[GRASS-user] v.to.db area calculations

[Blumentrath, Stefan]

Hi James,

QGIS uses a ellipsoidal area measurement, while GRASS (like most of the other GIS) uses a Cartesian measurement (others, please correct me if that is not the case in WGS84).

See amongst others:
https://issues.qgis.org/issues/12057 and
https://issues.qgis.org/issues/14675

Would you mind comparing area measurement of a third software (SAGA, PostGIS, ...)?

Cheers
Stefan
 
________________________________________
Von: grass-user [grass-user-bounces at lists.osgeo.org] im Auftrag von James Duffy [james.philip.duffy at gmail.com]
Gesendet: Donnerstag, 18. Mai 2017 16:22
An: grass-user
Betreff: [GRASS-user] v.to.db area calculations

Hello,

I'm trying to calculate the area of a polygon using v.to.db as follows (through R):

execGRASS("v.to.db", flags = c("p","quiet"), parameters = list(map = "test", units = "meters", option = "area", separator = "comma"),intern = TRUE)

I assume that the output of this command will be a meters squared calculation of the area inside the polygon 'test'.

However, in QGIS, I manually calculated the area the they do not match. QGIS (which I believe to be true) shows 0.128m2, and the output from v.to.db shows 22.42226m2.

I am working in WGS84.

Can anyone shed some light on how v.to.db calculates area and why it might be going wrong please?

Thank you

James



--
James Duffy
PhD Researcher
Environment and Sustainability Institute
Penryn Campus
University of Exeter
Penryn
Cornwall
TR10 9FE