[QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0

Tue Sep 25 06:31:48 PDT 2018

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 <qgis-developer-bounces at lists.osgeo.org> På vegne af Kristian Evers
Sendt: 25. september 2018 09:15
Til: Nyall Dawson <nyall.dawson at gmail.com>
Cc: qgis-developer <qgis-developer at lists.osgeo.org>
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 <nyall.dawson at gmail.com> 
Sendt: 25. september 2018 09:07
Til: Kristian Evers <kreve at sdfe.dk>
Cc: Paolo Cavallini <cavallini at faunalia.it>; qgis-developer <qgis-developer at lists.osgeo.org>
Emne: Re: [QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0

On Tue, 25 Sep 2018 at 16:59, Kristian Evers <kreve at 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-bounces at lists.osgeo.org> På vegne af Paolo Cavallini
> Sendt: 25. september 2018 08:23
> Til: qgis-developer at 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/
>
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