[GRASS-dev] area calculations in several GIS

[Stefan Blumentrath]

FYI:

Here are GRASS area measures for the given geometry:
In WGS84: 14718097678.6716
In UTM 33 (an appropriate UTM CRS): 14707741818.1113

So, in WGS84, GRASS is as close to Planimeter as most of the other GIS (that probably also use WGS84, "on the fly")...

curl https://gist.githubusercontent.com/kbevers/207b5bcb9be20e7554abe5f56742ec2c/raw/5bf6cd03bb1d03e86523f1602664567b4de8198e/ellipsoidal_area.geojson -o ellipsoidal_area.geojson

# In WGS84 location:
v.in.ogr input=/home/user/ellipsoidal_area.geojson layer=ellipsoidal_area output=area_test
v.to.db -p map=area_test option=area

# In UTM 33N location:
v.proj --overwrite location=wgs84 mapset=PERMANENT input=area_test smax=100 output=area_test
v.to.db -p map=area_test option=area

For those with special interest in historic discussion on area measurements in QGIS, see:
https://issues.qgis.org/issues/12057
(for example)

Cheers
Stefan


-----Original Message-----
From: grass-dev <grass-dev-bounces at lists.osgeo.org> On Behalf Of Helmut Kudrnovsky
Sent: tirsdag 25. september 2018 19:38
To: grass-dev at lists.osgeo.org
Subject: [GRASS-dev] area calculations in several GIS

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

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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