Sample raster

sample_raster.R

library(sp)
library(raster)
library(ggplot2)

# load example data
data("meuse.grid", package = "sp")
coordinates(meuse.grid) = ~x+y
proj4string(meuse.grid) <- CRS("+init=epsg:28992")
gridded(meuse.grid) = TRUE

# convert to raster
r <- raster(meuse.grid[, 'dist'])
# downscale resolution
r <- aggregate(r, fact = 4)
# convert raster to data.frame
df <- as.data.frame(r, xy = TRUE, na.rm = TRUE)

# run sampling
idx <- sample(
  1:nrow(df), 
  size = 30, 
  prob = df$dist,
  replace = TRUE
)

# extract random coords within cells
coords <- do.call(
  rbind,
  lapply(
    idx,
    function(i) {
      centre_x <- df$x[i]
      centre_y <- df$y[i]
      
      x <- centre_x + res(r) * (runif(1) - 0.5)
      y <- centre_y + res(r) * (runif(1) - 0.5)
      
      data.frame(x = x, y = y)
    }
  )
)

ggplot() +
  geom_raster(data = df, aes(x = x, y = y, fill = dist)) +
  scale_fill_viridis_c() +
  geom_point(data = coords, aes(x = x, y = y), shape = 23, fill = "red") +
  coord_equal() +
  theme_minimal()