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_time). 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.
sample_pseudoabs_time(
data,
raster,
n_per_presence,
coords = NULL,
time_col = "time",
lubridate_fun = c,
method = "random",
class_label = "pseudoabs",
return_pres = TRUE,
time_buffer = 0
)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)
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).
the terra::SpatRaster, stars or terra::SpatRasterDataset
from which cells will be sampled. If a terra::SpatRasterDataset, the
first dataset will be used to define which cells are valid, and which are
NAs.
number of pseudoabsence points to sample for each presence
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")
The name of the column with time; if time is not a lubridate
object, use lubridate_fun to provide a function that can be used to
convert appropriately
function to convert the time column into a lubridate object
sampling method. One of 'random', 'dist_min', 'dist_max', or 'dist_disc'.
the label given to the sampled points. Defaults to
pseudoabs
return presences together with pseudoabsences in a single tibble
the buffer on the time axis around presences that defines
their effect when sampling pseudoabsences. If set to zero, presences have
an effect only on the time step to which they are assigned in raster; if
a positive value, it defines the number of days before and after the date
provided in the time column for which the presence should be considered
(e.g. 20 days means that a presence is considered in all time steps
equivalent to plus and minus twenty days from its date).
#' @details Note that the time axis of the raster should be in POSIXct or
Date format, or use `tstep="years"'. See terra::time() for details on how
to set the time axis.