How To: Merge rasters using ArcGIS 8.x Spatial Analyst

Summary

Note:
This article is for ArcGIS Desktop 8.x versions only. Since then, a tool has been created for this process. 
For ArcGIS 10.x users, refer to the documentation for the Mosaic to New Raster tool.

Instructions provided describe how to merge rasters using ArcGIS Spatial Analyst's Raster Calculator. This method allows the combination of up to 23 rasters at a time.

Procedure

Follow the steps below.

1. Start ArcMap.
2. Add all of the rasters to combine into the current document.
3. Select Options from the Spatial Analyst dropdown menu.
4. Switch to the Extent tab.
5. Set the Analysis Extent dropdown list to Union of Inputs and click OK.
6. Select Raster Calculator from the Spatial Analyst dropdown list.
7. Type merge in the expression box.
8. Double-click one of the rasters in the Layers list and add a comma after the raster's name in the expression box.
9. Double-click the next raster in the Layers list and add a comma. Repeat until all rasters have been added.
10. End the expression with a close parenthesis ')'.

Code:
Merge([Raster1], [Raster2], [Raster3], [Raster 4])

Warning:
Do not press Return after entering the last parenthesis. The cursor must be to the right of it. Failure to do so causes the following error:
"Failed to evaluate the calculator equation"

11. Click Evaluate.
    The resulting raster appears with the name Calculation in ArcMap. To make it a permanent dataset, right-click Calculation and select Make Permanent.