[GRASS-dev] area calculations in several GIS

[Markus Metz]

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
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