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Is it possible using QGIS or GRASS to perform a Zone of Visual Impact assessment? E.g. where can a 100m high structure be seen at a viewing height of 1.8m within 30km, taking into account elevation (also maybe buildings and wooded areas)? I'm pretty new to GIS but am having to learn pretty quickly for work. Any help would be much appreciated.

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3 Answers 3

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Yes is the short answer. You can do this in GRASS using r.viewshed.

However, it sounds very much like you are doing some Windfarm ZTV (Zones of Theoretical Visibility) analysis. If that is correct, you could also try OpenWind. This is an excellent and free utility and may be right up your street. It doesn't have the full-on capabilities of the big three proprietary wind farm analysis packages, viz: WindFarmer (by Garrad Hassan), WindFarm (ReSoft) or WindPro(EMD). However, it does an excellent job for the most common tasks (including calculating a ZTV).

For buildings and woodland, you will have to get or estimate the heights and then much will depend on the exact system you are using. Some will account for vector obstructions, but if not, you can edit your DTM by adding the heights of the buildings and trees (be careful with your raster resolution though - so you don't over estimate the area of the obstruction inadvertently).

Finally, you may want to reconsider your view height of 1.8m. 1.8m represents an average man's height... to the top of their head. The eyes are a bit lower than that! I always use 1.5m as a more representative adult eye-height unless a client absolutely insists or when required to conform to specific planning guidance. It probably won't make much difference given the likely resolution of your DTM and the distances involved, but you can claim greater realism :)

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Thanks! Thats really helpful. It's actually for a mast so have used GRASS but Windfarm ZTV analysis is something we may be required to do in the future so thats really usefull. –  Simon Apr 30 '12 at 14:02
    
You can do a mast ZTV in any windfarm software by faking a turbine spec to suit your mast height. (BTW, it is encouraging to the board if you accept the reply that answers your question by clicking the little tick.) –  MappaGnosis Apr 30 '12 at 14:41
    
Opps! sorry, new to this board so thanks for the tip. I'm assuming that for a more accurate, less pixelated result you would just use a more accurate DTM such as Landform Profile Plus rather than Land-form Panorama? –  Simon May 1 '12 at 9:18
    
Exactly. Panorama (50m) is acceptable while Profile (10m) is preferred, certainly as far as Scottish Natural Heritage is concerned. Given the distances you are calculating a ZTV over (often up to 35km depending on turbine height), going for more detailed DTM data will probably kill most normal computers. –  MappaGnosis May 1 '12 at 10:03
    
Note: Scottish National Heritage guidance (here: snh.gov.uk/docs/A305436.pdf) recommends a standard viewer height of 2m, not 1.5m. –  veedub Oct 23 at 11:50

I'm assuming what you're after as a viewshed analysis tool. Check the wikipedia page for a brief introduction.

As far as i'm aware QGIS doesn't have this built in however there is a plugin available at http://pyqgis.org/repo/contributed . Not sure if this tool works with vector data however, but i suppose you could always do a conversion.

Similarly for GRASS there is a plugin available called r.viewshed ( http://grass.osgeo.org/wiki/GRASS_AddOns#r.viewshed ) that should be able to perform this task, however i believe it is only for raster data.

I can't vouch for either tool but hopefully this should point you in the right direction.

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Sextante has that feature, you can use it in gvSIG, Kosmo or qGIS; I've used it myself for some viewshed analysis of scenic roads and wildfires towers.

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Can you please expand? now that Sextante is integrated in QGIS I would like to use that to do a viewshed of a building, but I can't understand which is the right command. –  sanzoghenzo Oct 20 '13 at 13:29
    
@sanzoghenzo I am not sure if all the Sextante modules are implemented in QGis, I would suggest you use the Sextante packed in the gvSIG CE version (gvsigce.org); once in Sextante the tool you need is in Visibility and lightning>Visual exposure hope this helps –  jdeltoro1973 Nov 26 '13 at 16:19
    
thanks @jdeltoro1973! I cannot found a mac version of gvSIG, so I used the windows one in a virtual machine and everything went fine. Too bad I can't do it directly in QGIS! –  sanzoghenzo Jan 8 at 10:58

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