[GRASS-user] "ladders" in watershed delineation

[Micha Silver]

Markus Metz wrote:

> These single-cell strings in basins, did you get them with SFD or MFD 
> mode? If MFD mode, what convergence factor did you use?
>
My watershed delineation was done with r.watershed, not r.terraflow, so 
IIUC it's SFD.  I didn't see any "braided" streams in the output.
I'm using GRASS 6.4 RC4 which, I believe contains your newer watershed 
speedup. However size of the region and resolution were such that I had 
to use the -m option.
Regards,
Micha
> 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> 
>>>>>
>>>>>          _______________________________________________
>>>>>    grass-user mailing list
>>>>>    grass-user at lists.osgeo.org <mailto:grass-user at lists.osgeo.org>
>>>>>    http://lists.osgeo.org/mailman/listinfo/grass-user
>>>>>
>>>>>
>>>>>
>>>>> This mail was received via Mail-SeCure System.
>>>>>
>>>>>       
>>>>     
>>>
>>> This mail was received via Mail-SeCure System.
>>>
>>>
>>>
>>>   
>>
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>   
>
> This mail was received via Mail-SeCure System.
>
>