Probably I've found the bug. In Qgis the semi-minor axis of the ellipsoid is calculated with:
b = a - (f/a) where b = semi-minor axis a = semi-majot axis f = inverse flattening while it should be: b = a - (a/f) In Qgis the WGS84 semi-minor axis is 6378136,xxx while it should be 6356752.xxx giovanni 2012/4/6 G. Allegri <gioha...@gmail.com> > AFAICS, the Qgis formula [1] to calculate distances on ellipssoid is > Vincenty's. > I've reproduced it for the first set of points and I can confirm tha QGis > gives 540.9973919726176 m > > The online Vincenty calculator [2] gives 540.045 m > > I suppose it's a rounding issue. > I will investigate it more... > > giovanni > > > [1] > http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540 > > [2] http://www.movable-type.co.uk/scripts/latlong-vincenty.html > > 2012/4/6 blavet <didier.bla...@ird.fr> > >> ** >> Dear QGis Developpers and Users >> >> 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 >> >> 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. >> >> 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 >> >> *point name in the study* *East EPSG:27572 >> (m) >> * *North EPSG:27572 >> (m) >> * *Lat WGS84 (obtained from deprojection in latlon WGS 84 in QGIS 1.7.4) >> deg. dec. >> * *Long WGS84 (obtained from deprojection in latlon WGS 84 in QGIS 1.7.4) >> deg. dec. >> * *Simple Pythagore dist >> (m) >> * *Simple Pythagore dist QGIS 1.7.4 >> (m) >> * dist ellips Clarke 1880 estimated from linear alteration given by >> geodesic soft 1 (IGN Circé v4) >> (m) >> dist ellips WGS84 from geodesic Vincenty formula recommanded by proj4 >> (found in soft 2 at >> http://www.movable-type.co.uk/scripts/latlong-vincenty.html >> (m) >> *dist ellips QGIS 1.7.4 Clarke 1880 >> (m) >> * *dist ellips QGIS 1.7.4 WGS84 >> (m) >> * >> * >> * * >> * * >> * * >> * >> >> >> >> >> >> *B (-)9* 733080.2641 1847716.595 *43.61880918* *3.983106135* 540.795 >> 540.795 540.048 540.045 *541.016* *541.000* *AC (-)9* 732806.9273 >> 1848183.228 *43.62305376* *3.979845905* * >> * >> >> * >> * * >> * >> >> >> >> >> >> *A 0* 733245.932 1847790.425 *43.61944142* *3.985174410* 560.824 >> 560.824 560.049 560.046 *561.054* *561.038* *AC 0* 732962.4713 >> 1848274.34 *43.62384326* *3.981793539* >> >> >> * >> * * >> * >> >> >> >> >> >> *B 8* 733374.0699 1847888.697 *43.62030034* *3.986784862* 540.794 >> 540.794 540.047 540.043 *541.016* *541.001* *AC 8* 733100.7324 >> 1848355.328 *43.62454501* *3.983524812* >> >> >> * >> * >> >> >> >> >> >> >> *C (-)9* 733070.1404 1847733.878 *43.61896639* *3.982985394* 380.560 >> 380.560 380.034 380.031 *380.715* *380.692* *C 10* 733398.5116 >> 1847926.227 *43.62063297* *3.987096932* >> >> >> * >> * * >> * >> >> >> >> >> >> *AC (-)13* 732737.7966 1848142.733 *43.62270285* *3.978980306* 420.617 >> 420.617 420.036 420.034 *420.790* *420.765* *AC 8* 733100.7324 >> 1848355.328 *43.62454501* *3.983524812* >> >> >> Best regards >> >> >> _______________________________________________________________________________________ >> >> *Didier B**lavet* >> >> Institut de Recherche pour le Développement - IRD (http://www.ird.fr) >> >> UMR Eco&Sols - Ecologie Fonctionnelle & Biogéochimie des Sols & >> Agroécosystèmes >> >> (Montpellier SupAgro-CIRAD-INRA-IRD) ( >> http://www.montpellier.inra.fr/ecosols) >> >> Bâtiment 12, 2 place Viala, F-34060 Montpellier Cedex 2 , France >> >> Tel bureau : +33 (0)4 99 61 21 33 >> >> Secrétariat : +33 (0)4 99 61 21 01 / Fax : +33 (0)4 99 61 21 19 >> >> Courriel : didier.bla...@ird.fr >> >> >> _______________________________________________________________________________________ >> >> _______________________________________________ >> Qgis-user mailing list >> Qgis-user@lists.osgeo.org >> http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >
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