[Proj] What projections arguments to specify a custom ellipsoid ?
Frank Warmerdam
warmerdam at pobox.com
Fri Oct 6 11:57:53 EDT 2006
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,
--
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I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
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