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I want to make a visibility analysis of planned wind turbines in my region.

I have a DEM raster and a polygon describing the possible area where the wind turbines might be set up.

People are complaining, that the view to a prominent mountain in the region will be disturbed.

My "simple" plan is, to raise the DEM-values covered by the polygon up to 200 meters and then perform a visibility analysis from the top of that mountain to be able to identify the regions, where the wind turbines might "block" the view.

Problem: I'm not able to successfully combine the vector and the DEM.

Steps taken so far:

  • Polygon was clipped from the DEM
  • Raster values from the clipped part were set to 200

What doesn't work:

When using the raster calculator, trying to add DEM + clip, the resulting image only covers the clip.

As i understand, the extend of the two rasters should be the same, and that NoData values will prevent operations (googling for 3 hours without success).

I fail in resizing my clip to the DEM and also do not get rid of the NoData values.(Really tried every tool under Raster to no avail)

Unfortunately, the rastercalc tutorials i found do not cover these "basic" things, so i'm completely lost at the moment.

Thanx Bernd

p.s. using latest qgis master from osgeo4w

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2 Answers 2

I have done a LOT of work in this field and I strongly suggest that you DON'T pursue your current plan. It will completely lack credibility with developers, the community and planners alike. You need to do a proper Zones of Theoretical Visibility (ZTV) analysis rather than fudge the issue. I also strongly doubt that your turbines are 200m tall, so your analysis will be worse than the worst case. I know such turbines exist but they are almost exclusively used offshore (apart from a few location on continental Europe).

As you are using QGIS, I guess you favour Open Source tech. Therefore I suggest you use OpenWind and set up your turbines with correct hub and rotor dimensions at the correct coordinates plus a suitable DTM (at least 50m resolution is recommended by best practice and 'very-best practice' suggests a 10m resolution), account for the curvature of the Earth and refraction. A ZTV is typically done as a 'bald earth' calculation (no trees or buildings) but where these are vital to describe the visibility more accurately - then a hack to the DTM like the one you are suggesting can sometimes be acceptable (where conservative estimate of tree and buildings height are use or by estimating heights from LIDAR data for such obstructions)

You could use GRASS (as a plugin to QGIS or standalone) and indeed ArcGIS (with Spatial Analyst module) and there are various proprietary WindFarm software packages too but you should find OpenWind simpler and easier to use than the others.

Doing the ZTV in one of the packages mentioned here will get rid of your problems about combining two rasters (which you could simply get around by reclassifying your NoData as 0 and then simply add them. To do this, use the commandline in the dialog box to add an -init value of 0 when you rasterize the polygon - see link for syntax).

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Thanx for your hints and warnings, and i agree that it would be much better to use special software for that. But, 1. yep those turbines are planned with a max hight from 180 to 200 meters 2. I just wanted to have a quick and dirty overview as a starting point (and it's nothing official anyway at this moment) 3. My biggest concern now is that i seem not to be able to do stupid raster calculations, apart from the fact that the results might not be accurate. –  Bernd V. Jul 22 '13 at 15:26
    
Yikes! Those are monsters. I take it this is not the UK (they'll never get past planning here). Check my edits which I made while you were replying - which tells you how to rasterize the polygon properly. Are you doing this on behalf of the community or the developer? –  MappaGnosis Jul 22 '13 at 15:32
    
+1 There's a lot of good advice here. –  whuber Jul 22 '13 at 17:50
    
You guys really put me into a dilemma ;). The answers are really helpful and perfectly fitting for a Q&A on viewshed analyses, but the raster calc part which i was interested in mostly is really not that "simple" as described. I think i will rephrase my question to fit the answers and open a new one on the raster processing when i found a way through. –  Bernd V. Jul 23 '13 at 9:04

You might want to look at the following plugin for QGIS http://madchuckle.blogspot.ca/2010/05/effects-of-precision-in-viewshed.html it is for viewshed analysis. It is a little slow, if you are using large areas. I have done a lot of work in this area. My Masters research was related to viewsheds and perceptions. One thing you may want to consider is putting in any forested areas, as this will greatly impact the view of the turbines. If your interested I could share it.

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I think none will refuse to get hands on yet another possibly helpful plugin, so please share it :) As a basis, i only have a 200 meter free LIDAR grid, which i think i might pimp up a bit through interpolation with digitized structure and break lines. In addition, i downloaded the CORINE data for forest areas now, and will try to elevate my DEM at these parts with sth like 20 meters. Thanx for your input so far! –  Bernd V. Jul 23 '13 at 9:14

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