How to compute the presence of a minimum amount of green

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In many spatial planning cases, there are requirements not just for the presence of certain features, but also for those features to be of at least a specified minimum size. For example, when the presence of "usable" green in an urban environment usually requires the green features to be continuous and at least a few hundred square meters. Using Attributes, an Average Overlay, and a Combo Overlay, its possible to compute how much of a particular feature is in or nearby any given location.

The approach for this is to use the Average Overlay to average out a predictable value, and based on the averaging radius compute how much of the surface area within the radius contains the desired feature.

How to compute green locations of at least 200m²:

  1. Add an Average Overlay, set to the Attribute GREEN_M2, and a radius of 25m.
  2. Add a Combo Overlay, with the previously created Average Overlay as input A.
  3. Set the formula to "IF(GTE( MUL(A, MUL(POW(25, 2), 3.14)), 200 ),1,0)".
    This will compute whether at least 200m² nearby any computed cell contains the desired feature.
  4. Set the Combo Overlay to have a custom legend.
  5. Set the custom legend to have an entry for "0", and an entry for "1".
  6. Recalculate the project.
  7. Any locations where there is sufficient green nearby will have a value 1.

Notes

  • The formula multiples the average value of the calculating cell with the radius (25m) it is sought in, to compute the total surface area nearby. It is then tested against the desired amount of surface area (200m²).
    • The formula can be adapted by changing the 200 with whichever surface amount to find. The radius in which to look for the desired feature can be changed by changing both the search radius in the Average Overlay ánd the related parameter in the Combo Overlay formula.
  • Both the calculation and the results are not exact.
    • Due to rasterization, the computed amount of surface area found may differ from the actual amount of surface area present. However, there will always be at least one cell with a value of "1" for any qualifying location, and there will never be a cell with a value of "1" in a location which does not qualify.
  • The greater the radius configured in the Average Overlay, the more spread out the feature may be. The smaller the radius, the more clumped together the feature must be.
  • The method can be adapted to find features with more qualifying parameters by first using an Overlay, such as a Combo Overlay, to determine which locations match all the requirements the surface area must adhere to, outputting a boolean 1 or 0 to reflect the cell qualifying or not. The Average Overlay can then be set to look not for an Attribute but for a Grid Overlay.