Map Algebra

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Map Algebra
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Map algebra

• Map algebra is a high-level computational
  language for performing spatial analysis using
  gridded (raster) data.

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The Map Calculator
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Map Calculator Syntax Rules

• Grid names are always in square brackets (e.g.,
  Slope).
• Strings are in quotes.
• Optional blank spaces can be used to separate
  objects, requests, and parameters. Grid1
  Grid2
• Expressions are evaluated from left to right,
  regardless of the mathematical operator.
  Grid1(Grid2Grid3)
• You may nest expressions, as long as the entire
  expression evaluates to a grid. The inner most
  parentheses set is evaluated first, then outward.
  
• ((Grid1 Grid2) 0.40) (((Grid3
  Grid4) 0.10) ((Grid5 Grid6) 0.50))

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Map algebra operators

• Arithmetic operators
• basic math operators.
• Boolean operators
• logically examine the input grids. The integer
  output grid will contain values of 0 (FALSE) and
  1 (TRUE).
• Relational operators
• compare two numbers. If the result is true, 1 is
  returned, otherwise, a 0 is returned.

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Arithmetic Operators
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Relational Operators
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Boolean Operators
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Map Algebra Functions

• Local
• Neighborhood
• Zonal
• Global

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Local Functions

• Local functions perform cell-by-cell processing.
  The output value at each location is only
  dependent on the input cell at that location

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Local Functions

• arithmetic,
• trigonometric,
• exponential,
• logarithmic,
• reclassification,
• extraction,
• statistical.

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IsNull
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SetNull
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Neighborhood (Focal and Block) Functions

• Depends on value of pixel and its neighbors

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Resample
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Zonal functions

• A zone is defined as all cells in a grid that
  have the same value, regardless of whether or not
  they are contiguous.

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Zonal Functions
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Global functions

• Global functions compute an output grid where the
  value for each output cell is potentially a
  function of all the input cells

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Global functions
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RegionGroup

• The RegionGroup request creates regions for an
  input grid. A region is a group of connected
  cells with the same value. A zone is a group of
  connected or disconnected cells with the same
  value.

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