[GRASS-user] "ladders" in watershed delineation

[Micha Silver]

How can I avoid the problem of strings of single cells when creating 
basins with r.watershed? I think this is referred to as "ladders". 
Here's [1] an image showing what I mean.

In my example, the purple colored catchment has two "tails" of width 1 
cell. One tail separates between the light green and the pale blue 
catchments. The other (northern) tail splits the dark green catchment 
into two.

After running r.to.vect to get the catchment vectors, I'm left with the 
two "strings" or "ladders" of tiny vector areas. The southern string can 
be removed with v.clean tool=rmarea with no ill effects.

However when I remove those small areas in the northern "ladder" I'm 
left with the stream running *along the drainage divide* or even 
zigzagging across the divide, neither of which is correct.

Can this problem be avoided? I've tried with a couple of different dem 
sources, and at different resolutions and threshold values, but these 
ladder phenomena always seem to appear.

This example was done with the ASTER DEM data, using a threshold of 
11000 and resolution like the original data (1 arcsec ~= 30 m.)


Thanks,

Micha


[1] http://my.arava.co.il/~micha/ladders.html