I got this hillshade from my DHM (I used the hillshade-module from QGIS). For my purpose there is too much relief detail (roughness). How can I get rid of this in QGIS or GRASS?
QGIS – How to Generalize Rough Hillshade in 1” DHM
demgrasshillshadeqgis
Related Solutions
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:
- gdalcopyproj shaded_relief.tif relief_blurred.tif
There's a lot of room for adaptation in choosing what percentages to use, whether to use Overlay or Screen or Darken filters etc.
I usually post-process a copy of the raw hillshade in some external graphics editing program that I can directly over-write (save, instead of save-as)
Photoshop and GIMP are two that come too mind.
The process in a nutshell I follow:
Import GeoTiff'd Hillshade -> Tone -> Median -> Reduce Noise -> Gaussian Blur
- Your geotiff should be accompanied with a worldfile (like .tfw) for portability between the GIS and graphics program you use)
Use the tone tool to redistribute your min/max values of the greyscale hillshade. In GIMP you use the 'Levels' tool. I often use a very small range with a min of 5% grey and 25% grey. Rarely do I use full white to black greyscale ranges.
Median is used to generalize. This is the part where can easily misrepresent your hillshade. In GIMP the equivalent tool is 'Despeckle'.
The median/despeckle tool will reduce the sharp edges and make then rounded, like mountian tops and valleys. Thus often making it appear as though some valleys are filled-in and some mountain tops are flatter than they appear. Use this tool lightly, or not at all.
Reduce noise is used to remove any strange artifacts that median couldn't - often I don't even use reduce noise. Almost like a cheat way of de-striping your DEM if it suffers from 'stepping' - a result of manual profiling.
Gaussian blur will introduce the final smoothing of the hillshade and is often the last thing I do (this will also help remove any introduced artifacts).
The other option, is to run the above process over an HDR hillshade.
Create hillshades from various aspects like 360 (or 0 deg), 15, 30, 45. Those are the four angles I might use for my local region because that is the general direction the sun comes from during the morning to mid-day. HDR the four images using either GIMP or photoshop then use whatever generalizing filters you want like the aforementioned ones.
Experiment and see what works best for you.
What's good about working on a GeoTiff is that none of the georeferencing is lost as the pixel size and extent are not altered in the process.
There are lots of other filters/overlays to use and play with. The ones I've mentioned are the ones I use most frequently. Also remember that the less filters you have to apply the better so that the terrain doesn't become dramatically misrepresented.
When I apply the filters, they are done so in small doses. For instance, I find myself never applying a gaussian blur over 5%.
Useful resources:
http://www.shadedrelief.com/bumping/bumping.html
http://www.shadedrelief.com/shading/Swiss.html
Best Answer
Which hillshade module, Martin? In QGIS Master there are two under the Raster menu:
Raster -> Terrain analysis -> Hillshade
and
Raster -> Analysis -> DEM(Terrain models)
If you haven't already done so, I should try the second one (set 'Mode' to 'Hillshade'). Also, tick the 'Creation Options' box and enter 'TFW' under 'Name' and 'YES' under 'Value'. Doing this will make a world file for the hillshade tiff, which means that you will be able to edit it in GIMP or whatever to do things like applying blur, without losing the georeferencing (provided that you don't change the image size, that is). Nick.