On Tue, 25 Sep 2018 at 16:59, Kristian Evers <kr...@sdfe.dk> 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 <qgis-developer-boun...@lists.osgeo.org> På vegne af > Paolo Cavallini > Sendt: 25. september 2018 08:23 > Til: qgis-developer@lists.osgeo.org > 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 > QGIS-Developer@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > _______________________________________________ > QGIS-Developer mailing list > QGIS-Developer@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer _______________________________________________ QGIS-Developer mailing list QGIS-Developer@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer