[GRASS-dev] area calculations in several GIS

[Markus Metz]

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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