This function samples background points from a raster given a set of presences. The locations returned as the center points of the sampled cells, which can overlap with the presences (in contrast to pseudo-absences, see sample_pseudoabs_time). The following methods are implemented:
'random': background points randomly sampled from the region covered by the raster (i.e. not NAs).
'dist_max': background points 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".
'bias': background points are sampled according to a surface representing
the biased sampling effort. Note that the surface for each time step is
normalised to sum to 1;use n_per_time_step to affect sampling effort within
each time step.
sample_background_time(
data,
raster,
n_per_time_step,
coords = NULL,
time_col = "time",
lubridate_fun = c,
method = "random",
class_label = "background",
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 background points to sample for each time step (i.e. a vector of length equal to the number of time steps in raster)
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_max', or 'bias'.
the label given to the sampled points. Defaults to
background
return presences together with background in a single tibble
the buffer on the time axis around presences that defines
their effect when sampling background with method 'max_dist'. 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).
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.