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<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>Hello
Thibaut and Jan,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>The
towgs84 parameters you say that QGIS is using, seems to come from the file
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial> <A
href="http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/gcs.override.csv">http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/gcs.override.csv</A></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>that is
used by libgeotiff and GDAL. The datum shifts in this file have been
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial>constructed manually. For Belge 72 they seem to be wrong,
</FONT></SPAN><SPAN class=676244412-20012010><FONT face=Arial>as you
noted:</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial> The signs of DX, DY and DZ are
wrong,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial> The signs of RX, RY and RZ are
correct, provided that the COORD_OP_METHOD_CODE is changed to
9607,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial> The DS is 1.0000012747, but should be
-1.2747 (expressed in unity instead of parts per million, and wrong sign).
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>(This is
assuming that the datum shift EPSG:15928 is correct.)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>I think
the parameters in gcs.override.csv came from an information
source</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>that
gave </FONT></SPAN><SPAN class=676244412-20012010><FONT face=Arial>the datum
shift in the direction <EM>from</EM> WGS84 <EM>to</EM> Belge 72.
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=676244412-20012010><FONT face=Arial>Best
regards,</FONT></SPAN></DIV><!-- Converted from text/plain format -->
<P><FONT size=2>--<BR>Mikael Rittri<BR>Carmenta
AB<BR>SWEDEN<BR>www.carmenta.com</FONT> </P>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> proj-bounces@lists.maptools.org
[mailto:proj-bounces@lists.maptools.org] <B>On Behalf Of </B>Jan
Hartmann<BR><B>Sent:</B> Wednesday, January 20, 2010 11:29 AM<BR><B>To:</B>
PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] Belge
1972 / Belgian Lambert 72 (31370) - towgs84parameters<BR></FONT><BR></DIV>
<DIV></DIV>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
href="http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4.jpg"
moz-do-not-send="true">http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4.jpg</A>).
I already have a similar problem when writing a GPS software (<A
href="http://lists.maptools.org/pipermail/proj/2006-August/002447.html"
moz-do-not-send="true">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><PRE wrap=""><FIELDSET class=mimeAttachmentHeader></FIELDSET>
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