Mosaicking creates one large DEM from several smaller ones. They need not overlap.
1.) Acquire DEM files in ArcGRID format from a site like EarthExplorer (see ‘DEM Data Sources’ post).
2.) DEMs come in several formats. Chose GEOTIFF if available. If only .DEM format is available, you will need to convert them Arc-friendly rasters (ArcGRID format). Use Conversion tools > To Raster > DEM to Raster tool. You may need to correct the coordinate system, datum, projection, as well.
3.) ArcToolbox > Data Mgmt Tools > Raster > Raster Dataset > Mosaic to New Raster
– Chose input rasters from pulldown list or navigate to each
– Output location: Navigate to one folder level above the folder you wish to store output raster, click it
– Enter cell size for output; use the same value as your input (Properties > Source > Cell size)
– 8-bit unsigned (default)
– Spatial Reference Properties > Select…(choose a projection)
– Click Apply, then OK
– Number of Bands: 1
– First (default)
– Reject (default)
– Hit OK and processing window shows progress
4.) Try the Mosaic tool if Mosaic to New Raster fails. And quit whining, Skaar.
YouTube: GISTutorials – Creating a Seemless Raster Mosaic, http://www.youtube.com/watch?v=qJHCbEVbMv0