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

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 ?

Arnaud

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