1. Create 15 biome Human Influence Index (HII) rasters using biome masks. Based on WWF terrestrial biome delineation.
2. Compute the min, max values of each of the rasters created in 1. These are oldmin, oldmax in the equation
3. Plug in these values into the equation. the goal is to scale the biome HII raster values newmin = 0 and newmax = 100. 0 is least influenced in the biome and 100 is most influenced. The new rasters are Human Footprint index (HFP) rasters. x is the biome HII raster. say, your biome1 (tropical rainforests) HII file is b1HII.img and the min = 0 max 50 and output HFP is b1HFP.img, your equation in raster calculator will look like :- b1HFP.img = ((b1HII.img - 0) * (100 - 0) / (50 - 0) ) + 0
4. Mosaic the individual HFP rasters to global HFP raster.