Changing Global Environment Settings for Raster Processing
Change the global environment settings so that the raster processing you will perform in later steps will be constrained to a specific geographic extent. In this instance, you want all raster processing to match the extent of the Los Angeles County boundary. In ArcMap, select Geoprocessing from the main menu, then select Environments.
This step will open up the Environment Settings window. Here you will define the Processing Extent globally. Any changes you make here will affect the processing extent for tools you will use later on. Under Processing Extent, use the drop-down menu to select the LA County Boundary. Under Raster Analysis set the Mask settings to the Los Angeles County Boundary. By default, raster outputs are rectangular. With this setting, the output of the analysis will fill only the shape of the Los Angeles County boundary. Leave all other settings as default and click OK.