Re: [Qgis-user] about some ellipsoïdal distances

[blavet]

Thank you very much for this very fast and efficient diagnosis Best regards Didier Blavet Le 06/04/2012 12:01, G. Allegri a écrit : 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 _______________________________________________________________________________________ Didier Blavet 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 -- _______________________________________________________________________________________ Didier Blavet 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 _______________________________________________________________________________________

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