sample function

Take a regular sample

Take a spatial sample from a SpatRaster, SpatVector or SpatExtent. Sampling a SpatVector or SpatExtent always returns a SpatVector of points.

With a SpatRaster, you can get cell values, cell numbers (cells=TRUE), coordinates (xy=TRUE) or (when method="regular" and as.raster=TRUE) get a new SpatRaster with the same extent, but fewer cells.

In order to assure regularity when requesting a regular sample, the number of cells or points returned may not be exactly the same as the size requested. methods

## S4 method for signature 'SpatRaster'
spatSample(x, size, method="random", replace=FALSE, na.rm=FALSE, 
    as.raster=FALSE, as.df=TRUE, as.points=FALSE, values=TRUE, cells=FALSE, 
    xy=FALSE, ext=NULL, warn=TRUE, weights=NULL, exp=5, exhaustive=FALSE, 
        exact=FALSE, each=TRUE)

## S4 method for signature 'SpatVector'
spatSample(x, size, method="random", strata=NULL, chess="")

## S4 method for signature 'SpatExtent'
spatSample(x, size, method="random", lonlat, as.points=FALSE, exact=FALSE)

Arguments

• x: SpatRaster, SpatVector or SpatExtent
• size: numeric. The sample size. If x is a SpatVector, you can also provide a vector of the same length as x in which case sampling is done separately for each geometry. If x is a SpatRaster, and you are using method="regular" you can specify the size as two numbers (number of rows and columns). Note that when using method="stratified", the sample size is returned for each stratum
• method: character. Should be "regular" or "random", If x is a SpatRaster, it can also be "stratified" (each value in x is a stratum) or "weights" (each value in x is a probability weight)
• replace: logical. If TRUE, sampling is with replacement (if method="random")
• na.rm: logical. If TRUE, NAs are removed. Only used with random sampling of cell values. That is with method="random", as.raster=FALSE, cells=FALSE
• as.raster: logical. If TRUE, a SpatRaster is returned
• as.df: logical. If TRUE, a data.frame is returned instead of a matrix
• as.points: logical. If TRUE, a SpatVector of points is returned
• values: logical. If TRUE raster cell values are returned
• cells: logical. If TRUE, cell numbers are returned. If method="stratified" this is always set to TRUE if xy=FALSE
• xy: logical. If TRUE, cell coordinates are returned
• ext: SpatExtent or NULL to restrict sampling to a subset of the area of x
• warn: logical. Give a warning if the sample size returned is smaller than requested
• weights: SpatRaster. Used to provide weights when method="stratified"
• lonlat: logical. If TRUE, sampling of a SpatExtent is weighted by cos(latitude). For SpatRaster and SpatVector this done based on the crs, but it is ignored if as.raster=TRUE
• exp: numeric >= 1. "Expansion factor" that is multiplied with size to get an initial sample used for stratified samples and random samples with na.rm=TRUE to try to get at least size samples
• exhaustive: logical. If TRUE and (method=="random" and na.rm=TRUE) or method=="stratified", all cells that are not NA are determined and a sample is taken from these cells. This is useful when you are dealing with a very large raster that is sparse (most cells are NA). Otherwise, the default approach may not find enough samples. This should not be used in other cases, especially not with large rasters that mostly have values
• exact: logical. If TRUE and method=="regular", the sample returned is exactly size, perhaps at the expense of some regularity. Otherwise you get at least size many samples. Ignored for lon/lat rasters
• each: logical. If TRUE and method=="stratified", the sample returned is size for each stratum. Otherwise size is the total sample size
• strata: if not NULL, stratified random sampling is done, taking size samples from each stratum. If x has polygon geometry, strata must be a field name (or index) in x. If x has point geometry, strata can be a SpatVector of polygons or a SpatRaster
• chess: character. One of "", "white", or "black". For stratified sampling if strata is a SpatRaster. If not "", samples are only taken from alternate cells, organized like the "white" or "black" fields on a chessboard

Returns

numeric matrix, data.frame, SpatRaster or SpatVector

Examples

f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)
s <- spatSample(r, 10, as.raster=TRUE)
spatSample(r, 5)
spatSample(r, 5, na.rm=TRUE)
spatSample(r, 5, "regular")

## if you require cell numbers and/or coordinates
size <- 6
spatSample(r, 6, "random", cells=TRUE, xy=TRUE, values=FALSE)

# regular, with values 
spatSample(r, 6, "regular", cells=TRUE, xy=TRUE)

# stratified
rr <- rast(ncol=10, nrow=10, names="stratum")
set.seed(1)
values(rr) <- round(runif(ncell(rr), 1, 3))
spatSample(rr, 2, "stratified", xy=TRUE)

s <- spatSample(rr, 5, "stratified", as.points=TRUE, each=FALSE)
plot(rr, plg=list(title="raster"))
plot(s, 1, add=TRUE, plg=list(x=185, y=1, title="points"), col=rainbow(5))
 
## SpatExtent 
e <- ext(r)
spatSample(e, 10, "random", lonlat=TRUE)

## SpatVector
f <- system.file("ex/lux.shp", package="terra")
v <- vect(f)

# sample the geometries 
i <- sample(v, 3)

# sample points in geometries
p <- spatSample(v, 3)