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Mikael,<br>
<br>
All my observations about the towgs parameters have been tested with
real Belgian coordinates overlaid over Google. As I said before, the
EPSG database is not very systematical in the way it represents its
formulas, it just takes them from whatever source is available,
untested AFAIK. Never use them without testing them with real world
coordinates.<br>
<br>
Jan<br>
<br>
On 22-1-2010 13:31, Mikael Rittri wrote:
<blockquote
cite="mid:FAF6E56B26D2044696772EBE89B1535101BDE7B6@posty.carmenta.se"
type="cite">
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<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">Jan Hartmann wrote:</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">> </font><font face="Times New Roman">I have been
quoting from PROJ 4.7. The older towgs parameter is not exactly
erroneous, it's just a bit less exact.</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">But Thibaut's image showed an displacement of about 190
meters. I call that erroneous.</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">Well, if by "older", you mean the EPSG datum shifts </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial"> 1652 "BD72 to ETRS89 (1)" and 1609 "BD72 to WGS
84 (1)"</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">you are right that they are just a bit less exact than
EPSG datum shifts </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial"> 15928 "BD72 to ETRS89 (1)" </font></span><span
class="553020612-22012010"><font face="Arial">and 15929 "BD72 to WGS
84 (3)"</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">But the towgs84 found in the current gcs.override.csv, as
well as in PROJ's nad/epsg file </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">(at least in 4.6.1), are just wrong. I think the wrong
sign for DX, DY and DZ causes most </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">of the 190 meter error. </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">By the way, in your first letter, you gave the older
transforms as </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><span
class="553020612-22012010">>
+towgs84=-99.059,53.322,-112.486,0.419,-0.830,1.885,-1</span></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"><span
class="553020612-22012010"></span></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><span
class="553020612-22012010"><font face="Arial">which I think is
correct, but you gave the newer transforms as </font></span></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">> </font><font face="Times New Roman">+towgs84=-106.8686,52.2978,-103.7329,-0.3366,0.457,-1.8422,-1.2747
</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">which I think has the wrong sign for the rotation angles:
it should be </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">> </font><font face="Times New Roman">+towgs84=-106.8686,52.2978,-103.7329,0.3366,-0.457,1.8422,-1.2747
</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Times New Roman">instead. (I base this correction on the fact
that EPSG claims that their datum shifts 15928 and 15929</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Times New Roman">use the Coordinate Frame Rotation, so the
rotation signs have to be reversed for PROJ.4. If the </font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Times New Roman">claim of EPSG contradicts the official
documents - it happens now and then - please notify EPSG.) </font></span></div>
</font></span></div>
<div dir="ltr" align="left"><span class="553020612-22012010"></span> </div>
<div dir="ltr" align="left"><span class="553020612-22012010"><font
face="Arial">Regards,</font></span></div>
<!-- Converted from text/plain format -->
<p><font size="2">--<br>
Mikael Rittri<br>
Carmenta AB<br>
Box 11354<br>
SE-404 28 Göteborg<br>
Visitors: Sankt Eriksgatan 5<br>
SWEDEN<br>
Tel: +46-31-775 57 37<br>
Mob: +46-703-60 34 07<br>
<a class="moz-txt-link-abbreviated" href="mailto:mikael.rittri@carmenta.com">mikael.rittri@carmenta.com</a><br>
<a class="moz-txt-link-abbreviated" href="http://www.carmenta.com">www.carmenta.com</a></font> </p>
<div> </div>
<br>
<div dir="ltr" class="OutlookMessageHeader" align="left" lang="en-us">
<hr tabindex="-1"><font face="Tahoma" size="2"><b>From:</b> Jan
