Hi,

And will contiguous raster cells have the same value, as Arnaud expected?

2011/10/13 Rolf Turner <r.turner@auckland.ac.nz>

> On 14/10/11 09:01, Arnaud Mosnier wrote:
>
>> Dear all,
>>
>> In searching a solution from the internet, I found someone who had the
>> same
>> problem as me but ... who had no answers.
>> I hope I will have better chance!  :-)
>>
>> Here is her explanation:
>> http://r-sig-geo.2731867.n2.**nabble.com/Converting-raster-**
>> to-polygons-with-just-one-**polygon-for-cells-with-common-**
>> value-td6065464.html<http://r-sig-geo.2731867.n2.nabble.com/Converting-raster-to-polygons-with-just-one-polygon-for-cells-with-common-value-td6065464.html>
>>
>> In the same way, I want to convert a raster with several values to
>> SpatialPolygons.
>> As in the previous message, I used the function "rasterToPolygons"
>> (package
>> raster) ... but it is really slow and memory hungry.
>> Moreover, I want to have polygons that include contiguous raster cells
>> with
>> the same value, not one polygon by raster cell.
>> The function unionSpatialPolygons can do that.
>>
>> Is there another (more efficient) way to do this in R ?
>>
>
> If you can convert your "raster" to a spatstat object of class "owin",
> (and type "mask" of course) then the spatstat function "as.polygonal" will
> convert it to an owin object of type "polygonal".  Which you will then
> want, I presume, to convert to an object of class "SpatialPolygons".
>
> The latter can be done easily via:
>
> require(maptools)
> Z <- as(Y,"SpatialPolygons")
>
> where Y is the output of as.polygonal().
>
> I don't know about the raster --> owin conversion though.  Others may
> be able to tell you.
>
> Note that you will need to have "gpclib" enabled to use as.polygonal()
> and hence will need to be operating under the terms allowed by the
> gpclib license.
>
> HTH.
>
>    cheers,
>
>        Rolf Turner
>
>
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