[GRASS-dev] area calculations in several GIS

[Markus Metz]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-dev/attachments/20181002/278c8d4f/attachment-0001.html>