NoData and how it affects analysis
Every cell location in a raster has a value assigned to it. When information is unavailable for a cell location, the location will be assigned as NoData. Note that NoData and 0 are not the same—0 is a valid numerical value.
The fact that an input location can have NoData instead of a numerical value has ramifications for how tools handle them. NoData means that not enough information is known about a cell location to assign it a value. There are two ways that a location with NoData can be treated in the computation of an expression:
- Always return NoData for that specified cell location.
- Ignore the NoData and compute with the available values for that specified cell location.
It is important to understand how NoData is handled in a particular tool before making a decision. You may need to know if a location with NoData in the output ever had a value or if it received a value of NoData as a result of the tool's algorithm. In addition, when locations receive values, it may be important to know if the output value really is the actual minimum or maximum value or if it is the minimum or maximum value of the existing known values.
Depending on the particular tool, one of the above approaches will make greater sense than the other. For example, when adding two raster datasets together, if a cell location in one of the datasets contains NoData, there is no basis for assigning a new value to the corresponding location on the output raster dataset.
In contrast, when looking for the minimum value in a neighborhood that contains a NoData value, an assumption can be made (or a risk taken) that the cell location with the NoData value will not be the minimum value. The focal operation can thus be used to return the minimum value of the remaining valid values in the neighborhood.
Spatial Analyst fully supports the NoData concept. If NoData exists in any of the input raster datasets in the operation, the output values will be affected. The behavior of NoData is addressed for each tool in its respective tool reference documentation.