[Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84parameters

[Jan Hartmann]

No, if QGIS uses PROJ, this is just an error. PROJ and EPSG  use 
opposite rotational formulas, and PROJ uses degrees,  EPSG radians. 
Could you report this bug to the the QGIS team?

Jan

On 20-1-2010 14:04, Mikael Rittri wrote:
> Hello Thibaut and Jan,
> The towgs84 parameters you say that QGIS is using, seems to come from 
> the file
> http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/gcs.override.csv
> that is used by libgeotiff and GDAL.  The datum shifts in this file 
> have been
> constructed manually.  For Belge 72 they seem to be wrong, as you noted:
>       The signs of DX, DY and DZ are wrong,
>       The signs of RX, RY and RZ are correct, provided that the 
> COORD_OP_METHOD_CODE is changed to 9607,
>       The DS is 1.0000012747, but should be -1.2747 (expressed in 
> unity instead of parts per million, and wrong sign).
> (This is assuming that the datum shift EPSG:15928 is correct.)
> I think the parameters in gcs.override.csv came from an information source
> that gave the datum shift in the direction /from/ WGS84 /to/ Belge 72.
> Best regards,
>
> --
> Mikael Rittri
> Carmenta AB
> SWEDEN
> www.carmenta.com
>
> ------------------------------------------------------------------------
> *From:* proj-bounces at lists.maptools.org 
> [mailto:proj-bounces at lists.maptools.org] *On Behalf Of *Jan Hartmann
> *Sent:* Wednesday, January 20, 2010 11:29 AM
> *To:* PROJ.4 and general Projections Discussions
> *Subject:* Re: [Proj] Belge 1972 / Belgian Lambert 72 (31370) - 
> towgs84parameters
>
> 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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>
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