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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