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

[Gerald I. Evenden]

On Friday 06 October 2006 11:28 am, 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]],

Suggest:
gie at charon:~$ lproj -le | grep Clarke
   clrk66 a=6378206.4      b=6356583.8      Clarke 1866
  clrk66M a=6378693.704    b=6357069.451    Clarke 1866 Michigan
   clrk80 a=6378249.145    b=6356514.960    Clarke 1880 (Arc)
  clrk80M a=6378249.139    rf=293.4663      Clarke 1880
  clrk80B a=6378300.79     b=6356566.43     Clarke 1880 (Benoit)
  clrk80I a=6378249.2      rf=293.46602     Clarke 1880 (IGN)
  clrk80R a=6378249.145    rf=293.465       Clarke 1880 (RGS)
  clrk80S a=6378249.2      rf=293.46598     Clarke 1880 (SGA 1922)
gie at charon:~$
Thus it is apparent that the proj option (third from bottom)
   +ellps=clrk80I
would be appropriate for matching specified projection.  You were a little too 
selective in your grep search.

>         PRIMEM["Greenwich",0.0],
>         UNIT["Degree",0.0174532925199433]],

Pi/180 or value to multiply degrees by to get radians.  I wonder what the use 
is as the angular values in the specs appear to be degrees.

>     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 ?
>
> What about this UNIT["Degree",0.0174532925199433]], ?
>
>
>
> Thanks in advance
>
>
> Best regards