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 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 : [email protected]

_______________________________________________________________________________________

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