[Qgis-user] Re: [GRASS-user] Perimeter of lakes

[Micha Silver]

On 12/04/2010 11:37 AM, Emma wrote:
> Dear all,
>
> Is there a way to calculate from the DEM the total perimeter of all 
> lakes (+reservoirs, etc.)? In other words, I need to calculate the 
> total length of lake banks and so far could only determine the total 
> lakes area, which is of no use in this case.
>

 From a DEM directly? No, not that I know. How would you determine what 
is a lake, and what is a flat land area? How did you get the area??  
Furthermore, sometimes lakes have a slight slope...
Maybe have a look at r.param.scale:
r.param.scale in=dem out=terrain param=feature s_tol=0.01
(or some other very small value)
This creates terrain features: peaks, channels, saddles and flat areas. 
The s_tol (slope tolerance) sets how flat an area needs to be before 
it's classified as flat.

Once you have devised a method to identify the lakes, then convert those 
raster areas to a vector, and the v.to.db module will give you the 
perimeter.
It would go something like:

v.db.addcol lakes col="perim double percision"
v.to.db lakes opt=perimeter col=perim units=hectares


> Thanks a lot for your help!
>
> Sincerely yours,
> Emma
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-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il