complexity of combinations

Grouping raster or vector data by attribute in order to SIMPLIFY the OVERLAY process

Need to ask 2 questions first;”With what complexity?” then “How to assign values?”

in order of Increasing Complexity simplification of continuous layers yields discrete categorical layers

1. binary layers (yes/no, on/off, 1/0, suitable/unsuitable)
2. multiple category layers( yes/maybe/no, on/bordering/off, highly suitable/less suitable/not suitable)
3. “index” layers (1, 2, 3, 4, 5, or 1, 2, 3,…10, etc)

How does one divide up a continuous variable into discrete categories? (open simplify project from your …GIS\demo\simplify folder)

1. Raster Calculator Map Algebra
   1. “IF” statements (plus “if… then…else”) in traditional programming
      1. BINARY DISCRETE CATEGORIES : a simple “Con” statement has this syntax
         Con (conditional statement, yesGrid, noGrid) . . . . .see help — it means “conditional testing”
         Con("elev" > 500, 1, 0) copy and paste this into the Spatial Analyst Raster Calculator
      2. MULTIPLE DISCRETE CATEGORIES: a “nested” .Con statement (where the “no” for the first conditional sends you to another conditional test)
         Con("elev" > 600, 2, Con("elev" > 300, 1, 0))
   2. Using arithmetic (works for Binary, Multiple, or Index Category Layers)
      1. Determine the range and divide by the number of categories required
         index category width = ([range]/[# of divs needed])
         for our elevation example; 240 to 830 = 590 / 10 classes = 59 m per index level
      2. subtract the minimum from each cell or attribute value to set the origin of the index at the lowest value
          ("elev" - 240) / 59
      3. find the next lowest integer (or highest, depending if you want the index values to start at 0 or 1) using the Round functions (up or down)
          RoundDown((“elev” – 240) / 59) however to get it into an INTEGER raster, you must use
         Int(RoundDown((“elev” – 240) / 59))
         which yields 10 categories from 0 to 9
         or
          Int(RoundUp((“elev” – 240) / 59))
         which yields 10 categories from 1 to 10What if you want the same number of cells in each category? (called “quantile” or “equal area”)
2. Reclassify geoprocessing is on the next page