terra (version 1.4-22)

spatSample: Take a regular sample

Description

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 type="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.

Usage

# S4 method for 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)

# S4 method for SpatVector
spatSample(x, size, method="random", strata=NULL, chess="")
# S4 method for SpatExtent
spatSample(x, size, method="random", lonlat, as.points=FALSE)

Arguments

x

SpatRaster

size

numeric. The sample size. If x is a SpatVector, you can also provide a vector of the same length as x in wich case sampling is done seperately 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)

method

character. Should be "regular" or "random"

replace

logical. If TRUE, sampling is with replacement (if method="random"

na.rm

logical. If TRUE, codeNAs 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 cell values are returned

cells

logical. If TRUE, cell numbers are returned

xy

logical. If TRUE, cell coordinates are returned

ext

SpatExtent or NULL to restrict sampling to a a subset of the area of x

warn

logical. Give a warning if the sample size returned is smaller than requested

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

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

Value

numeric or SpatRaster

Examples

# NOT RUN {
f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)
s <- spatSample(r, 10, as.raster=TRUE)
spatSample(r, 10)
spatSample(r, 10, "random")

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

v <- r[cells]
xy <- xyFromCell(r, cells)
cbind(xy, v)

# regular 
cells <- spatSample(r, 6, "regular", cells=TRUE, values=FALSE)
cells <- as.vector(cells)
v <- r[cells]
xy <- xyFromCell(r, cells)
cbind(xy, v)

## SpatExtent 
e <- ext(r)
spatSample(e, 10, "random", lonlat=TRUE)

## SpatVector
f <- system.file("ex/lux.shp", package="terra")
v <- vect(f)
#sample geometries 
i <- sample(nrow(v), 5)
vv <- v[i,]

# }