On Fri, 14 Aug 2009, Wesley Roberts wrote:
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")
Coersion can be fun, but don't overdo it without checking. Thr output
object is a SpatialPolygonsDataFrame object, but has an empty data frame
in the data slot (actually, this coersion needs blocking, or changing to
prevent the problem):
library(rgdal)
scot_BNG <- readOGR(system.file("vectors", package = "rgdal")[1],
"scot_BNG")
grd<-spsample(scot_BNG, cellsize=c(40000, 40000), type="regular")
grd1 <- as(grd,"SpatialPixels")
grd2 <- as(grd1, "SpatialPolygons")
grd3 <- as(grd2, "SpatialPolygonsDataFrame")
str(grd3, max.level=2)
Try:
df <- data.frame(FID=row.names(grd2), row.names=row.names(grd2))
grd3 <- SpatialPolygonsDataFrame(grd2, data=df)
str(grd3, max.level=2)
Hope this helps,
Roger
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
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: roger.biv...@nhh.no
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