Null values are a particular type of values that are used to indicate cells where the value for a given layer is not defined. Understanding how to use them is important to avoid wrong results when performing analyses but also to use them as a tool to get better and more correct results. This recipe explains some of the fundamental ideas about null values in raster layers.
The watershed.tif layer contains the area of a watershed. Cells inside the watershed are cells from which water will eventually flow into the outlet point of the watershed. The remaining cells belong to a different watershed. To mask the DEM with the watershed mask, follow these steps:
Open the
watershed.tiflayer.Open the identify tool and check whether the cells that belong to the watershed have a value of 1, and the ones outside, have a value of
no data.Try clicking inside and outside the watershed; in your Identify Results dialog, you will see the results, as shown in the following screenshot...