[GRASS-user] Determining area of poligon vector layer around circle of points in other vector layer

GRASS GRASS user
1

Hi

I have two vector layers - one with polygons and one with points, both
in UTM 34 S.

Now I would like to calculate the area of the polygons in circles
around the points, i.e. how much area in a 500m circle around the
points is covered by the polygons, and store this in the attribute
table of the point layer.

Is there any easy way of achieving this? If I simply buffer the
points, the circles overlap sometimes and form one polygon, and I want
to have the results for each point - so that doesn't work.

Thanks

Rainer

--
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

Plant Conservation Unit
Department of Botany
University of Cape Town
Rondebosch 7701
South Africa

2

Hello Rainer,

On Donnerstag, 12. Juni 2008 19:13:18 Rainer M Krug wrote:

Hi

I have two vector layers - one with polygons and one with points, both
in UTM 34 S.

Now I would like to calculate the area of the polygons in circles
around the points, i.e. how much area in a 500m circle around the
points is covered by the polygons, and store this in the attribute
table of the point layer.

Is there any easy way of achieving this? If I simply buffer the
points, the circles overlap sometimes and form one polygon, and I want
to have the results for each point - so that doesn't work.

perhaps if you create a loop:

v.to.rast your points output=points use=cat

max=`r.info -r "points" | grep max | cut -f2 -d=`

COUNTER=`r.info -r "points" | grep min | cut -f2 -d=`

while [ "$COUNTER" -le "$max" ]; do

  r.mapcalc "tttemp_1 = if(points == "$COUNTER",1,null())"

  r.buffer input=tttemp_1 output=tttemp_buffer distances=100 --o

# your query
# patch result with previous query
# redo it until all patches are analysed

  let "COUNTER=COUNTER+1"

done

hope this helps, Martin

3

Thanks - that sounds interesting. So there is no way of doing that
without a loop?

Rainer

On Thu, Jun 12, 2008 at 7:25 PM, Martin Wegmann
<wegmann@biozentrum.uni-wuerzburg.de> wrote:

Hello Rainer,

On Donnerstag, 12. Juni 2008 19:13:18 Rainer M Krug wrote:

Hi

I have two vector layers - one with polygons and one with points, both
in UTM 34 S.

Now I would like to calculate the area of the polygons in circles
around the points, i.e. how much area in a 500m circle around the
points is covered by the polygons, and store this in the attribute
table of the point layer.

Is there any easy way of achieving this? If I simply buffer the
points, the circles overlap sometimes and form one polygon, and I want
to have the results for each point - so that doesn't work.

perhaps if you create a loop:

v.to.rast your points output=points use=cat

max=`r.info -r "points" | grep max | cut -f2 -d=`

COUNTER=`r.info -r "points" | grep min | cut -f2 -d=`

while [ "$COUNTER" -le "$max" ]; do

       r.mapcalc "tttemp_1 = if(points == "$COUNTER",1,null())"

       r.buffer input=tttemp_1 output=tttemp_buffer distances=100 --o

# your query
# patch result with previous query
# redo it until all patches are analysed

       let "COUNTER=COUNTER+1"

done

hope this helps, Martin

--
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

Plant Conservation Unit
Department of Botany
University of Cape Town
Rondebosch 7701
South Africa

4

Rainer:

Re: Determining area of poligon vector layer around circle of points
in other vector layer

...

Thanks - that sounds interesting. So there is no way of
doing that without a loop?

maybe v.rast.stats, v.what.rast, or v.what.vect helps?

(+ v.buffer/r.buffer)

Hamish