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.