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I am using QGIS.

I have 20 large rasters showing land cover over 20 years (MCD12Q1). Each pixel contains a value between 1 and 17 that corresponds to a land cover class.

I am trying to create a new raster that shows how many unique land cover classes each pixel has experienced across the 20 yearly rasters. For example, if a pixel was forested, then became grassland for a few years, and then reverted back to forest, that pixel in the new raster would have a value of 2.

I am relatively new to all of this, but I've tried exploring online and asking others and haven't found a solution yet. I have tried using a loop to index each pixel across the rasters, then use the R unique and length commands to "count" unique values like this:

for (i in 1:nrow(layer2001)){
  for (j in 1:ncol(layer2001)){
    
    sub = as.numeric(unique(s[[1:20]][i,j]))
    sub = unique(sub)
    temp = as.numeric(length(sub))
    
    newrast[cellFromRowCol(newrast,i,j)][1] <- temp
    
  }
}

but because of the size of the rasters this is not a solution. I have also been pointed towards the stackApply command but haven't been able to use it successfully.

1 Answer 1

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You can use Menu Processing > Toolbox > Cell statistics. For Statistics, select Variety (count of unique values). See details in the documentation.

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