Probably I've found the bug.<div>In Qgis the semi-minor axis of the ellipsoid is calculated with:</div><div><br></div><div>b = a - (f/a)</div><div><br></div><div>where </div><div>b = semi-minor axis</div><div>a = semi-majot axis</div>
<div>f = inverse flattening</div><div><br></div><div>while it should be:</div><div><br></div><div>b = a - (a/f)</div><div><br></div><div><br></div><div>In Qgis the WGS84 semi-minor axis is 6378136,xxx</div><div>while it should be 6356752.xxx</div>
<div><br></div><div>giovanni</div><div><br></div><div><br></div><div><br></div><div><br><br><div class="gmail_quote">2012/4/6 G. Allegri <span dir="ltr"><<a href="mailto:giohappy@gmail.com">giohappy@gmail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"><span>AFAICS, the Qgis formula [1] to calculate distances on ellipssoid is Vincenty's.</span><div>I've reproduced it for the first set of points and I can confirm tha QGis gives 540.9973919726176 m</div>
<div><br></div><div>The online Vincenty calculator [2] gives 540.045 m</div><div><br></div><div>I suppose it's a rounding issue. </div><div>I will investigate it more...</div><div><br>
</div><div>giovanni</div><div><br><div><br></div><div>[1] <a href="http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540" style="color:rgb(17,85,204)" target="_blank">http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540</a> </div>
<div>[2] <a href="http://www.movable-type.co.uk/scripts/latlong-vincenty.html" style="color:rgb(17,85,204)" target="_blank">http://www.movable-type.co.uk/scripts/latlong-vincenty.html</a> </div></div><br></div><div class="gmail_quote">
2012/4/6 blavet <span dir="ltr"><<a href="mailto:didier.blavet@ird.fr" target="_blank">didier.blavet@ird.fr</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div class="h5"><u></u>
<div text="#000000" bgcolor="#ffffff">
Dear QGis Developpers and Users <br>
<br>
First of all many thanks to the Developpers of Qgis for this wonderful
and user friendly Sofwaret. I'm sure that Qgis will become more and
more a "standard" in the next few years<br>
<br>
So, just a small point (and I hope to be wrong with this, but if it
is the case i don't see where) : as i needed to find quite exactly some
ellipsoïdal distances, I'm afraid to have also found that in some
cases the ellipsoïdal distance could have some problem in QGIS 1.7.4
(tested versions Windows and Debian). This could depend on the
algorithm used and the geographical position of the points.<br>
<br>
The following table suggests for few studied points that the
ellipsoïdal distances computed in Qgis 1.7.4 was overestimated and
quite different than ellipsoïdal distances computed with the help of
two geodesic estimations<br>
<br>
<table cellspacing="0" cols="11" rules="NONE" border="0" frame="VOID">
<colgroup><col width="86"><col width="89"><col width="89"><col width="102"><col width="102"><col width="74"><col width="84"><col width="141"><col width="167"><col width="96"><col width="84"></colgroup>
<tbody>
<tr>
<td valign="MIDDLE" width="86" align="LEFT" height="131"><b>point
name in the study</b></td>
<td valign="MIDDLE" width="89" align="CENTER"><b><font color="#0000ff">East EPSG:27572<br>
(m)<br>
</font></b></td>
<td valign="MIDDLE" width="89" align="CENTER"><b><font color="#0000ff">North EPSG:27572<br>
(m)<br>
</font></b></td>
<td valign="MIDDLE" width="102" align="CENTER"><b><font color="#800080">Lat WGS84 (obtained from deprojection in latlon WGS 84
in QGIS 1.7.4)<br>
deg. dec.<br>
</font></b></td>
<td valign="MIDDLE" width="102" align="CENTER"><b><font color="#800080">Long WGS84 (obtained from deprojection in latlon WGS
84 in QGIS 1.7.4)<br>
deg. dec.<br>
</font></b></td>
<td valign="MIDDLE" width="74" align="CENTER" bgcolor="#94bd5e"><b><font color="#ffffff">Simple Pythagore dist<br>
(m)<br>
</font></b></td>
<td valign="MIDDLE" width="84" align="CENTER" bgcolor="#94bd5e"><b><font color="#ffffff">Simple Pythagore dist QGIS 1.7.4<br>
(m)<br>
</font></b></td>
<td valign="MIDDLE" width="141" align="CENTER" bgcolor="#355e00"><font color="#00ff00">dist ellips Clarke 1880 estimated from linear
alteration given by geodesic soft 1 (IGN Circé v4)<br>
(m)<br>
</font></td>
<td valign="MIDDLE" width="167" align="CENTER" bgcolor="#355e00"><font color="#00ff00">dist ellips WGS84 from geodesic Vincenty formula
recommanded by proj4 (found in soft 2 at
<a href="http://www.movable-type.co.uk/scripts/latlong-vincenty.html" target="_blank">http://www.movable-type.co.uk/scripts/latlong-vincenty.html</a><br>
(m)<br>
</font></td>
<td valign="MIDDLE" width="96" align="CENTER" bgcolor="#ff950e"><b><font color="#ffff00">dist ellips QGIS 1.7.4 Clarke 1880<br>
(m)<br>
</font></b></td>
<td valign="MIDDLE" width="84" align="CENTER" bgcolor="#ff950e"><b><font color="#ffff00">dist ellips QGIS 1.7.4 WGS84<br>
(m)<br>
</font></b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><br>
</td>
<td valign="TOP" align="CENTER"><b><font color="#0000ff"><br>
</font></b></td>
<td valign="TOP" align="CENTER"><b><font color="#0000ff"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td align="CENTER"><font color="#00ff00"><br>
</font></td>
<td align="LEFT"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">B (-)9</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733080.2641</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1847716.595</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.61880918</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.983106135</font></b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">540.795</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">540.795</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">540.048</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">540.045</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>541.016</b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>541.000</b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">AC (-)9</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000"><a href="tel:732806.9273" value="+17328069273" target="_blank">732806.9273</a></font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1848183.228</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62305376</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.979845905</font></b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000"><br>
</font></b></td>
<td valign="MIDDLE" align="LEFT"><font color="#800000"><br>
</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000"><br>
