Hello,
I had the same problem in the past and so I developped a function that
allow to convert directly a raster to a SpatialPolygonsDataFrame.
The function ('polygonize') is available in a small package I wrote :
http://sourceforge.net/projects/geoconv/
The package is written in French (sorry about that), so if the
documentation is not understandable, ask me.
I hope this can help.
Vlad
----
Vladislav Navel
Ingénieur d'études
Atmo Poitou-Charentes
05 46 44 83 88
Le 14/10/2011 07:24, Mathieu Rajerison a écrit :
Hi,
And will contiguous raster cells have the same value, as Arnaud expected?
2011/10/13 Rolf Turner<r.tur...@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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