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

Thu Sep 27 13:46:00 PDT 2018

Stefan,

You mentioned getting different results when you densify the polygon. The
results should be the same. With geographiclib all distances are based on
geodesic shortest paths between vertices to calculate the area. I tried
your experiment but used a geodesic densifier on the polygon, calculated
the area, and in both cases they were almost identical. The only difference
was way down in the noise. What this tells me is that whatever densify
algorithms you were using, they were not geodesic.

Regards

On Wed, Sep 26, 2018 at 4:36 PM Stefan Blumentrath <
Stefan.Blumentrath at nina.no> wrote:

> Hi,
>
> Even if the discussion already moved on, for the record below some more
> numbers from other OSGeo tools:
> PostGIS (in UTM 33N): 14727443738.0 m2
> GRASS 7.4 (in WGS84): 14707741818.0911
> GRASS 7.4 (in UTM33): between 14707741818.0911 m2 and 14727443738.0262 m2
> (depending on how much one densifies vertices on the boundaries during
> reprojection, the latter value measured without boundary vertex
> densification)
>
> Please see also: https://issues.qgis.org/issues/12057 for earlier
> discussion of area calculations in QGIS...
> And https://trac.osgeo.org/grass/ticket/3356 for recent changes in area
> calculations in GRASS.
>
> Cheers
> Stefan
>
> -----Original Message-----
> From: QGIS-Developer <qgis-developer-bounces at lists.osgeo.org> On Behalf
> Of Kristian Evers
> Sent: tirsdag 25. september 2018 15:32
> To: Kristian Evers <kreve at sdfe.dk>; Nyall Dawson <nyall.dawson at gmail.com>
> Cc: qgis-developer <qgis-developer at lists.osgeo.org>
> Subject: Re: [QGIS-Developer] [Qgis-user] New Features in Shape Tools 3.2.0
>
> 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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