[GRASS-dev] area calculations in several GIS

Tue Oct 2 06:01:24 PDT 2018

On Tue, Oct 2, 2018 at 12:08 PM Kristian Evers <kreve at sdfe.dk> wrote:
>
> Use GeographicLib/PROJ as the reference.

Which one? The results of Planimeter from GeographicLib and
geod_polygonarea() from PROJ are different. If in doubt, use Planimeter as
reference?

Markus M

> I would say that this polygon is large, yes. If you want to get a better
sense for what constitutes a large polygon in this sense, try creating a
series of polygons of varying size (e.g. polygons of your house, your city,
your county, your country and your continent) and calculate the area with
both GRASS and GeographicLib. At some point the calculated areas should
start to diverge significantly. I suspect it happens somewhere between
county and country.
>
>
>
> /Kristian
>
>
>
> Fra: Markus Metz <markus.metz.giswork at gmail.com>
> Sendt: 2. oktober 2018 11:51
> Til: Kristian Evers <kreve at sdfe.dk>
> Cc: GRASS developers list <grass-dev at lists.osgeo.org>; Helmut Kudrnovsky <
hellik at web.de>
> Emne: Re: [GRASS-dev] area calculations in several GIS
>
>
>
>
>
> On Tue, Oct 2, 2018 at 8:47 AM Kristian Evers <kreve at sdfe.dk> wrote:
> >
> > Markus,
> >
> >
> > Thanks for extending the list. I do wonder why Planimeter gives
different results than geod_polygonarea(). I’ll run that by Charles Karney
when I get a chance.
> >
>
> for completeness, I used geod_polygonarea() from proj-5.2.0
>
>
>
> >> Regarding areas in projected space: UTM is not area true, laea is but
is instead scewing angles. You can check that stuff by running proj in very
verbose mode.
> >
>
> [...]
>
> >
>
> > So yes, the LAEA is the better choice of the two but it is never going
to represent the true area, especially for large polygons, and I would not
advice using it as reference for ellipsoidal area calculations.
>
>
>
> The question is, what can be used as reference? And is the test polygon a
"large" polygon, causing "large" deviations from the true area when
measured in LAEA?
>
>
>
> Markus M
>
> >
>
> > /Kristian
> >
> >
> >
> >
> >
> > Fra: Markus Metz <markus.metz.giswork at gmail.com>
> > Sendt: 1. oktober 2018 23:22
> > Til: GRASS developers list <grass-dev at lists.osgeo.org>; Kristian Evers <
kreve at sdfe.dk>
> > Cc: Helmut Kudrnovsky <hellik at web.de>
> > Emne: Re: [GRASS-dev] area calculations in several GIS
> >
> >
> >
> > Updated list with area calculations for
> >
> > https://gist.github.com/kbevers/207b5bcb9be20e7554abe5f56742ec2c
> >
> >
> >
> > PROJ [1]:    14,737.935 km^2
> > 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
> > GRASS GIS:   14.718.098 km^2
> >
> > EU LAEA [2]: 14,718.098 km^2
> > UTM 33 N:    14,707.742 km^2
> >
> > QGIS 3.2:    14,652.181 km^2
> > QGIS 2.8:    14,652.181 km^2
> >
> > [1] geodesic.h:geod_polygonarea()
> > [2] EPSG:3035
> >
> >
> >
> > In this case, GRASS GIS provides the best match of geodesic area to
metric area.
> >
> >
> >
> > @Kristian: are the metric area measurements in "EU LAEA" and "UTM 33 N"
suitable as reference?
> >
> >
> >
> > Discussion started on
> >
> >
http://osgeo-org.1560.x6.nabble.com/Re-Qgis-user-New-Features-in-Shape-Tools-3-2-0-td5378898.html
> >
> >
> >
> > Markus M
> >
> >
> >
> >
> >
> > On Mon, Oct 1, 2018 at 2:56 PM Markus Metz <
markus.metz.giswork at gmail.com> wrote:
> >
> >
> >
> > On Tue, Sep 25, 2018 at 7:38 PM Helmut Kudrnovsky <hellik at web.de> wrote:
> > >
> > > fyi see
> > >
https://lists.osgeo.org/pipermail/qgis-developer/2018-September/054644.html
> > >
> > > with GRASS mentioned
> > > ------------------
> > > Kristian Evers:
> > >
> > > 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
> >
> >
> >
> > adding to the confusion:
> >
> >
> >
> > I used the geographiclib API as included in PROJ 5.2.0 following the
example for geod_polygonarea() in geodesic.h and get
> >
> > geographiclib: 14,737.935 km^2
> >
> > quite different from
> >
> > Planimeter: 14,722.522 km^2
> >
> >
> >
> > GRASS native gives 14,718.097679
> >
> > as also reported by Helmut and Stefan
> >
> >
> >
> > Markus M
> >
> > >
> >
> > > 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
> > > -----------------
> > >
> > >
> > >
> > >
> > >
> > > -----
> > > best regards
> > > Helmut
> > > --
> > > Sent from: http://osgeo-org.1560.x6.nabble.com/Grass-Dev-f3991897.html
> > > _______________________________________________
> > > grass-dev mailing list
> > > grass-dev at lists.osgeo.org
> > > https://lists.osgeo.org/mailman/listinfo/grass-dev
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