I am trying to sample random points according to 2 conditions:
I tested the terra::spatSample()
function, but I can't meet the second condition.
Here is a reproducible example:
library(terra)
set.seed(1)
f <- system.file("ex/elev.tif", package="terra")
f <- terra::rast(f)
v <- system.file("ex/lux.shp", package="terra")
v <- vect(v)
v <- subset(v, v$NAME_2 != "Mersch")
test <- terra::crop(f, v, mask = TRUE)
sp <- terra::spatSample(test, 1000, method="random", replace=FALSE, na.rm=TRUE, as.df=TRUE, as.points=TRUE, xy=TRUE, warn=TRUE, weights=NULL)
plot(test)
plot(v, add = TRUE)
plot(sp, add = TRUE)
One option is to convert f to a point SpatVector first, and then intersect()
the resulting points with v. That way, only points that intersect with v will be returned. You can then sample()
your points. Note that the use of set.seed()
in this reprex means the sample()
will not be 'random'.
library(terra)
library(tidyterra)
library(ggplot2)
set.seed(1)
f <- system.file("ex/elev.tif", package="terra")
f <- rast(f)
# Convert f to points SpatVector and remove NAs
p <- as.points(f, na.rm = TRUE)
v <- system.file("ex/lux.shp", package="terra")
v <- vect(v)
v <- subset(v, v$NAME_2 != "Mersch")
# Return p if inside v
p1 <- intersect(p, v)
# Return difference between p and p1 (for illustrative purposes)
p2 <- erase(p, p1)
# Sample p1
sp <- sample(p1, 1000)
# Plot
ggplot() +
geom_spatraster(data = f) +
geom_spatvector(data = v, alpha = 0.6) +
geom_spatvector(data = p2, colour = "#920000") +
geom_spatvector(data = sp) +
coord_sf(xlim = c(5.8, 5.9),
ylim = c(49.5, 49.6)) +
scale_fill_continuous(na.value = "transparent") +
scale_x_continuous(breaks = seq(5.8, 5.9, length.out = 3)) +
theme(panel.background = element_blank())