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

[Markus Metz]

On Thu, May 18, 2017 at 4:22 PM, James Duffy <james.philip.duffy at gmail.com>
wrote:
>
> 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.

This is a large difference. Is it possible that several polygons share the
same category? In this case the entry in the attribute table is the sum of
all polygons with that category.

>
> I am working in WGS84.

GRASS uses geodesic distance and area calculation for latlon, I believe
QGIS as well.

Markus M

>
> 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
>
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