Visual Exposure: Alternative Approach

Cumulative Visibility
First, a cumulative viewshed is created in TNTmips. A viewshed for each turbine is created at 10m height intervals (up to the tip of the rotor as a worst case scenario). These are then added together using a raster function (I have to use TNTmips since it's impractical to do it in Manifold using vector data structures). I then import this cumulative viewshed into Manifold.

Viewshed Grid
This part is very RAM intensive and can be painfully slow. Copy and past the cumulative viewshed as a Table, then cut and paste the Table as a Drawing ([Viewshed Grid] in this case). The result in this case is almost 3 million points.

Minimum Distance Query
I normally create a separate ID column for use in these queries. Create new 16bit Integer column [VGRID_ID] and fill it with the intrinsic [ID] column values. The query to find the minimum distance of each grid point to the wind farm is as follows:

I had/have to use the wind farm area rather than the turbines (i.e. an area object rather than a set of points) otherwise the query takes too long to be practical. All points of [Viewshed Grid] inside the wind farm area [Turbine Area] will therefore have a minimum distance of 0m from the turbines which isn't as accurate as one would like, but for now that will have to do (hoping a combination of more RAM, version 9 of Manifold and a CUDA enabled graphics card somewhere in the future will make a point based query more viable).

 Rank Column
Now I use the Manifold Decision Support System to calculate visual exposure from visibility [VISIBILITY] and minimum distance [MIN_DIST] columns. High Proximity (the closer you are to the turbines the higher the visual exposure):

The Low preset assigns 100 percent desirability for numbers up to one standard deviation below the average, and then decreases desirability in a smooth ramp to zero for numbers above the standard deviation.

 High Visibility (visibility increases as you see more of the wind farm):

The High preset assigns zero desirability to all numbers below the average and then increases desirability in a smooth ramp to 100 percent for numbers that are greater than one standard deviation above the average.

I added the Hedge value of very to the High Visibility criterion in an attempt to compensate for the 0m minimum distances within the wind farm area (see Minimum Distance Query above), otherwise this area showed a visual exposure that was too low to make sense (to me). It also improved visual exposure ratings in other areas of the study area as far as I could tell.

Visual Exposure Surface
Once the Rank Column did its thing (which can take a while as it churns through 3 million points) I then copied the Drawing [Viewshed Grid] and pasted as a Surface without using interpolation and using the same pixel size as the original cumulative viewshed. The visual exposure surface may be offset a couple of meters due to the transformations involved in going from Surface to Drawing to Surface, but it's easy enough to shift it to fit accurately onto the original cumulative viewshed (since nothing has changed in terms of visibility - that is, if the turbines are not visible from an area, the visual exposure will also be 0).

Finally, the map. Does this approach make sense? I'd appreciate comments and suggestions. I realise this is not entirely robust science here, but I feel this approach makes more sense than the standard one where distance buffers are used to represent visual exposure. The distance buffers do not take into consideration the size of the development.

Visual Exposure

Visual exposure is based on distance from the project to selected viewpoints. Exposure or visual impact tends to diminish exponentially with distance. - Oberholzer 2005

I think this is at least one way of indicating the distance effect (see first image - it's not the best example but it does give one an idea?). I created buffers around the development site point (100m, 250m, 500m, 1000m, 2500m, 5000m and 10000m). In the following way (a bit of a mission, but doable):

• Start with 10000m buffer. Copy and past to new drawing. Call it something like Exposure Buffers (EB)
• 5000m buffer -> into EB. All the way to 100m
• In EB, create a column called Distance.
• Enter the buffer distance for each record.
• Make sure the records are in exactly that order (from 10000 down to 100). If it's not in that order you're going to have to cut buffer polygons and paste them back into the drawing to get the right order, because when you paste polygons into the drawing they are placed on top of each other so that the larger 10000m buffer will be placed over the smaller ones.
  



• If they are in the correct order, open the drawing (EB) and run Normalise Topology on it (in the Transform Toolbar - normally bottom right hand of Manifold window). This will 'cut' each polygon through the larger one behind it (not sure if that describes it adequately but if you select e.g. the 10000 polygon before Normalise Topology and then afterwards you'll see what I mean). If the order wasn't correct this will mess up the drawing and not all the buffers will be present anymore.
  
• Now place the Viewshed (VS) drawing in a Map component with EB (i.e. create a new Map with EB and VS in it). Order here is not important. Select Topology Overlay from the Drawing menu at the top. You'll get something like the third picture.
  
• In the dialog that opens up select the viewshed VS in the Data combo box, EB in Overlay and choose Intersect (areas - areas) as the Method.
• When you run this a new drawing will be created in the Project pane on the right called [VS 2]. This should contain the viewshed polygons split into the buffer zones and the Distance column should be available in the drawing. Use the Distance column to set up a range of colours. I coloured the Fill and Outlines of the polygons using the same theme so it sort of grades smoothly.
• I may have missed some points along the way. Let me know if it doesn't work.

Oberholzer, B., 2005. Guideline for involving visual & aesthetic specialists in EIA processes, Cape Town: CSIR, Provincial Government of the Western Cape, Department of Environmental Affairs & Development. Available at: http://www.capegateway.gov.za/Text/2005/10/5_deadp_visual_guideline_june05.pdf.