[GRASS-user] buffer vector within a given area

[Martin Schweizer]

Annalisa
Thanks for your help, but there is a little problem: I calculate the
agricultural land around a city, then i buffer the vector, transform it
into a raster map and subtract all the "not possible land". At the end,
there is less agricultural land than I calculated. There must be a
buffering operation which excludes already the NULL values of the "agri
map" in the calculation.... is there something like that?

Thanks a lot

Martin

Annalisa Minelli schrieb:
> Martin,
> after you have done the buffering operation (as Moritz said), and
> transformed vector map (buffer_areas) in raster (v.to <http://v.to>
> rast), you can easily extract only the interested areas by using a
> mapalgebra operation.. for example:
>
> if "agri" is the map of your agricultural areas (green map)
> and "buff" is the map of the buffer around the vector points
>
> you can type:
>
> r.mapcalc "interested_areas = agri * buff"
>
> all cells with a not-zero value are the interested areas..
> (I usually prefer to use NULL rather than ZERO.. if you prefer it too,
> you can use r.null to transform ZERO to NULL values)
>
> Hope this helps!
>
> Annalisa
>
>
>
> 2009/4/23 Martin Schweizer <schweizermartin at students.unibe.ch
> <mailto:schweizermartin at students.unibe.ch>>
>
>     Hi Moritz
>
>     Thanks for your help. The problem is, that not each pixel around a
>     point
>     (city) can be used as agricultural land. So I can't simply buffer the
>     cities with a circle. To concretize what i mean, i made a picture:
>     http://www.fotos-hochladen.net/view/landandcity1hjbk4qy3.jpg in green
>     you see the potential agricultural land and the black point is a
>     city....
>
>     Thanks a lot
>
>     Martin
>
>     Moritz Lennert schrieb:
>     > On 22/04/09 20:39, Martin Schweizer wrote:
>     >> Hi
>     >>
>     >> I have a question which is a bit complex. Hope i can describe
>     it ;-).
>     >>
>     >> I have a point-vector layer (the citis) and a raster layer with
>     1 and
>     >> ZERO cells (The potenial agricultural land). One categorie in
>     the vector
>     >> layer is the land (Squaremeter) that hast to be cultivated for each
>     >> point (city). The question is now: How can I calculate and display,
>     >> which cells of the raster map (potential agricultural land)
>     hast to be
>     >> cultivated? Must be some sort of buffer or neighbourhood.....?
>     >
>     > Assuming that this agricultural land is the same in all
>     directions of
>     > the city, create a column which contains the radius of the
>     circle that
>     > encompasses the land needed (i.e. if my memories of geometry are
>     > correct: sqrt(surface/pi) and feed the name of this column to the
>     > bufcol= parameter of v.buffer (or feed the formula directly to the
>     > bufcol parameter, but this only works if you use a db-backend which
>     > has a sqrt function, i.e. PostgreSQL).
>     >
>     > Moritz
>     >
>
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