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

daljeet sachdev_dj at yahoo.com
Tue Jul 28 10:02:11 EDT 2009

```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.

Thanks
daljeet

Moritz Lennert wrote:
>
> On 27/07/09 21:38, daljeet wrote:
>> Hi,
>>
>> I have the point rainfall data of a region. Few of the point data are
>> outside the region of interest.
>>
>> I am making the voronoi polygons using the v.voronoi on the vector
>> imported
>> from this point ascii (x,y,z) data.
>>
>> The polygon may be fully inside the region of interest or may be
>> partially
>> outside the region of interest.
>>
>> In case of the polygon which is partially outside the region of interest,
>> the area which is inside the region of interest need to be calculated.
>> How
>> to calculate this?
>
> - Define region of interest with g.region.
> - Create polygon of region with v.in.region.
> - Overlay voronoi polygons with region polygon with v.overlay operator=AND
>
>> How can i calculate the area of voronoi polygon.
>
> v.report or v.to.db
>
> Moritz
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> grass-user at lists.osgeo.org
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>
>

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