Working with QGIS and Grass I would like to produce a ZTV. I have the bare land DEM happily ready to go but would like to add buildings and woodland to the calculation. Can anyone please point me in the direction of how to add buildings and woodland to the DEM upon which I will be doing the ZTV?

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    Could you give a quick explanation about ZTV - what is it? – Simbamangu Nov 27 '14 at 17:20
  • Simbamangu - ZTV stands for Zone of Visual Influence. Within Grass its a Visibility study (r.los) that locates a target point on a DEM and queries Grass to review from which other locations within the DEM that target is visible. Classically used to determine the extent to which wind turbines will be visible, here in the UK ZTVs are also used for many other types of development as well. – joverall Nov 27 '14 at 20:01

The way I have done this in the past is to convert buildings, forested areas, etc. to a raster (rasterize the polygon using the height column for the raster value, using the same pixel resolution as the DEM). Then merge the two rasters (DEM and rasterized buildings). This will add the building height to the DEM. One thing to note is that you will probably want to have the areas under buildings flat prior to adding the building features, otherwise you may have buildings with different slopes for roofs.

  • Ryan - thanks for the help I will give this a go. One question re: buildings because I have never yet rasterised a vector set of buildings: Is it possible to set the floor height of the buildings to be below the ground level provided by the existing DEM and so avoid the building roof from aligning itself with the ground plane? – joverall Nov 27 '14 at 20:04
  • @joverall That would require some additional raster calculation. Adding the two together simply puts a straight extrusion from existing values into the DEM, as Ryan mentions. What you'd first have to do is a zonal statistics using your buildings as zones on the DEM. You could get mean/min/max DEM value in the zone, then with another raster calc modify the DEMs in the zones to those values, and then add your bldg height raster to that. Note that mean/min/max will affect the ultimate height. If placing boxes for buildings onto the DEM, mean/max would have a gap between ground and floor. – Chris W Nov 27 '14 at 21:40
  • Chris is absolutely right, and it is something that is done in 3D visualization very often – Ryan Garnett Nov 27 '14 at 22:26

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