[OSRS-PROJ] Pb with conversion betwenn WGS84 and Lambert

Frank Warmerdam warmerdam at pobox.com
Thu Nov 20 14:08:47 EST 2003

Aurélie Lebreton wrote:
> Hello
> I want to transform a pair of angular coordinates in datum WGS84 to NTF
> Lambert 2. I have written this line :
> ./cs2cs +proj=latlong +datum=WGS84 +ellps=GRS80 +to +proj=lcc +lat_0=46.8000
> +lat_1=46.8000 +lon_0=2.3372291 +k_0=0.99987742 +x_0=600000 +y_0=200000
> +a=6378249.2 +b=6356515 +ellps=clrk80
> The input coordinates are :
> 2d39'48882"E 48d32'8575"N
> But the coordinates transformed come out wrong and i don't understand why :
> 1575621.19      743408.43 0.00
> (the coordinates should turn around these values :
>  600000 2400000
> )
> Does any one have the solution to this, or at least some in depth
> documentation on how to use proj ?
> Aurelie


The main problem is your DMS specification: 2d39'48882"E.  I imagine
you mean 2d39'48.882"E (that is 48.882 seconds, not 48882 seconds).
You may also find you need to include +towgs84=0,0,0 in the NTF coordinate
system to ensure that the ellipsoid conversion is done, even though no datum
shift info is available.

With the DMS specs fixed I get:

warmerda at gdal2200[265]% cs2cs +proj=latlong +datum=WGS84 +ellps=GRS80 +to +proj=lcc +lat_0=46.8000 +lat_1=46.8000 
+lon_0=2.3372291 +k_0=0.99987742 +x_0=600000 +y_0=200000 +a=6378249.2 +b=6356515 +ellps=clrk80 +towgs84=0,0,0
2d39'48.882"E 48d32'85.72"N
624110.53       395817.08 83.91

This value looks reasonable.  I am not sure why you were thinking the
northing should be in the range of 2400000m.  48d32' is less than 2 degrees
north of the reference point (46.8) and a degree is roughly 100000m.  So
the value I got (395817m N) is roughly 2 degrees north of the reference point
which makes sense.

Note that without good datum shift information the precision of the
conversion will not be great.

Good luck,

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    | Geospatial Programmer for Rent

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