[GRASS-user] "ladders" in watershed delineation

[Markus Metz]

These single-cell strings in basins, did you get them with SFD or MFD 
mode? If MFD mode, what convergence factor did you use?

Markus M


Micha Silver wrote:
>
> Milton Cezar Ribeiro wrote:
>
>> 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!
>>
>>   
> After writing my first (silly) response, I thought thru and realized 
> what you were suggesting.
> I did r.neighbors twice: once with the interspersion method, and a 
> second time with the mode method to create two new rasters.
> Then, exactly as you explained above, I set 68 as the maximum 
> interspersion value [6 different value cells in the 3X3 window gives 
> (6/9)*100+1=67%].
>
> Then the mapcalc expression:
> if(catch_inter<=68, catch, catch_mod)
> gave me the new catchment raster. I looked it over and *most* but not 
> all of the problematic strings are gone. So this seems like a very 
> reasonable way to improve the catchment raster from r.watershed.
>
> Thanks for putting me on the right track,
> Micha
>> 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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>>>>
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>>>>       
>>>     
>>
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>>
>>   
>
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