On Mon, 1 Oct 2018 at 23:09, Helmut Kudrnovsky <[hidden email]> wrote:
> >I'm struggling to work out where the difference lies between the code
> >paths. Do you mind sharing the values from the st state struct when
> >G_ellipsoid_polygon_area is called for this polygon?
> delivered by Markus Metz, GRASS dev:
> st->AE: 40408299984661.4
> st->QA: 0.00446291999342754
> st->QB: 2.68888340714427e-05
> st->QC: 1.71432450253425e-07
> st->QbarA: -1.00448998025995
> st->QbarB: 0.0015057373196404
> st->QbarC: -5.48062628444059e-06
> st->QbarD: 2.44903500362035e-08
> st->Qp: 1.00448998025995
> st->E: 510065621153775
> precision: %.15g
> by adding a debug message in GRASS: in lib/gis/area_poly1.c add debug
> messages in G_begin_ellipsoid_polygon_area().
thanks a lot for this. It's a beautiful example of how open source
works, finding and solving problems through collaboration and highly
skilled people. It would be nice if someone would write about this for a
All the best.
Il 10/02/2018 05:12 AM, Nyall Dawson ha scritto:
> On Mon, 1 Oct 2018 at 23:01, Helmut Kudrnovsky <[hidden email]> wrote:
>> Helmut Kudrnovsky wrote
>>>> 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
>>>> My suspicion is that the geodesic algorithm used by QGIS (and apparently
>>> GRASS) is to blame here.
>>> a quick check in GRASS 7.7.svn shows :-)
> Ok, with https://github.com/qgis/QGIS/pull/8090 QGIS now exactly
> matches GRASS' value (14.718 km2).
> (For reference -- not a regression, this bug has been around since
> ellipsoidal calculations were introduced to QGIS!)
> QGIS-Developer mailing list
> [hidden email] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer
Thanks so much for investigating and fixing this. Also thank you all who have contributed to this discussion. This was an important bug to find and fix. I think we will find that the geographiclib calculations are going to be slightly more accurate coming in at
14.737 km^2, but the GRASS calculations are quite accurate.