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.
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