sampleStratified: Stratified random sample

Description

Take a stratified random sample from the cell values of a Raster* object (without replacement). An attempt is made to sample size cells from each stratum. The values in the RasterLayer x are rounded to integers; with each value representing a stratum.

Usage

# S4 method for RasterLayer
sampleStratified(x, size, exp=10, na.rm=TRUE, xy=FALSE, ext=NULL, sp=FALSE, ...)

Value

matrix of cell numbers (and optionally coordinates) by stratum

Arguments

x: Raster* object, with values (rounded to integers) representing strata
size: positive integer giving the number of items to choose
exp: numeric >= 1. 'Expansion factor' that is multiplied with size to get an intial sample. Can be increased when you get an insufficient number of samples for small strata
na.rm: logical. If TRUE (the default), NA values are removed from random sample
xy: logical. Return coordinates of cells rather than cell numbers
ext: Extent object. To limit regular sampling to the area within the extent
sp: logical. If TRUE, a SpatialPointsDataFrame is returned
...: Additional arguments. None implemented

Details

The function may not work well when the size (number of cells) of some strata is relatively small.

Examples

Run this code

 r <- raster(ncol=10, nrow=10)
 names(r) <- 'stratum'
 values(r) <- round((runif(ncell(r))+0.5)*3)
 sampleStratified(r, size=3)