[GRASS-user] Building topologically correct stream network

[stephen sefick]

re: inline

On Wed, Jul 22, 2009 at 2:01 AM, Markus
Metz<markus.metz.giswork at googlemail.com> wrote:
>
> stephen sefick wrote:
>>
>> Well, I am not sure what I am going to do.  I would look at the code,
>> but I am not adept at C.  Also, I don't know if I understand the
>> algorithm, but I think if the topology were complete then it would
>> work OK.  I think the 0 orders are unconnected streams- danglers.
>>
>
> Should not happen unless you specified real depressions in r.watershed. How
> did you extract streams?

flow accumulation from r.watershed -> multiplied by gridcell area and
multiplied by average runoff for that hydrologic unit code in map
calc-> r.thin -> r.to.vect

>>
>> This makes sense because they are neither a tributary nor a main stem.
>>  The topology is the problem,
>
> What exactly is wrong with topology? The existence of these dangles?

Maybe there is nothing wrong.  On further reflection the area that I
am trying to use this procedure on is very flat and there are
intermittent streams and wetlands (site visit) all over the study
area.  These non-connected lines maybe showing intermittentancy?

>
> Markus M
>>
>> and I can't figure out how to correct
>> this.
>> thanks
>>
>> Stephen Sefick
>>
>> On Tue, Jul 21, 2009 at 12:34 PM, Markus
>> Metz<markus.metz.giswork at googlemail.com> wrote:
>>
>>>
>>> Margherita Di Leo wrote:
>>>
>>>>
>>>> I'm sorry but the procedure that I proposed does not produce good
>>>> results.
>>>> In fact the ascii file produced by v.strahler has very strange orders,
>>>> and
>>>> also many 0 (that does not have sense in strahler order!). I think the
>>>> problem is generated in finding the outlets
>>>>
>>>
>>> Outlet points are where stream is not null and drainage direction is
>>> negative, e.g.
>>>
>>> r.mapcalc "outlet_points = if(!isnull(streams) && drainage < 0, streams,
>>> null())"
>>>
>>> That would give you outlet points whose cell value is the stream segment
>>> number leading to the outlet point.
>>>
>>>
>>>>
>>>> and/or confluences.
>>>>
>>>
>>> All nodes of the stream vector, excluding outlet points?
>>>
>>> Markus M
>>>
>>>
>>>>
>>>> Waiting for the response of the List (please)..
>>>>
>>>> margherita
>>>>
>>>>
>>>> Jarek Jasiewicz wrote:
>>>>
>>>>>
>>>>> Hi
>>>>>
>>>>> maybe this will help:
>>>>>
>>>>> http://gis.vsb.cz/GIS_Ostrava/GIS_Ova_2009/sbornik/Lists/Papers/060.pdf
>>>>>
>>>>> it require another gis program but gives great results!
>>>>>
>>>>> Jarek
>>>>>
>>>>>
>>>>> Margherita Di Leo pisze:
>>>>>
>>>>>>
>>>>>> Hi Stephen,
>>>>>> I actually am fighting against almost the same problem, so let's do it
>>>>>> together :)
>>>>>> I found that r.strahler.sh works well to delineate streams and correct
>>>>>> topology, (I use a treshold of 1km^2, it works well but don't ask me
>>>>>> why :)
>>>>>> ), but (in my little experience) does not work to order streams, so I
>>>>>> must
>>>>>> run v.strahler after r.strahler.sh.
>>>>>> I know it's a very small hint but hope this helps.
>>>>>>
>>>>>> Best
>>>>>> margherita
>>>>>>
>>>>>>
>>>>>> I have a low relief area in the costal plain of georgia that I am
>>>>>> trying
>>>>>> to produce a topologically correct stream network. There are many
>>>>>> breaks in
>>>>>> the flow accumulation grid and I would like to delineate the streams
>>>>>> and
>>>>>> "correct" the topology after r.thin and r.to.vect have been preformed
>>>>>> so
>>>>>> that I can then use v.strahler to order the streams. Thanks for any
>>>>>> help and
>>>>>> I will provide and data that would be helpful. thanks,
>>>>>>
>>>>>>
>>>>>> -- Stephen Sefick Let's not spend our time and resources thinking
>>>>>> about
>>>>>> things that are so little or so large that all they really do for us
>>>>>> is puff
>>>>>> us up and make us feel like gods. We are mammals, and have not
>>>>>> exhausted the
>>>>>> annoying little problems of being mammals. -K. Mullis
>>>>>>
>>>>>>
>>>>
>>>>
>>>
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>>>
>>>
>>
>>
>>
>>
>



-- 
Stephen Sefick

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods.  We are mammals, and have not exhausted the
annoying little problems of being mammals.

								-K. Mullis