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

[Mikael Rittri]

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.c
sv
 
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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