for 6378249.2 but what is  the argument for  
> 293.46602 ?
> 
> NB: I already tryied :
> $ proj -le | grep clrk80
>    clrk80 a=6378249.145    rf=293.4663      Clarke 1880 mod.
 >
> But it seam that I can't use the +ellps=clrk80 because the values don't 
> match exactly with mine.
> 
> May I use +rf=293.46602 ?

I would suggest explicitly listing the +a and +rf values instead of
trying to use a predefined ellipsoid since there doesn't seem to
be an exact match.

> What about this UNIT["Degree",0.0174532925199433]], ?

This doesn't have any application to the PROJ.4 string.  The angular values
are already implicitly in decimal degrees.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org