[GRASS-user] r.basin ? for plane geometry of polygon shapefiles

[Marco]

Hi Margherita,

Grazie. Its not related to hydrology. I just want to get some shape indexes to characterize networks of polygons (periglacial patterned ground).

I came to your Grass Add-on because the indexes used in 2D basin morphometry are really adequate to characterize each individual polygon.


I have already used SAGA GIS and GVSig, which automatically supply shape parameters relating the perimeter of the polygon to its area in various ways. And that´s already fine information. 

From that I know if the polygon is more elongated or more circular. And maybe more information won´t add to much... But I still would like to try. Mainly because of the indexes which grab the axis and  which relate the polygon to and ideal shape. 

Maybe I should focus on doing one thing at a time: 1) axis calculation; and 2) deriving the ideal shape from the centroid of the polygon.Then I´d have great information. 


My next step is to go inside you python code (of which I know little) and try to extract the relevant pieces. 

Thank you.
Marco

(gonna start a MSc related to cryosphere geomorphology next Fall)





________________________________
 From: Margherita Di Leo <diregola at gmail.com>
To: Marco Jorge <marco_gjv at ymail.com> 
Cc: "grass-user at lists.osgeo.org" <grass-user at lists.osgeo.org> 
Sent: Thursday, June 7, 2012 11:26 AM
Subject: Re: [GRASS-user] r.basin ? for plane geometry of polygon shapefiles
 

Hi Marco,


On Tue, Jun 5, 2012 at 4:54 PM, Marco Jorge <marco_gjv at ymail.com> wrote:

Hi,
>
>
>I have 10k small polygons stored in a shapefile and in raster format (nr classes = nr of basins).   
>I am interested in getting some planar geometry information for each polygon.
I have 3 problems, not considering the lack of knowledge: 
>
>
>- no DTM; 
>- no channels; 
>- try to foul r.basin with the shapefile
>
>
>Can you think of a way to pick the module and its code to calculate the indices in the end of the post? 
>Can anyone point me a way to pick up the major axis instead of the lenght of the main channel ? 
>Any hint on major (or minor) axis extraction?
>
>
>Should I try other approach? 

To me, it's not clear what you are trying to get. If your aim is to perform a hydrological analysis, you do need a DEM, and you can easily download i.e. an ASTER GDEM for your area and then use r.basin. If your analysis it's not related to hydrology, and you just need some "shape indexes", what you should do is probably related to what you are trying to achieve..  Can you explain a little more?

Regards,
madi

-- 
Dr. Margherita Di Leo
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