On Wed, 13 May 2009 22:21:51 -0700 (PDT), Hamish <hamis...@yahoo.com> wrote:
> Seb wrote: >> If one were to calculate an average of several rasters, one could >> simply do: >> r.mapcalc "ave = (A + B + C) / 3" >> But how can we get around the problem of null values in any of the >> rasters, which would propagate it to the result? What is an >> efficient way to calculate both the numerator and denominator for >> each pixel so that it corresponds only to rasters with non-null >> values. > be careful with those sorts of tricks because different regions of the > result will end up with different levels of statistical strength and > so the result may be misleading. >> A second problem is how to script this (shell) so that a large number >> of rasters can be included in this calculation. I would appreciate >> some pointers to some scripts where something along these lines can >> be read, or any thoughts about alternative/better approaches. > use r.series method=average. The -n flag will Propagate NULLs. Thanks Hamish, that's exactly the tool for this job. Basically, r.series without the -n flag would do what I described, although the point about uneven representation of cells across rasters is something to worry about. -- Seb _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
