Re: [Qgis-user] about some ellipsoïdal distances

[G. Allegri]

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.blavet at 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.blavet at ird.fr
>
>
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>
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