randomPoints: Random points

Description

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.

Usage

randomPoints(mask, n, p, ext=NULL, extf=1.1, excludep=TRUE, cellnumbers=FALSE, tryf=5, warn=2)

Arguments

mask

Raster* object; Cells with NA in this object (or first layer of the object) are excluded

Integer. Number of points

Presence points (if provided, random points won't be in the same cells (as defined by mask)

ext

Exent; to restrict sampling to a spatial extent

extf

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)

excludep

Logical. If TRUE, presence points are exluded from background

cellnumbers

Logical. If TRUE, cell numbers for ]code{mask} are returned rather than coordinates

tryf

numeric > 1. Multiplyer used for initial sample size from which the requested sample size is extracted after removing NA points (outside of mask)

warn

integer. 2 or higher give most warnings. 0 or lower give no warnings if sample size n is not reached

Value

matrix with coordinates, or, if cellnumbers=TRUE, a vector with cell numbers.