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I am performing a Viewshed analysis in ArcGIS (10.1), and I use an OFFSETA (observer) of 1.7m and an OFFSETB (target) of 6m. The latter is the height of a typical building for which I wish to estimate the visibility across the landscape of a given study area. I would like to know: if a location (actually, a pixel) in the resulting binary Viewshed Raster is flagged as visible, I assume that a 6m-tall building placed at that location would be visible. But I also understand that we would not know if it is the whole building that would be entirely visible, or just (say) its top 1 meters.

In the context of a Viewshed analysis, is there a way to assess the amount of the building that would be actually visible?

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Visibility is in standard viewshed analysis is true if any part of the building is visible.

To do the sort of calculation you want, try using Viewshed2. This gives you an out-agl layer that is the amount you would need to raise the ground by to see that cell. So a cell where the ground is visible already will have a value of zero and, in your case any cells with values less than 6 will have some portion of the building visible and the proportion can then be calculated. For instance, if the out_agl value is 3 then 50% of the building will be visible.

Note however, that this is vertical proportion. To calculate horizontal proportion you will need actual building footprints and some definition to handle buildings at an angle. The horizontal proportion is more usually calculated as the percentage of the included angle of view or horizon (so you might be able to see all of a building from its narrow gable end, but it has a much smaller visual impact than the same building if you could see the whole of its long side (assuming a simple rectangle for the sake of argument here).

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