This function samples presences within a species distribution, either randomly or with a sampling bias. The sampling bias can be defined manually or with a set of pre-defined biases.
1 2 3 4 5
an integer. The number of occurrence points to sample.
a character string, a
a numeric value between 0 and 1, corresponding to the probability of detection of the species. See details.
a positive numeric value. The strength of the bias to be applied
How the function works:
The function randomly selects
n cells in which samples occur. If a
is chosen, then the selection of these cells will be biased according to the type and
strength of bias chosen. If the sampling is of
type "presence only", then only
cells where the species is present will be chosen. If the sampling is of
type "presence-absence", then all non-NA cells can be chosen.
The function then samples the species inside the chosen cells. In cells
where the species is present the species will always be sampled unless
detection.probability is lower than 1. In that case the
species will be sampled with the associated probability of detection.
In cells where the species is absent (in case of a
sampling), the function will always assign absence unless
is greater than 1. In that case, the species can be found present with the
associated probability of error. Note that this step happens AFTER the detection
step. Hence, in cells where the species is present but not detected, it can
still be sampled due to a sampling error.
How to restrict the sampling area:
Use the argument
Provide the name (s) (or a combination of names) of country(ies), region(s) or continent(s). Examples:
sampling.area = "Africa"
sampling.area = c("Africa", "North America", "France")
Provide a polygon (
How the sampling bias works:
bias.strength indicates the strength of the bias.
For example, a value of 50 will result in 50 times more samples within the
bias.area than outside.
Conversely, a value of 0.5 will result in half less samples within the
bias.area than outside.
How to choose where the sampling is biased:
You can choose to bias the sampling in:
a particular country, region or continent (assuming your raster has the WGS84 projection):
Set the argument
"continent", and provide the name(s) of the associated countries,
regions or continents to
bias.area (see examples).
List of possible
levels(getMap()@data$SOVEREIGNT) in the console
Regions: "Africa", "Antarctica", "Asia", "Australia", "Europe", "North America", "South America"
Continents: "Africa", "Antarctica", "Australia", "Eurasia", "North America", "South America"
"polygon", and provide your
an extent object:
"extent", and either provide your
extent object to
bias.area, or leave it
NULL to draw an extent on
Otherwise you can define manually your sampling bias, e.g. to create
biases along roads. In that case you have to provide to
weights a raster layer in which
each cell contains the probability to be sampled.
list with 3 (unbiased sampling) to 4 (biased sampling) elements:
sample.points: the data.frame containing the coordinates of
samples, the real presence-absences (or presence-only) and the sampled presence-
detection.probability: the chosen probability of detection of
the virtual species
error.probability: the chosen probability to assign presence
in cells where the species is absent
bias: if a bias was chosen, then the type of bias and the
area will be included.
Boris Leroy firstname.lastname@example.org
with help from C. N. Meynard, C. Bellard & F. Courchamp
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
# Create an example stack with six environmental variables a <- matrix(rep(dnorm(1:100, 50, sd = 25)), nrow = 100, ncol = 100, byrow = TRUE) env <- stack(raster(a * dnorm(1:100, 50, sd = 25)), raster(a * 1:100), raster(a * logisticFun(1:100, alpha = 10, beta = 70)), raster(t(a)), raster(exp(a)), raster(log(a))) names(env) <- paste("Var", 1:6, sep = "") # More than 6 variables: by default a PCA approach will be used sp <- generateRandomSp(env, niche.breadth = "wide") # Sampling of 25 presences sampleOccurrences(sp, n = 25) # Sampling of 30 presences and absebces sampleOccurrences(sp, n = 30, type = "presence-absence") # Reducing of the probability of detection sampleOccurrences(sp, n = 30, type = "presence-absence", detection.probability = 0.5) # Further reducing in relation to environmental suitability sampleOccurrences(sp, n = 30, type = "presence-absence", detection.probability = 0.5, correct.by.suitability = TRUE) # Creating sampling errors (far too much) sampleOccurrences(sp, n = 30, type = "presence-absence", error.probability = 0.5) # Introducing a sampling bias (oversampling) biased.area <- extent(0.5, 0.7, 0.6, 0.8) sampleOccurrences(sp, n = 50, type = "presence-absence", bias = "extent", bias.area = biased.area) # Showing the area in which the sampling is biased plot(biased.area, add = TRUE) # Introducing a sampling bias (no sampling at all in the chosen area) biased.area <- extent(0.5, 0.7, 0.6, 0.8) sampleOccurrences(sp, n = 50, type = "presence-absence", bias = "extent", bias.strength = 0, bias.area = biased.area) # Showing the area in which the sampling is biased plot(biased.area, add = TRUE)
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.