[R-sig-Geo] krige problem: IDW + irregular blocks

[Mauricio Zambrano]

Dear List,

I need to carry out an IDW interpolation of precipitation values
within a basin. I want to obtain the mean value in each one of the
subcatchments, which are defined as polygons in a shapefile.

I would like to know:

How can I transform the subcatchments polygons (class
"SpatialPolygonsDataFrame" ) into a "data.frame" that can be used by
the "block" parameter of the "krige" function ?

So far, I've tried:

1) SubCatchments.shp <- readShapePoly(SubCatchmentsShp.fname, proj4string=p4s)
2) centroids <- coordinates(SubCatchments.shp)
3) pp.idw.block <- krige(value~1, locations=centroids, block=SubCatchments.shp)

Steps 1 and 2 are ok, but in step 3 I get the error:

Error en function (classes, fdef, mtable)  :
  unable to find an inherited method for function "krige", for
signature "formula", "matrix"

because "SubCatchments.shp" class is "SpatialPolygonsDataFrame' and
not 'data.frame'


Any help will be appreciated.

Thanks in advance,

Mauricio
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