[GRASS-user] "ladders" in watershed delineation

[Jarek Jasiewicz]

Micha Silver pisze:
> 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
>
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you cannot in this case.
jarek