</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">A 0</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733245.932</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1847790.425</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.61944142</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.985174410</font></b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">560.824</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">560.824</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">560.049</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">560.046</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>561.054</b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>561.038</b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">AC 0</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000"><a href="tel:732962.4713" value="+17329624713" target="_blank">732962.4713</a></font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1848274.34</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62384326</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.981793539</font></b></td>
</tr>
<tr>
<td align="LEFT" height="17"><br>
</td>
<td align="LEFT"><br>
</td>
<td align="LEFT"><br>
</td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">B 8</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733374.0699</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1847888.697</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62030034</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.986784862</font></b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">540.794</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">540.794</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">540.047</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">540.043</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>541.016</b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>541.001</b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">AC 8</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733100.7324</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1848355.328</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62454501</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.983524812</font></b></td>
</tr>
<tr>
<td align="LEFT" height="17"><br>
</td>
<td align="LEFT"><br>
</td>
<td align="LEFT"><br>
</td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td align="LEFT"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">C (-)9</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733070.1404</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1847733.878</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.61896639</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.982985394</font></b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">380.560</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">380.560</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">380.034</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">380.031</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>380.715</b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>380.692</b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">C 10</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733398.5116</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1847926.227</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62063297</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.987096932</font></b></td>
</tr>
<tr>
<td align="LEFT" height="17"><br>
</td>
<td align="LEFT"><br>
</td>
<td align="LEFT"><br>
</td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080"><br>
</font></b></td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td valign="MIDDLE" align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
<td align="CENTER"><br>
</td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">AC (-)13</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000"><a href="tel:732737.7966" value="+17327377966" target="_blank">732737.7966</a></font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1848142.733</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62270285</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.978980306</font></b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">420.617</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#94bd5e">420.617</td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">420.036</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#355e00"><font color="#00ff00">420.034</font></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>420.790</b></td>
<td rowspan="2" valign="MIDDLE" align="CENTER" bgcolor="#ff950e"><b>420.765</b></td>
</tr>
<tr>
<td valign="MIDDLE" align="LEFT" height="17"><b><font color="#800000">AC 8</font></b></td>
<td valign="MIDDLE" align="RIGHT"><font color="#800000">733100.7324</font></td>
<td valign="MIDDLE" align="CENTER"><font color="#800000">1848355.328</font></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">43.62454501</font></b></td>
<td valign="MIDDLE" align="CENTER"><b><font color="#800080">3.983524812</font></b></td>
</tr>
</tbody>
</table>
<br>
<br>
<br>
Best regards<br>
<br>
<div>
<p style="margin-bottom:0cm"><font>_______________________________________________________________________________________</font></p>
<p style="margin-bottom:0cm"><font color="#008000"><font><b>Didier
B</b><b>lavet</b> </font></font>
</p>
<p style="margin-bottom:0cm"><font color="#000000"><font>Institut
de
Recherche pour le Développement - IRD (<font color="#0000ff"><a href="http://www.ird.fr" target="_blank">http://www.ird.fr</a></font>)
</font></font></p>
<p style="margin-bottom:0cm"><font color="#000000"><font>UMR
Eco&Sols - Ecologie Fonctionnelle & Biogéochimie des Sols &
Agroécosystèmes </font></font>
</p>
<p style="margin-bottom:0cm"><font color="#000000"><font>(Montpellier
SupAgro-CIRAD-INRA-IRD)
(<font color="#0000ff"><a href="http://www.montpellier.inra.fr/ecosols" target="_blank">http://www.montpellier.inra.fr/ecosols</a></font><span style="font-style:normal">)</span></font></font></p>
<p style="margin-bottom:0cm"><font color="#000000"><font>Bâtiment
12,
2 place Viala, F-34060 Montpellier Cedex 2 , France </font></font>
</p>
<p style="margin-bottom:0cm"><font color="#000000"><font>Tel
bureau : <a href="tel:%2B33%20%280%294%2099%2061%2021%2033" value="+33499612133" target="_blank">+33 (0)4 99 61 21 33</a> </font></font>
</p>
<p style="margin-bottom:0cm"><font color="#000000"><font>Secrétariat
:
<a href="tel:%2B33%20%280%294%2099%2061%2021%2001" value="+33499612101" target="_blank">+33 (0)4 99 61 21 01</a> / Fax : <a href="tel:%2B33%20%280%294%2099%2061%2021%2019" value="+33499612119" target="_blank">+33 (0)4 99 61 21 19</a> </font></font>
</p>
<p style="margin-bottom:0cm"><font color="#000000"><font>Courriel
:
<font color="#0000ff"><a href="mailto:didier.blavet@ird.fr" target="_blank">didier.blavet@ird.fr</a></font> </font></font>
</p>
<p style="margin-bottom:0cm"><font>_______________________________________________________________________________________</font></p>
</div>
</div>
<br></div></div><div class="im">_______________________________________________<br>
Qgis-user mailing list<br>
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<br></div></blockquote></div><br>
</blockquote></div><br></div>