randomPoints: Random points
In dismo: Species Distribution Modeling

randomPoints

R Documentation

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, prob=FALSE, 
             cellnumbers=FALSE, tryf=3, warn=2, lonlatCorrection=TRUE)

Arguments

`mask`	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)
`n`	integer. Number of points
`p`	Presence points (if provided, random points won't be in the same cells (as defined by mask)
`ext`	Extent object. Can be used 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
`prob`	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)
`cellnumbers`	logical. If `TRUE`, cell numbers for `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 gives most warnings. 0 or lower gives no warnings if sample size `n` is not reached
`lonlatCorrection`	logical. If `TRUE` then correct for the fact that longitude/latitude is not a planar coordinate system