Subset and sort a SpatialPolygonsDataFrame object by one variable

Question

I took a sample of a spatial polygon dataframe with 2256 elements using:

valdata <- ref_10[sample(ref_10@data$ID, length(ref_10)/3),]

now I would like to sort my new spatial dataframe by my ID-colum and always get this error message:

valdata[sort(valdata@data$ID),]

Error in valdata[sort(valdata@data$ID), ] : cannot get a slot ("Polygons") from an object of type "NULL"

Answer 1

Try

valdata<-ref_10[which(ref_10@data$ID %in% sample(ref_10@data$ID, length(ref_10)/3)),]