Right, here are the calculated areas as returned by a number of different GIS applications and the planimeter app of GeographicLib for reference:
Caris LOTS: 14.737 km^2 ArcMap: 14.727,446 km^2 MapInfo: 14.727,352 km^2 GeoMedia: 14.726,443 km^2 Planimeter: 14.722,522 km^2 QGIS 3.2: 14.652,181 km^2 QGIS 2.8: 14.652,181 km^2 The polygon that I have used to get the numbers above can be found here: https://gist.github.com/kbevers/207b5bcb9be20e7554abe5f56742ec2c I am quite confident that GeographicLib delivers the most accurate result (if you have doubts, this reference [0] should convince you). As can be seen from the table above all but QGIS come fairly close. I expect some variation in the results as these are numerical approximations, although I think QGIS is too far of the mark. My suspicion is that the geodesic algorithm used by QGIS (and apparently GRASS) is to blame here. /Kristian [0] https://arxiv.org/pdf/1102.1215.pdf -----Oprindelig meddelelse----- Fra: QGIS-Developer <[email protected]> På vegne af Kristian Evers Sendt: 25. september 2018 09:15 Til: Nyall Dawson <[email protected]> Cc: qgis-developer <[email protected]> Emne: Re: [QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0 It's not a small area, that's why I have been nitpicking ellipsoid area calculation algorithms! I did end up doing the calculations with geographiblib to get a reference to judge various other calculations by. It's a pain in the butt to extract the data from a polygon shapefile (or whatever) and format it to suit the planimeter app of geographiclib. It would be so much cooler if I could get the same results by clicking info on a polygon in QGIS :-) /Kristian -----Oprindelig meddelelse----- Fra: Nyall Dawson <[email protected]> Sendt: 25. september 2018 09:07 Til: Kristian Evers <[email protected]> Cc: Paolo Cavallini <[email protected]>; qgis-developer <[email protected]> Emne: Re: [QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0 On Tue, 25 Sep 2018 at 16:59, Kristian Evers <[email protected]> wrote: > > Hi Paolo, > > I have some rought notes that I can try to polish up a bit and share here. > I'll see if I can find the time for that today. I didn't get to the bottom of > the problem but I am leaning towards the issue being that QGIS uses an > inferior algorithm to determine geodesics. Unfortunately I couldn't find out > from either the source code or the documentation which algorithm is used. > As far as I remember it differs from the otherwise popular algorithm by > Vincenty. That may be because a different algorithm is used or the > Implementation is wrong. > > I'll get back with a better write-up later and some test data and results. We use a direct port from GRASS: https://github.com/qgis/QGIS/blob/master/src/core/qgsdistancearea.cpp#L690 vs GRASS upstream: https://trac.osgeo.org/grass/browser/grass/trunk/lib/gis/area_poly1.c?rev=71260 It's not very stable for very small areas -- better to use cartesian calculations there. Or bring in geographiclib and punt the responsibility elsewhere ;) Nyall > > /Kristian > > -----Oprindelig meddelelse----- > Fra: QGIS-Developer <[email protected]> På vegne af > Paolo Cavallini > Sendt: 25. september 2018 08:23 > Til: [email protected] > Emne: Re: [QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0 > > Hi Kristian, > > > Il 09/25/2018 07:24 AM, Kristian Evers ha scritto: > > > > > > On a related note, I have been meaning to raise a similar issue based > > on QGIS’s not so accurate polygon-area-on-the-ellipsoid calculations. > > A colleague of mine came across some differences between QGIS and > > CARIS LOTS while doing area calculations in relation to a border > > dispute at sea. After that I tested several popular closed source GIS > > apps and they all performed better than QGIS. > this is worth exploring in detail. could you please be more specific? do > you have a test case? could you share the results, or give us a way to > replicate them? > Thanks. > > -- > Paolo Cavallini - www.faunalia.eu > QGIS.ORG Chair: > http://planet.qgis.org/planet/user/28/tag/qgis%20board/ > > _______________________________________________ > QGIS-Developer mailing list > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > _______________________________________________ > QGIS-Developer mailing list > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer _______________________________________________ QGIS-Developer mailing list [email protected] List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer _______________________________________________ QGIS-Developer mailing list [email protected] List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer
