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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?

  • Copy-pasting commands from the tools would help. Merging DEMs together would definitely help. Merging creates a new actual raster (not a virtual one) that includes all of the inputs without overlaps. – Maxim Dubinin Oct 31 '15 at 1:44
  • Merge does just what you expect - it merges your 6 adjacent DEMs into one big DEM. Overlap is handled by the input order - the last input is on top. – Kersten Oct 31 '15 at 7:29

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