Merge two or more raster from a set of images in ArcGIS, is a very simple and very useful process when you need to merge multiple digital elevation models or raster images inside into one, it should be noted that the input data must contain the same amount bands, bit depth; Otherwise, the tool will exit with an error message.
This example shows how to merge four DEM files downloaded from Aster Gdem with the Mosaic To New Raster tool.
ArcToolbox > Data Management Tools > Raster > Raster Dataset > Mosaic To New Raster
In this tool in the Input Raster field add rasters, define the output directory, the extension of the new file (img), spatial reference (WGS84_17S) and the number of bands (1), the other fields are optional (set of according to our needs), accept and the result is displayed at the end.
Nice