Working with NoData

Similar to logical values, NoData values also influence the evaluation of expressions. The NoData value is the only non-zero value that is not interpreted as a true condition. This special value indicates that there is no information associated with the cell. In general, a Map Algebra expression will return NoData for a cell if any of the corresponding input cells have NoData.

In general, Map Algebra expressions assign the value NoData to the output cell if any corresponding input cell is NoData.

In a few instances, NoData will be returned when the input condition is evaluated as false. For example, the CON function will return NoData when no value is given for the false argument.

Most functions or expressions simply ignore NoData cells, making it difficult to test for NoData or to assign NoData. The ISNULL and SETNULL functions make it possible for you to perform these two tasks in Map Algebra.

The ISNULL function tests the current cell to see if it contains NoData and returns either 1 or 0 (true or false). It is normally used in conjunction with the CON function to replace the NoData value with something else.

The SETNULL function applies a test against the current cell and, if the test is true, replaces its value with NoData. If the test is false, it writes the results of the expression to the cell.