Map Algebra functions

The term "functions" might sound like programmers' speak, but whether you realize it or not, you've probably already used them. In the previous module, you derived hillshade, slope, and aspect rasters using choices on the Surface Analysis menu. These choices are simply dialogs that implement Map Algebra functions—programs that perform specific tasks, such as calculating slope.

While you can do a lot through the Spatial Analyst user interface, many more functions are available through Map Algebra. Functions are the major element of the Map Algebra language, and there are over a hundred of them.

The ArcGIS online help system provides a listing of all of the functions and a help topic for each. Here is where you can find a description of what the function does, as well as its syntax.

Below is an alphabetical listing of the functions you'll use throughout the remainder of this course:

• ASPECT: identifies the direction of maximum rate of change in z value from each cell.
• BOUNDARYCLEAN: smoothes the boundary between zones by expanding and shrinking the boundary.
• CON: performs one or more conditional if/else evaluations.
• EQUALTO: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input grid values are equal to the value specified by the first argument.
• GREATERTHAN: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input grid values are greater than the value specified by the first argument.
• HILLSHADE: creates a shaded relief grid from a grid by considering the sun illumination angle and shadows.
• INT: converts input floating-point values to integer values through truncation.
• ISNULL: returns ‘1’ if the input value is NODATA, and ‘0’ if it is not.
• LESSTHAN: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input grid values are less than the value specified by the first argument.
• MEAN: uses multiple input grids to determine the mean value on a cell-by-cell basis.
• MERGE: merges multiple, possibly non-adjacent input grids into a single grid based upon order of input.
• MOSAIC: merges multiple adjacent continuous grids and performs interpolation in the overlapping areas.
• NIBBLE: replaces areas in a grid corresponding to a mask, with the values of the nearest neighbors.
• REGIONGROUP: records for each cell in the output the identity of the connected region to which it belongs. A unique number is assigned to each region.
• SETNULL: returns NODATA if the evaluation of the input condition is ‘TRUE’; if it ‘FALSE’, returns the value specified by the second input argument.
• SLICE: ‘slices’ (or changes) a range of values of the input cells by specified ranges, zones of equal area, or zones with equal intervals.
• SLOPE: identifies the rate of maximum change in z value from each cell.
• ZONALAREA: calculates the area of each zone in the input grid.