[GRASS-user] Flow line [r.flow]

[Huidae Cho]

Valter,

The default value for the skip= parameter is max(1, nrows/50, ncols/50), so
the density of flowlines depends on the resolution of your elevation map.
If you want better defined flowlines, reduce skip (skip=1 for all cells and
best, but can be slow). I believe, in your case, those scattered flowlines
all start from cells that are "skip" cells apart. I can see those short
flowlines in a grid-like manner (distance between starting cells = max(1,
nrows/50, ncols/50) by default). So try skip=1 (in UI, Optional => Number
of cells between flowlines), then you should get better and continuous
flowlines. The better result in one of your emails is a smaller region and
a less number of rows and cells, so a smaller skip, I think.

Best,
Huidae

On Fri, Jan 31, 2020 at 9:00 AM Valter Albino <valteralbino at gmail.com>
wrote:

> Hi,
>
> It's done, running from GRASS GIS 7.8.2 GUI, in a Windows 64bits OS.
> There ares differences when simulating for small areas or bigger areas,
> with default options.
> When testing for bigger areas I wasn't setting the number of cells
> bettween flowlines & maximum number of segments per flowline.
> Thanks
>
> Cumprimentos,
> *Valter Albino -* Geógrafo Físico, M.Sc.
> Modelação H&H / Riscos ambientais / OT&U
> www.valteralbino.wixsite.com/hydrodynamics
>
>
> Valter Albino <valteralbino at gmail.com> escreveu no dia quinta, 30/01/2020
> à(s) 09:55:
>
>> Hi Huidae
>> Yes, the purple and black lines flow lines. Wondering if using units in
>> "cm", instead of "m" gives correct results (the online manual sais that, i
>> suppose) in terms of: more number and length of flow lines.
>> Using r.flow (https://grass.osgeo.org/grass78/manuals/r.flow.html) with
>> the following raster file:
>> [image: image.png]
>> Sorry to moderator (sent 2 files with extra weight)
>>
>> Cumprimentos,
>> *Valter Albino -* Geógrafo Físico, M.Sc.
>> Modelação H&H / Riscos ambientais / OT&U
>> www.valteralbino.wixsite.com/hydrodynamics
>>
>>
>> Huidae Cho <grass4u at gmail.com> escreveu no dia terça, 28/01/2020 à(s)
>> 02:56:
>>
>>> Valter,
>>>
>>> Which module did you use to generate flow lines? It wouldn't be easy to
>>> troubleshoot your problem without sample data and detail steps. Are those
>>> purple and black lines flow lines?
>>>
>>> Best,
>>> Huidae
>>>
>>>
>>>
>>> On Wed, Jan 15, 2020 at 3:58 PM Valter Albino <valteralbino at gmail.com>
>>> wrote:
>>>
>>>> Is it normal results, scattered flow lines in a landscape with
>>>> altimetric amplitude of 50 to 60 meters (see following image). Is it an
>>>> erratic result?
>>>>
>>>>
>>>> [image: imagem.gif]
>>>>
>>>> Cumprimentos,
>>>> *Valter Albino -* Geógrafo Físico, M.Sc.
>>>> Modelação H&H / Riscos ambientais / OT&U
>>>> www.valteralbino.wixsite.com/hydrodynamics
>>>> _______________________________________________
>>>> grass-user mailing list
>>>> grass-user at lists.osgeo.org
>>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>>
>>>
>>>
>>> --
>>> Huidae Cho, Ph.D., GISP
>>> GRASS GIS Developer
>>> https://idea.isnew.info
>>>
>>

-- 
Huidae Cho, Ph.D., GISP
GRASS GIS Developer
https://idea.isnew.info
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