[GRASS-user] elevation values of watershed basins

[temiz]

Dylan Beaudette wrote:
> On Thursday 01 February 2007 08:27, temiz wrote:
>   
>> Dylan Beaudette wrote:
>>     
>>> On Wednesday 31 January 2007 05:14, Markus Neteler wrote:
>>>       
>>>> temiz wrote on 01/31/2007 06:53 PM:
>>>>         
>>>>> hello
>>>>>
>>>>> Can you make a suggestion about how I can find the highest and lowest
>>>>> elevation points of all watershed basins
>>>>> as vector points ?
>>>>>
>>>>> I mean, I have watershed map as vector. I need it's table includes
>>>>> like "cat", "highest elevation value", "lowest elevation value"
>>>>>
>>>>> How can I do that ?
>>>>>           
>>>> If I understand your question correctly, you can do this easily with
>>>>
>>>> v.rast.stats
>>>> Description:
>>>>  Calculates univariate statistics from a GRASS raster map based on
>>>> vector objects
>>>>
>>>> The help page contains a similar example.
>>>>
>>>> cheers
>>>> Markus
>>>>         
>>> If you have problems with this approach (no reason to suspect problems),
>>> I have found that 'starspan' coupled with GRASS to be an excellent
>>> raster+vector reporting tool.
>>>
>>> main idea:
>>>
>>> start grass
>>> starspan --vector location/mapset/vector/yourvector \
>>> --raster location/mapset/cellhd/your_raster \
>>> --stats output.csv min max avg mode (...)
>>>
>>> Output is saved to a CSV file, not quite as nice as v.rast.stats saving
>>> results back to the attribute table though... The nice thing about this
>>> approach is that the raster and vector sources can be any GDAL-readable
>>> data type. For example, I use starspan to compute raster statistics
>>> within a given radius of a set of points. The raster files are all stored
>>> in GRASS, and the points are stored in PostGIS.
>>>
>>> Cheers,
>>>       
>> /*thank you
>> */
>>     
>
> Hi Ahmet,
>
>   
>> /*I have never met  */starspan so far. I am looking at its home page and
>> it will certain to be very useful.
>> I will work on it.
>>     
>
> Be sure to, it is a simple compile from source code if you have both GDAL and 
> GEOS installed with their include files.
>
>   
>> As I see, you are a soil scientist, so you will understand me better.
>>     
>
> :) possibly...
>
>   
>> The logic of my question is based on finding slopes' length as landslide
>> susceptibility parameter ( I accept slope is waterhed area).
>>     
>
> so: landslide susceptibility ~ f( slope length ) --> this is a simple 
> raster-based operation: calculate the upslope contributing area for each cell 
> of a DEM. Check the literature for ideas. 
>
> I am not entirely sure I understand what you are trying to do... Are you 
> trying to estimate a landslide potential for each watershed boundary polygon?
>   
     I am going to build the model using logistic regression ( in R - Stat).
      slope  length  is not only parameter.
     -- existing landslides are response variable
     -- rock type,slope angle, slope shape, aspect, slope length (** 
??**)  are the explatory variables.
     I have already constructed all maps except slope length.
     I am going to use watershed basin as slope unit in one model. So 
you are right, each watershed
     basin ( slope)  I will generate unique landslide potential value. 
something like you see, landslide is
     a slope movement so I have to say, this slope is unstable but 
adjacent slope is stable)
 
    Unfortunately, while r.terraflow generates very good and realistic 
watershed basin map, it does not generate
    half basin map. ( as far as I see r.watershed's basin is not as good 
as r.terraflow's basin)
>   
>> Firstly, I used PostGis's extent function and got width and length of
>> polygons (say watershed or  landslide as polygon). But this
>> polygon's longest dimension doesn't fit dimension in downslope
>> direction, which is the slope length I am looking for. So, if I found
>> max and min values of elevation of the polygons, I could calculate
>> slopes' length.
>>     
>
> This does not sound like a realistic solution, as the extent() function will 
> return a minimum bonding box -- which provides the maximum 'length' 
> and 'width' of some polygon, but not much useful information.
>
>   
>> /*v.rast.stats */gives max and min values of elevation of the polygons(
>> /*v.rast.stats*/ has  still been running for most 20 hours).
>> maybe from these values I generate vector points containing min & max
>> values .
>>     
>
> Do you have a number of polygons, and the specifications for your DEM: cell 
> size, region size? You may have hit some upper boundary for v.rast.stats. 
> Starpan seems to scale very well, so you may not run into this problem.
>
>   
>> I will appreciate if you supply your ideas and recommendations
>>
>> regards
>>
>>     
>
> Sure. With a clearer statement of the problem, and some information on your 
> source data it may be possible to come up with a workable solution.
>
> Cheers,
>
>   


-- 
Ahmet Temiz


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