[GRASS-user] dilation & erosion (mathematical morphology)

[Robert Nuske]

Hi there,

looking at some noisy raster map, I would like to clean up the patches (map 
consists only of one class and NULL) using basic mathematical morphology 
(dilation & erosion) in GRASS 7.0.


The dilation can easily be achieved with r.grow.
But the erosion seems not to be as straight forward.

Since the example about shrinking was removed from the help page of r.grow in 
GRASS 7.0 (http://grass.osgeo.org/grass70/manuals/r.grow.html), I hoped for an 
easier way to carry out the erosion. Looking in vain for a raster command 
named shrink or erode, I tried a negative distance in r.buffer (-> error) and 
negative radius in r.grow (treated as absolute value). 

The following produce identical output with grown patches.
  r.grow in=mod_10_33 out=mod_10_33_1 radius=1.1
  r.grow in=mod_10_33 out=mod_10_33_2 radius=-1.1


Any suggestions for a quick erosion?


thanks 
  Robert