Sometimes it is necessary to merge multiple GeoTIFFs into a single raster. For example, if the image you are trying to orthorectify <envi-image-orthorectify> overlaps multiple disjoint SRTM DEM GeoTIFFs, then you can combine the SRTM DEM GeoTIFFs into a single image using the gdal_merge.py script available with the GDAL package.
The gdal_merge.py script takes the paths of the GeoTIFFs you want to combine as input and creates a single GeoTIFF as output. If you do not specify a filename with the -o option, the script will save the result as out.tif. :
gdal_merge.py image1.tif image2.tif -o merged.tif