Generate random points that can be used to extract background values ("random-absence"). The points are sampled (without replacement) from the cells that are not 'NA' in raster 'mask'.
If the coordinate reference system (of mask) is longitude/latitude, sampling is weighted by the size of the cells. That is, because cells close to the equator are larger than cells closer to the poles, equatorial cells have a higher probability of being selected.
randomPoints(mask, n, p, ext=NULL, extf=1.1, excludep=TRUE, prob=FALSE,
cellnumbers=FALSE, tryf=3, warn=2, lonlatCorrection=TRUE)matrix with coordinates, or, if cellnumbers=TRUE, a vector with cell numbers.
Raster* object. If the object has cell values, cells with NA are excluded (of the first layer of the object if there are multiple layers)
integer. Number of points
Presence points (if provided, random points won't be in the same cells (as defined by mask)
Extent object. Can be used to restrict sampling to a spatial extent
numeric. Multiplyer to adjust the size of extent 'ext'. The default increases of 1.1 increases the extent a little (5% at each side of the extent)
logical. If TRUE, presence points are exluded from background
logical. If TRUE the values in mask are interpreted as probability weights (and the values should be positive numbers (or NA)). NOTE: this currently only works for rasters of a relatively modest size (that can be loaded into RAM)
logical. If TRUE, cell numbers for mask are returned rather than coordinates
numeric > 1. Multiplyer used for initial sample size from which the requested sample size is extracted after removing NA points (outside of mask)
integer. 2 or higher gives most warnings. 0 or lower gives no warnings if sample size n is not reached
logical. If TRUE then correct for the fact that longitude/latitude is not a planar coordinate system
Robert J. Hijmans