I am using QGIS 2.12 on Macbook Pro running on Mac OS X Yosemite 10.10.5.
I have six adjacent DEMs of the 7.5-minute quadrangle size (National Elevation Data or NED, I think). I loaded them into a QGIS project, then created a .vrt file by using
Raster -> Miscellaneous -> Build Virtual Raster (Catalog) . This created and loaded onto the map canvas the expected seamless .vrt layer DEM.
Then I created a hillshade from the .vrt layer (using
Raster -> Analysis -> DEM (Terrain Models)
Only 1/6 of the .vrt layer -- a part that corresponds exactly to one of the original 7.5-minute DEM layers -- appears as hillshade. The other areas of the .vrt layer (corresponding to the other 5 7.5-minute DEMs) look just as they did before (DEMs).
How can I get the entire .vrt layer to make a contiguous hillshade?
Would it be better to merge the 6 DEMs, rather than create the raster catalog .vrt layer? What does "Merge" actually do with two or more rasters?