R: Stratified random sample

sampleStratified {raster} | R Documentation

## 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 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.

### Value

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)

[Package raster version 3.3-7 Index]