[GRASS-user] sum one raster map conditional on another raster map

[Martin Wegmann]

Hello Stephen,

On Mittwoch, 17. März 2010 23:06:55 stephen sefick wrote:
> I would like to sum all of the positive values of one raster map
> (NDVI) based on a raster containing many watersheds that have values
> 1-22.  Is there a way to do this in GRASS?  Or a way to export this
[..]

Glynn already pointed you to r.statistics hence you only need to combine it 
with a few if-else statement in r.mapcalc:

r.mapcalc "watershed_new = if(watershed >=1 && watershed <=22,1,0)"
# if watersheds values are larger-equal 1 AND smaller-equal 22, assign value 1 
otherwise value 0

r.mapcalc "ndvi_new = if(ndvi_raster >=0,ndvi_raster,null())"
#all values above 0 keep their values, below 0 are converted to null() 
(nothing)

r.mapcalc "ndvi_new_clip = if(watershed_new == 1,ndvi_new,null())"
# if watershed_new is equal 1 (see above), then assign the values of ndvi_new, 
otherwise nothing

now you can perform the r.statistics method=sum analysis on NDVI values above 
0 only and for areas where watershed is above 1 and below 22.

hope this helps and I understood your question right, Martin

P.S.: in case you want to do the analysis on single watersheds (1-22) then you 
would need another statement like: 
for i in 1 2 3 4 5 ... 22 ; do 
r.mapcalc "new = if(watershed == $i,1,null())"
r.statistics ....
; done



-- 
Dr. Martin Wegmann

University of Wuerzburg
Institute of Geography
Department of Remote Sensing
Remote Sensing and Ecosystem Research
Am Hubland
97074 Wuerzburg, Germany
@
German Aerospace Center (DLR)
German Remote Sensing Data Center (DFD)

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