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Hi Thibaut,<br>
<br>
the most recent version of the EPSG database, 7.4, (<a class="moz-txt-link-abbreviated" href="http://www.epsg.org">www.epsg.org</a>) 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<br>
<br>
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<br>
<br>
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 <br>
<br>
The second one if the official one from the Belgian National Geographic
Institute (<a class="moz-txt-link-freetext" href="http://www.ngi.be/FR/FR4-4.shtm">http://www.ngi.be/FR/FR4-4.shtm</a>). 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.<br>
<br>
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 <br>
<br>
Jan<br>
<br>
On 20-1-2010 10:18, Thibaut Gheysen wrote:
<blockquote
cite="mid:94e91cde1001200118x16b58727x21ce687c3598e4d3@mail.gmail.com"
type="cite">Hi,<br>
<br>
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 : <a moz-do-not-send="true"
href="http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4.jpg">http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4.jpg</a>).
I already have a similar problem when writing a GPS software (<a
moz-do-not-send="true"
href="http://lists.maptools.org/pipermail/proj/2006-August/002447.html">http://lists.maptools.org/pipermail/proj/2006-August/002447.html</a>).
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.<br>
<br>
Best regards,<br>
Thibaut.<br>
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