sample_pseudoabs {tidysdm}

Sample pseudo-absence points for SDM analysis

Description

This function samples pseudo-absence points from a raster given a set of presences. The locations returned as the center points of the sampled cells, which can not overlap with the presences (in contrast to background points, see sample_background). The following methods are implemented:

• 'random': pseudo-absences randomly sampled from the region covered by the raster (i.e. not NAs).

• 'dist_min': pseudo-absences randomly sampled from the region excluding a buffer of 'dist_min' from presences (distances in 'm' for lonlat rasters, and in map units for projected rasters).

• 'dist_max': pseudo-absences randomly sampled from the unioned buffers of 'dist_max' from presences (distances in 'm' for lonlat rasters, and in map units for projected rasters). Using the union of buffers means that areas that are in multiple buffers are not oversampled. This is also referred to as "thickening".

• 'dist_disc': pseudo-absences randomly sampled from the unioned discs around presences with the two values of 'dist_disc' defining the minimum and maximum distance from presences.

Usage

sample_pseudoabs(
  data,
  raster,
  n,
  coords = NULL,
  method = "random",
  class_label = "pseudoabs",
  return_pres = TRUE
)

Arguments

data

An sf::sf data frame, or a data frame with coordinate variables. These can be defined in coords, unless they have standard names (see details below).

raster

the terra::SpatRaster from which cells will be sampled

n

number of pseudoabsence points to sample

coords

a vector of length two giving the names of the "x" and "y" coordinates, as found in data. If left to NULL, the function will try to guess the columns based on standard names c("x", "y"), c("X","Y"), c("longitude", "latitude"), or c("lon", "lat")

method

sampling method. One of 'random', 'dist_min', 'dist_max', or 'dist_disc'. Threshold distances are set as additional elements of a vector, e.g c('dist_min',70000) or c('dist_disc',50000,200000).

class_label

the label given to the sampled points. Defaults to pseudoabs

return_pres

return presences together with pseudoabsences in a single tibble

Value

An object of class tibble::tibble. If presences are returned, the presence level is set as the reference (to match the expectations in the yardstick package that considers the first level to be the event)