Merging Rasters (requires Spatial Analyst extension)

1. Go to the options of the Spatial Analyst (from the Spatial Analyst toolbar drop down menu)
2. Set the Extent to "Union of Inputs"
3. Choose the cell size (can be same as another raster or typed in manually) then exit the options.
4. Go to the Raster Calculator (also from the Spatial Analyst toolbar drop down menu).
5. Type... MERGE (
6. Double click on the rasters you want to merge separating the inserted text by commas then close the brackets
7. IMPORTANT ensure that the cursor is then moved back a space so that it is flashing within the brackets! This operation usually does not complete if the cursor is outside the brackets.
8. Click "Evaluate"
9. A new temporary raster is generated and appears in the Layers pane.
10. If you are happy with this new raster make the temporary raster permanent by right clicking on the raster in the Layers pane, selecting "data >" and then "Make permanent". Give your new raster a name and location and click save, then add it to your map if you wish to complete any further tasks with this raster.

NOTE: The order of priority when merging rasters is determined by the order in which the layers are selected in the raster calculator. The first raster you select whilst merging defines the settings which the output raster will default to (e.g. projection). All subsequent rasters when merged will conform to these settings.