[GRASS-user] Presence/Absence error

[Joel Pitt]

Hi all,

I have a raster map that indicates the predicted area of where a
species exists, plus a vector map of points where the species has
actually been recorded.

I'm trying to work out a simple method for working out the number of
correctly predicted occurences (where the vector points overlap with
non-null cells in the raster), the number of false positives (where a
non-null raster cell exists but no vector point), the number of false
negatives (where the point exists but raster cell is null), and the
number of correct negatives (the raster cell is null and no point
exists).

If possible I would like to get a list of the raster values in each of
the above categories, since the map is one of probabilities (So in the
above discussion the "null" raster cells have probability values but I
use mapcalc to cut-off values less then a given threshold.

Any help would be appreciated. I'm trying to look for a pretty
efficient method since this needs to be done thousands of times - but
any suggestion would be helpful.

Thanks!

-- 
-Joel

"Wish not to seem, but to be, the best."
                -- Aeschylus