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I often add LiDAR data to a map along with a shaded relief. Most of the time i have to generalize the LiDAR dem depending on the scale of the map. Still, there is often too much detail and mountain ridges and valleys are not well pronounced. I came across an article recently that describes the process of combining 2 hillshade models, one detailed and the other generalized. The resulting relief is very nice. I would like to produce such effect in my maps: Assuming i have 2 hillshade (detailed and generalized) rasters, what is the best way of combining them to acheive such effect?

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up vote 8 down vote accepted

This sounds like Tom Patterson's work on Resolution Bumping GTOPO 30 in Photoshop. The theory is described well enough to be adaptable to other software, though work needs to be done coming up with the specifics. The basic idea is to generalize (blur) one data set, a lot, to emphasize the general shape and hide specific detail and then blend the hi-res and lo-res images together using alpha channels or opacity percentages.

While Patterson's described process is applied to the elevation models themselves, I adapted it to reliefs as follows.

Load the relief image in GIMP:

  • duplicate layer
  • upper layer == the detail layer, leave alone, set Mode to Overlay
  • bottom == the shape layer, apply Gaussian blur with setting 20
  • save a copy (relief_blurred.tif)

Re-assign projection:

There's a lot of room for adaptation in choosing what percentages to use, whether to use Overlay or Screen or Darken filters etc.

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very good method. –  Mapperz Sep 15 '10 at 20:00
ohhh I'm going to try this, looks cool. –  Nathan W Sep 16 '10 at 3:55
i was hoping for an ArcGIS solution but will try this too... –  Jakub Sep 16 '10 at 12:45
I am starting to think that the only way to do this must be as you have suggested in an image editor like GIMP, Photoshop, etc. (Since ArcGIS color ramp cannot be set from transparent to black but always defaults to white to black) –  Jakub Sep 16 '10 at 19:05
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I see "ArcGIS" is a tag, Jakub. Using Spatial Analyst you would simply compute a weighted average of the two hillshades. E.g., the 60-40 mixture could be generated with a calculation like this:

(60*[Detailed hillshade] + 40*[Generalized hillshade]) / 100

If you need it, the Gaussian blur can be executed by running a few circular focal means over a raster. The effective radius of doing m focal means, each of radius r, is r * Sqrt(m).

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Thanks Bill. My input detailed hillshade is a cell size 5, 5 but the generalized hillshade is 20, 20. The raster calculator result is therefore 20, 20 as well. It seems that it enhances the primary shadows but the fine detail is lost. What would you recommend to retain the original cellsize? Resample the generalized hillshade? Which resapling technique to use? –  Jakub Sep 16 '10 at 18:30
I tried it, everything works but the final hillshade product is not much different from the detailed original other that a little smoother... (Rather than intensifying the original detailed hillshade) –  Jakub Sep 16 '10 at 18:57
the central though unspoken idea in Bill's answer is that just about everything you can do in Gimp/Photoshop/etc. with regards to mixing layers and overlays can be done with raster calculation -- as long as you know the math. It's not easy though which is why there aren't a lot of ready made solutions. Going on theory alone, I'm not a math head, I think what's missing from the example above is a third raster to act as a poor man's opacity/transparency slider. –  matt wilkie Sep 16 '10 at 21:32
@Jakub: You're right, you need to retain the finer cellsize or else all is lost. In ESRI products this is usually done by preliminarily setting some "environment" variable somewhere to specify the output cell size, extent, and projection. Look for a Spatial Analyst "options" menu or an ArcToolBox "environment" menu in ArcGIS. –  whuber Sep 16 '10 at 23:06
@Matt: I may have missed something in the Tom Patterson description. (It's hard to follow because he only says what buttons to push, not why or what they're really doing.) Would you mind elaborating a little on what the "opacity/transparency slider" is doing for us? –  whuber Sep 16 '10 at 23:08
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