types of combinations

Summary of Overlay Types when simplifying continuous data in layers

    1. Binary Layers
      1. overlay by multiplying two binary risk layers (“quality A” and “quality B”)
      Quality B
      low hazard
      0      		 Quality B
      high hazard
      1
      Quality A
      low hazard
      0      		 low risk low risk
      Quality A
      high hazard
      1      		 low risk high risk

      or adding them

      Quality B
      low hazard
      0      		 Quality B
      high hazard
      1
      Quality A
      low hazard
      0      		 low risk moderate risk
      Quality A
      high hazard
      1      		 moderate risk high risk

      Try these with a binary_slope and binary_elev

    2. Multiple Category Layers
      1. multiple boolean operators become very complex but can be done with a script
      2. overlay by adding or multiplying;
      B: High
      Hazard
      B: Mod
      Hazard
      B: Low 
      Hazard
      A: High
      Hazard
      high risk
      moderate risk
      low risk
      A: Mod
      Hazard
      high risk
      low risk
      no risk
      A: Low
      Hazard
      moderate risk
      low risk
      no risk

But how would you “hand pick” various levels of risk as shown above, when the weighting is not symmetrical as shown? (in ArcGIS Pro, look at the “combine” tool).

    1. Index Layers
      1. use reclassify or map algebra to create indices
      2. overlay by adding or by multiplying has a different effect on the distribution of high and low results.
    2. Layer Weighting (can be combined with Overlay Types #1-3)
      1. weighting is applied to one layer (or more) when it is more important or has more influence on the outcome than others. This is an “expert opinion” rather than a GIS determinant.  Here’s an example from a multi-criteria selection tutorial from Ujaval Gandhi’s great web site

        Note: In this example, we are giving equal weight to both road and water proximity. In real-life scenario, you may have multiple criteria with different importance….For example, if proximity to roads is twice as importance as proximity away from water, instead of (("roads_reclass@1" + "water_reclass@1")/2),
        you can use the expression ((2*"roads_reclass@1" + "water_reclass@1")/3).

      2. use raster calculator to overlay the weighted layers
      3. In ArcGIS Pro, you can use weighted overlay tool
        (Note that the input is complicated, because this tool does both the simplification/reclass step and the overlay with weighting step in a single toolbox).