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It really looks like a QGIS problem. Perhaps you should send a
bug-report to QGIS, that the correct +towgs84 parameter is the one
which displays your points correctly. If possible, add both images to
it.<br>
<br>
Jan<br>
<br>
On 22-1-2010 13:34, Thibaut Gheysen wrote:
<blockquote
cite="mid:94e91cde1001220434je5a6991m4c25195e536f2aa5@mail.gmail.com"
type="cite">Hi Jan, <br>
<br>
When I replace towgs84 parameters in QGIS by
'+towgs84=-99.059,53.322,-112.486,0.419,-0.83,1.885,-0.999999' the
points are correctly placed (red points in the image). When I remove
towgs84 parameters, I have a error similar to this with original QGIS
parameters but the points are not placed at same place (<a
moz-do-not-send="true"
href="http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4-2.jpg">http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4-2.jpg</a>).
I have thus supposed that error come from towgs84 parameters but I'm
not a proj.4 specialist at all.<br>
<br>
Thibaut.<br>
<br>
<br>
<br>
<div class="gmail_quote">2010/1/22 Jan Hartmann <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:j.l.h.hartmann@uva.nl">j.l.h.hartmann@uva.nl</a>></span><br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">Hi Thibaut, your problem
doesn't have anything to do with the two
different Belgian parameters; the error in your picture is too big. It
looks as if the WGS84 datum shift has not been applied at all. You
could test this by projecting the points from the Belgian Lambert
projection to Google without any towgs parameter.<br>
<font color="#888888"><br>
Jan</font>
<div>
<div class="h5"><br>
<br>
On 22-1-2010 9:37, Thibaut Gheysen wrote:
<blockquote type="cite">Thanks Jan and Mikael for your responses. <br>
As suggested, I have forwarded this bug to the QGIS developer mailing
list.<br>
<br>
Thibaut.<br>
<br>
<br>
<div class="gmail_quote">2010/1/21 Jan Hartmann <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:j.l.h.hartmann@uva.nl"
target="_blank">j.l.h.hartmann@uva.nl</a>></span><br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">
<div><br>
<br>
On 21-Jan-10 12:52, Mikael Rittri wrote:
<blockquote type="cite">
<div dir="ltr" align="left"><span><font face="Arial">Jan
Hartmann
wrote: </font></span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><font face="Arial"><font
face="Times New Roman"><span>> </span>No,
if QGIS uses PROJ, this is just an error. </font></font></div>
<div dir="ltr" align="left"> </div>
<div><span><font face="Arial">Okay, you
may be right that QGIS does not use the file gcs.override.csv. </font></span></div>
<div><span><font face="Arial">But I see
that </font></span><span><font face="Arial">the
nad/epsg file of PROJ.4 contains </font></span><span><font face="Arial">the
same
erroneous </font></span></div>
<div><span><font face="Arial">+towgs84
parameters for Belge 1972 </font></span><span><font face="Arial">as
the
gcs.override.csv. </font></span></div>
<div><span></span><span><font face="Arial">(At least PROJ
version
4.6.1). </font></span></div>
<div><font face="Arial"><span></span></font></div>
</blockquote>
</div>
I have been quoting from PROJ 4.7. The older towgs parameter is not
exactly erroneous, it's just a bit les exact
<div><br>
<blockquote type="cite">
<div> </div>
<div><font face="Arial"><span><font face="Times New Roman">>
PROJ
and EPSG use opposite rotational
formulas, and PROJ uses degrees, EPSG radians. </font></span></font></div>
<div><font face="Arial"><span></span></font> </div>
<div><font face="Arial"><span>I don't
agree in the general case. PROJ uses the Position Vector </span></font></div>
<div><font face="Arial"><span>Transform, </span></font><font
face="Arial"><span>while EPSG is neutral </span></font><font
face="Arial"><span>on the rotation sign
convention: </span></font></div>
<div><font face="Arial"><span>they use the
same sign convention as the original source. </span></font></div>
<div><font face="Arial"><span>And PROJ
uses arc seconds for rotations, while EPSG is neutral </span></font></div>
<div><font face="Arial"><span>on the angle
unit: </span></font><font face="Arial"><span>they
use the same angle unit as the original source</span></font></div>
<div><font face="Arial"><span>(usually arc
seconds, but sometimes </span></font><font face="Arial"><span>microradians
or
radians). </span></font></div>
<div><font face="Arial"><span> For the
EPSG transforms you quote, EPSG use arc-seconds</span></font></div>
<div><font face="Arial"><span>for the
rotations, </span></font><font face="Arial"><span>but either the
Position Vector Transform or
the</span></font></div>
<div><font face="Arial"><span>Coordinate
Frame Rotation </span></font><font face="Arial"><span>depending on
whether they got the </span></font></div>
<div><font face="Arial"><span>transform
from Eurogeographics or directly </span></font><font face="Arial"><span>from
Belgium.
</span></font></div>
<div><font face="Arial"><span></span></font> <br>
</div>
</blockquote>
</div>
My information was for the Dutch and Belgian cases, as from the
official documents. I don't know on what principles EPSG operates, I
guess they just take it as they get it. It is not an easy-to-use
database.<br>
<br>
Jan<br>
</div>
<br>
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