[Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84 parameters

[Jan Hartmann]

Hi Thibaut,

the most recent version of the EPSG database, 7.4, (www.epsg.org) has 
two definitions for the datum shift from Belge Lambert to WGS84 (they 
call it a coordinate transformation). I'l give the parameters as a PROJ 
string, with rotations converted from radians to degrees and sign-converted

nr 162 and 164 (accurate to a meter) :  +proj=lcc 
+lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90 
+lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl 
+towgs84=-99.059,53.322,-112.486,0.419,-0.830,1.885,-1 +units=m +no_defs

nr 163 and 166: (accurate to 20 cm): +proj=lcc +lat_1=51.16666723333333 
+lat_2=49.8333339 +lat_0=90 +lon_0=4.367486666666666 +x_0=150000.013 
+y_0=5400088.438 +ellps=intl  
+towgs84=-106.8686,52.2978,-103.7329,-0.3366,0.457,-1.8422,-1.2747 
+units=m +no_defs

The second one if the official one from the Belgian National Geographic 
Institute (http://www.ngi.be/FR/FR4-4.shtm). Both should give 
approximately the same results. I have used epsg:31370, which uses the 
second definition without errors. The errors in your picture, about 100 
meters, look as if no datum transformation has been applied at all.

You could test this by transforming your data points manually with cs2cs 
for both towgs84 strings, and compare the results with the position of 
the WGS84 coordinates in QGIS

Jan

On 20-1-2010 10:18, Thibaut Gheysen wrote:
> Hi,
>
> I have a problem in QGIS 1.4.0 when I reproject a gpx file into 
> Belgian lambert 72 coordinate system : the waypoints are not correctly 
> placed on the map (see this image : 
> http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4.jpg). I already 
> have a similar problem when writing a GPS software 
> (http://lists.maptools.org/pipermail/proj/2006-August/002447.html). To 
> solve this I have used this towgs84 parameters : 
> +towgs84=-99.059,53.322,-112.486,0.419,-0.83,1.885,-0.999999. In QGIS, 
> the towgs84 parameters are different 
> (+towgs84=106.869,-52.2978,103.724,-0.33657,0.456955,-1.84218,1). When 
> I use my GPS software to reproject into Belgian Lambert 72 the 
> waypoints are correctly placed (red points in the image) but not when 
> I use QGIS (yellow points). I think thus there is a problem in the 
> epsg 31370 definition.
>
> Best regards,
> Thibaut.
>
>
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