[GRASS-user] [GRASSLIST:1141] Re: Qn. on extracting from multiple rasters, data corresponding to vector polygons

[Hamish]

Vishal Mehta wrote:
> I have a vector polygon shapefile which contains 8 polygons (each
> watershed is a polygon). I also have some 100 GRASS raster files of
> environmental data (rainfall, temperature etc).
>
> Whats the best way to extract for each watershed, the corresponding
> average data from each raster file and put it in a text file?
>
> I've looked at v.what.rast but it seems to be quite cumbersome for
> this application. I'm using Grass 6.0.1 on Ubuntu linux.


I think this can be easily done.

why not just

g.region rast=biggest_map
v.to.rast in=vectmap out=watersheds use=cat # or attr if ID column exists
r.mapcalc "MASK=if(watersheds == 1, 1, null() )"
r.univar rainfall




putting that in a loop:

g.region rast=typical_map
v.to.rast in=vectmap out=watersheds use=cat

r.info -r watersheds
min=1
max=8
eval `r.info -r watersheds`

#if IDs are not sequential use
# for WATERSHED in `r.stats -qn watersheds` ; do


# this is untested, but the general method works

echo "watershed|map|average_val" > results.txt

for WATERSHED in `seq $min $max` ; do
  g.remove MASK
  unset MEAN

  echo "Watershed: $WATERSHED   Map: $MAP"
  r.mapcalc "MASK=if(watersheds == $WATERSHED, 1, null() )"

  for MAP in `g.mlist type=rast pat=* fs=space` ; do
     #g.region rast=$MAP  # adjust to match res of each map
     MEAN=`r.univar -g $MAP | grep '^mean=' | cut -f2 -d=`
     echo "$WATERSHED|$MAP|$MEAN" >> results.txt 
  done

done
g.remove MASK



if maps are all different sizes and resolutions do the v.to.rast
step at the finest resolution to avoid aliasing problems.


good luck,
Hamish