[GRASS-user] "ladders" in watershed delineation

[Milton Cezar Ribeiro]

Hi Micha,

May be with *r.neighbors *combined with *interspersion*  method you can
solve this.

good luck

milton
brazil=toronto

2009/8/2 Micha Silver <micha at arava.co.il>

> 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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