Hartmann [<a class="moz-txt-link-freetext" href="mailto:j.l.h.hartmann@uva.nl">mailto:j.l.h.hartmann@uva.nl</a>] <br>
<b>Sent:</b> Thursday, January 21, 2010 11:51 PM<br>
<b>To:</b> Mikael Rittri<br>
<b>Cc:</b> PROJ.4 and general Projections Discussions<br>
<b>Subject:</b> Re: [Proj] Belge 1972 / Belgian Lambert 72 (31370) -
towgs84parameters<br>
</font><br>
</div>
<br>
<br>
On 21-Jan-10 12:52, Mikael Rittri wrote:
<blockquote
cite="mid:FAF6E56B26D2044696772EBE89B1535101BDE688@posty.carmenta.se"
type="cite">
<meta name="GENERATOR" content="MSHTML 8.00.6001.18854">
<div dir="ltr" align="left"><span class="176502811-21012010"><font
face="Arial">Jan Hartmann wrote: </font></span></div>
<div dir="ltr" align="left"><span class="176502811-21012010"></span> </div>
<div dir="ltr" align="left"><font face="Arial"><font
face="Times New Roman"><span class="176502811-21012010">> </span>No,
if QGIS uses PROJ, this is just an error. </font></font></div>
<div dir="ltr" align="left"> </div>
<div><span class="176502811-21012010"><font face="Arial">Okay, you
may be right that QGIS does not use the file gcs.override.csv. </font></span></div>
<div><span class="176502811-21012010"><font face="Arial">But I see
that </font></span><span class="176502811-21012010"><font face="Arial">the
nad/epsg file of PROJ.4 contains </font></span><span
class="176502811-21012010"><font face="Arial">the same erroneous </font></span></div>
<div><span class="176502811-21012010"><font face="Arial">+towgs84
parameters for Belge 1972 </font></span><span
class="176502811-21012010"><font face="Arial">as the
gcs.override.csv. </font></span></div>
<div><span class="176502811-21012010"></span><span
class="176502811-21012010"><font face="Arial">(At least PROJ version
4.6.1). </font></span></div>
<div><font face="Arial"><span class="176502811-21012010"></span></font></div>
</blockquote>
I have been quoting from PROJ 4.7. The older towgs parameter is not
exactly erroneous, it's just a bit les exact<br>
<blockquote
cite="mid:FAF6E56B26D2044696772EBE89B1535101BDE688@posty.carmenta.se"
type="cite">
<div> </div>
<div><font face="Arial"><span class="176502811-21012010"><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 class="176502811-21012010"></span></font> </div>
<div><font face="Arial"><span class="176502811-21012010">I don't
agree in the general case. PROJ uses the Position Vector </span></font></div>
<div><font face="Arial"><span class="176502811-21012010">Transform,
</span></font><font face="Arial"><span class="176502811-21012010">while
EPSG is neutral </span></font><font face="Arial"><span
class="176502811-21012010">on the rotation sign convention: </span></font></div>
<div><font face="Arial"><span class="176502811-21012010">they use
the same sign convention as the original source. </span></font></div>
<div><font face="Arial"><span class="176502811-21012010">And PROJ
uses arc seconds for rotations, while EPSG is neutral </span></font></div>
<div><font face="Arial"><span class="176502811-21012010">on the
angle unit: </span></font><font face="Arial"><span
class="176502811-21012010">they use the same angle unit as the
original source</span></font></div>
<div><font face="Arial"><span class="176502811-21012010">(usually
arc seconds, but sometimes </span></font><font face="Arial"><span
class="176502811-21012010">microradians or radians). </span></font></div>
<div><font face="Arial"><span class="176502811-21012010"> For
the EPSG transforms you quote, EPSG use arc-seconds</span></font></div>
<div><font face="Arial"><span class="176502811-21012010">for the
rotations, </span></font><font face="Arial"><span
class="176502811-21012010">but either the Position Vector Transform or
the</span></font></div>
<div><font face="Arial"><span class="176502811-21012010">Coordinate
Frame Rotation </span></font><font face="Arial"><span
class="176502811-21012010">depending on whether they got the </span></font></div>
<div><font face="Arial"><span class="176502811-21012010">transform
from Eurogeographics or directly </span></font><font face="Arial"><span
class="176502811-21012010">from Belgium. </span></font></div>
<div><font face="Arial"><span class="176502811-21012010"></span></font><br>
</div>
</blockquote>
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>
</blockquote>
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