How to Clip a Raster by Mask in QGIS

Step 1: Prepare Your Polygon and Raster

To start clipping a raster layer by a mask layer, you need both a polygon and a raster. The polygon can represent a specific area of interest, such as a watershed boundary. Ensure you have created your polygon and have a raster layer ready for processing.

Step 2: Access the Clip Raster Tool

Navigate to the raster tools in QGIS. Go to the Extraction menu and select Clip Raster by Mask Layer. You can also find this tool by using the search bar in QGIS.

Step 3: Set Input Layers

In the Clip Raster tool, choose your raster as the input layer. It’s important to check that the coordinate reference systems (CRS) of the raster and polygon match. If they differ, be mindful of this for any area calculations later.

Step 4: Specify No Data Value

Setting a no-data value is crucial to avoid displaying unwanted values in the output raster. Use a value like -9999.0, which is unlikely to exist in your data. This value will help in defining areas outside the polygon.

Step 5: Match Extents

Ensure that the output extent matches that of your mask layer (polygon). This step is vital to ensure that the output raster aligns correctly with the area of interest.

Step 6: Run the Process

Once all parameters are set, you can run the process. The output will generate a raster layer that only contains data within the polygon area.

Step 7: Handling No Data Values

If you forget to set a no data value, the output may display a black box around the raster. This occurs because rasters must be rectangular, and if not defined, the default value is zero, which is displayed as black. Always ensure to set a no data value to avoid this issue.