[R-sig-Geo] writeOGR unknown data type

[Wesley Roberts]

Dear r-sig-geo,

I am having trouble exporting an sp object created in R using writeOGR. Below is the code I am using, basically it creates a regular grid to cover a point data set opened using readShapePoints. I then convert the spatialpoints to polygons and then finally to a SpatialPolygonsDataFrame. I would now like to export this sp data set as a shapefile so that I can add some attributes to it. Unfortunately I receive the following error each time I try to run the export,

> poly<-readShapePoints("Poly_one_pts.shp", proj4string=CRS("+proj=tmerc +south +ellips=WGS84 +datum=WGS84 +lon_o=31 +k_0=1 +units=m +no_defs"))
> grd<-spsample(poly,cellsize=c(4,4),type="regular")
> grd<-as(as(as(grd,"SpatialPixels"),"SpatialPolygons"),"SpatialPolygonsDataFrame")
> tp<-tempdir()
> writeOGR(grd,tp,"grd",driver="ESRI Shapefile",verbose = FALSE)
Error in writeOGR(grd, tp, "grd", driver = "ESRI Shapefile", verbose = FALSE) : 
  NULL NULL unknown data type


An export of poly runs fine but it seems when I try to export grd it is an unknown data type. Both classes seem to be similar and writeOGR should be able to export both data sets. 

> class(poly)
[1] "SpatialPointsDataFrame"
attr(,"package")
[1] "sp"
> class(grd)
[1] "SpatialPolygonsDataFrame"
attr(,"package")
[1] "sp"


Any help on this issue would be greatly appreciated.
Many thanks and kind regards,
Wesley

Wesley Roberts MSc.
Researcher: Earth Observation
Natural Resources & the Environment (NRE)
CSIR
Tel: +27 (0)21 888-2490
Fax: +27 (0)21 888-2693
"To know the road ahead, ask those coming back."
- Chinese proverb



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