# [GRASS-user] how to calculate area of voronoi polygons considering the boundary of the catchment

Moritz Lennert mlennert at club.worldonline.be
Tue Jul 28 10:46:03 EDT 2009

```On 28/07/09 16:02, daljeet wrote:
> Thanks Moritz.
>
> I was wrongly using the term "region" in my question above. The requirement
> is slightly different. Let me explain you in detail.
>
> Inputs:
> 1. I have got the shapefile data of the country. I imported it using
> v.in.ogr and extracted (v.extract) the vector of the desired "State" using
> the "where" clause on the attribute which contains the name of the States.
> So, this means i have the vector of a particular State and its area is
> available using v.report.
>
> 2. I have also got the rainfall point data for the entire country in the
> ascii file (x,y,z) for one week (z being the amount of rainfall)
>
> Output required:
> The volume of rainfall in the particular State (which is extracted above).
>
> What I am doing:
> I wanted to apply both, the Thiessen polygon and the Isohyetal Method to
> calculate the volume  of rainfall.
>
> Till now I am only working on Thiessen, and this is what i have done:
> 1. Imported the ascii rainfall point data (v.in.ascii) in the vector. This
> is the vector for the whole country.
> 2. Created the voronoi vector on the above rainfall vector (v.voronoi)
> 3. Overlaid the voronoi rainfall vector of the country (from step 2) on the
> extracted vector of the State (from input 1 above) with AND operator.
>
> To verify, the sum of "area of each polygon" on the overlaid vector should
> be same as area of the extracted State vector.
>
> But this is not coming out to be same. The area of the voronoi overlaid map
> is smaller than the area of the extracted shape of State.  One observation,
> the voronoi overlaid map shows some white grids (both inside and at the
> boundary of the map) and I suspect if these are being taken into account for
> the area. Attached is the png image of the overlaid vectors.
>
> http://n2.nabble.com/file/n3342895/rainfall_theissen_png.png
>
> Any idea what could be my mistake. What I am doing which is making the loss
> of area in the case of polygons.

Sorry, cannot help you as such. Start by giving the exact command lines
used with each command. Also show the results of v.voronoi. And how did
you calculate the areas ?

Moritz
```