[GRASS-user] "ladders" in watershed delineation

[Milton Cezar Ribeiro]

Hi Micha,

Make a test. If you run a 3x3 or 5x5 filter and get the percentage, and
after check the values for those isolated pixels, you perceive that when
pixels are isolated the percentage values are very different from its
neighbours. I forgot to give the full suggestion:

1. run r.neighbors with interspersion
2. check the values for isolated pixels and define a threshold
3. run r.neighbors with majority
4. run r.mapcalc "newmap=if(map_inter< XXX, map_majority, map)"
    (I dont remember if is map_inter<   or map_inter> - check it).

Try this!

cheers

milton




2009/8/3 Micha Silver <micha at arava.co.il>

> Milton Cezar Ribeiro wrote:
>
> Hi Micha,
>>  May be with *r.neighbors *combined with /interspersion/  method you can
>> solve this.
>>
>>
> Hi Milton
> Thanks for your help.
> If my reading of the manual is correct, the "interspersion" option gives
> each cell the percentage of different cells surrounding it. I'm not clear
> how this will help with the string of single cells.
> But maybe running r.neighbors on the catchments raster with the default
> "average" option will get rid of those strings...
> I'll give it a try.
> Best regards,
> Micha
>
>  good luck
>>  milton
>> brazil=toronto
>>
>> 2009/8/2 Micha Silver <micha at arava.co.il <mailto: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
>>    <http://my.arava.co.il/%7Emicha/ladders.html<http://my.arava.co.il/~micha/ladders.html>
>> >
>>
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