sampleStratified function

Stratified random sample

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.

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

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.

Returns

matrix of cell numbers (and optionally coordinates) by stratum

See Also

sampleRandom, sampleRegular

Examples

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