[GRASS-user] Rasterize polygons, multiple polygons per grid cell

[Lars Dalby]

Hi 

I have been desperately trying to figure out how to solve the task described
below in GRASS (via R using spgrass6). I have searched the list, but did not
find exactly what I was looking for.

I have a large polygon file, app. 250000 polygons. It is the GLWD-2 data set
available  here
<http://worldwildlife.org/publications/global-lakes-and-wetlands-database-small-lake-polygons-level-2>   
The map is in lat long. My goal is to end up with a raster layer in Behrmann
projection with app 96486.28m by 96486.28m grid cells showing the area per
grid cell covered by polygons in the GLWD-2 layer. 

So as I see it the task is to, for each grid cell, "clip" the underlying
polygon file, calculate the area and return that value in the resulting
raster. 

So far I have successfully imported the shape file and reprojected it to a
location with the desired projection and resolution. So far so good. 

Then I imported a raster with the desired grid cell size and dimensions. So
now I seem to have the layers I need in the same location.

Then I some how want to do the actual clipping and calculation of the area
of polygons in each cell. This is where I get stuck. Any suggestions to a
newbie?

Initially I thought I somehow could specify which function(e.g. sum, mean
etc) should be used in v.to.rast in the case of multiple polygons falling in
the same grid cell. But I could not find anything here. 

I'm running GRASS 7.0 on OSX btw.

Any help and suggestions would be highly appreciated!

Best regards,
Lars



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