Creating a raster mosaic
Frequently, raster data is made available in tiles. In fact, some consider the Murphy's Law of GIS to be that your study area lies at the intersection of four topographic quadrangles. In this situation, the input rasters will need to be combined into a seamless raster that covers the study area. When doing this, the individual input rasters must all be in the same coordinate reference system and have the same number of bands.
Assuming that these two conditions have been met, the Merge tool that can be found by navigating to Raster | Miscellaneous can be used to merge the individual rasters together. This tool is a GUI version of the
GDAL_merge command-line tool. Overlap among the input rasters is allowed. If this happens, ...