I have a raster cropped to the extent of some glaciers and divided it into three masks to address inner glacial cells, border cells and non-glacier cells adjacent to the glacier. Data can be found here: https://drive.google.com/open?id=1ERFdsqDGLH1a_FbxwawE_gPm0Au0Q9vT

EDIT: Data preparation was as follows

glaciers <- readOGR("***", "Glacier_Clip_Polygon")
DEM <- raster("***Glacier_Clip1.tif")
glaciers_sf <- st_as_sf(glaciers)
glaciers_comb <- st_union(glaciers_sf) 
glaciers_comb <- st_cast(glaciers_comb, "POLYGON") 
glaciers_large <- glaciers_comb[st_area(glaciers_comb) > units::set_units(50000,"m^2")]
glaciers_large_sp <- as(glaciers_large, Class =  "Spatial")
glaciers_ras <- mask(DEM,glaciers_large_sp)

To divide the masked DEM into my three classes I used the function proposed by @obrl_soil:

outer_cells <- function(x = NULL, diag = TRUE) { 
  focal(x, w = matrix(1, ncol = 3, nrow = 3),
        fun = function(win) {
          if(!is.na(win[5])) {

          if(diag == TRUE) {
            if(any(!is.na(win[c(-5)]))) { 
            } else {
          } else {
            if(any(!is.na(win[c(2,4,6,8)]))) {  
            } else {


## presence/absence layer with no DEM-information
g_fp <- glaciers_ras
g_fp[!is.na(getValues(g_fp))] <- 1L

## glacier-adjacent cells 
y_msk <- outer_cells(g_fp)
glacier_adj_cells <- y_msk + DEM

## glacier marginal cells 
rcl <- data.frame('f' = c(1L, NA_integer_), 't' = c(NA_integer_, 1L))
g_fp_inv <- subs(g_fp, rcl)
g_msk <- outer_cells(g_fp_inv)
glacier_marg_cells <- g_msk + DEM

## inner glacier cells
lb_msk <- subs(g_msk, rcl)
lb_msk <- lb_msk + g_fp
glacier_inn_cells <- DEM + lb_msk

Now I have three raster layers with my three classes for various glaciers. From the inner cells mask of each glacier I now want to draw random cells (in order to lay a buffer and calculate the slope and do some more stuff). The ratio between the random cells and the number of glacier marginal cells should be approx. 0.3.

I want to draw the same ratio from each of my glaciers, so I would have to address them separately. Is there a possibility of doing this in a raster object, maybe by using the polygons as markers of some kind?

I hope, this edit makes my problem a bit clearer.

  • I suspect you're not getting a response here because its a bit unclear. Could you describe what your data files are (with their names) and maybe read them in and plot them so we can see what you want?
    – Spacedman
    Jul 6, 2019 at 7:33
  • Thanks for the advice! I hope, my question is clearer now? Jul 6, 2019 at 9:27

1 Answer 1


Here is a suggestion for a framework. I assume it could be done more effectively and I might have misunderstood some aspects of your question. A couple of things to note is the case where the 0.3 ratio of marginal cells exceeds the number of inner cells, where I have simply defined all inner raster cells as the sample. There are also a couple of polygons that hold glacier rasters, but do not hold any inner raster cells (as can be seen in the figure below).

I originally provided the answer drawing random raster cells, but seeing as you want to create buffers around the samples for further analysis it seemed better to create random points.


# Reads in the data, provided it is located in the working directory
glaciersPoly <- readOGR(".", "Glacier_Clip_Polygon")
adjacent <- raster('adjacent_cells.tif')
marginal <- raster('border_cells.tif')
inner <- raster('inner_cells.tif')

# Combine glacier rasters into one
glaciersRast <- merge(marginal, inner)

# Plot for visualisation
plot(glaciersRast, add = TRUE, alpha = 0.5)

# Loop over each polygon to draw samples
for(i in 1:length(glaciersPoly)){
  # Ignore polygons not overlapping any raster cells
  if(all(is.na(extract(glaciersRast, glaciersPoly[i,])[[1]]))){
  } else {
    # Find the number of cells of the marginal raster that fall within the polygon
    margin <- extract(marginal, glaciersPoly[i,])
    # Find the number of cells that the desired 0.3 ratio equates to (rounded)
    marginRatio <- round(length(na.omit(margin[[1]])) * 0.3)
    # Clip the inner raster by the polygon
    innerClipped <- mask(inner, glaciersPoly[i,])
    # If the number of cells in innerClipped is less than what is required to meet 
    # the ratio I simply create a point for each raster cell and mark it red, as I'm
    # not sure how you would like to handle this case
    if(ncell(na.omit(innerClipped[])) < marginRatio){
      samplePts <- rasterToPoints(innerClipped)
      points(samplePts, pch = 3, cex = 0.4, col = "red") 
    } else {
      # Draw the sample of random points from the inner raster cells
      samplePts <- sampleRandom(innerClipped, size = marginRatio, sp = TRUE)
      points(samplePts, pch = 3, cex = 0.4)

Here is the output, where the sample points are in black and red. enter image description here

  • 1
    Thanks, that brought me on the right track Jul 8, 2019 at 14:20

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