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I am confused by the description of OFFSETB in wiewshed analysis (ArcGIS 10.1).

The help documentation says:

The OFFSETB item indicates a vertical distance in surface units to be added to the z-value of each cell as it is considered for visibility.

Still, to me is rather obscure.

I understood that OFFSETA should be the value representing the height of the observer point: that is, if an observer is 2 meters high, and (s)he is on a spot 10 meters above sea level, the total elevation of the observer point will be 12m.

As to OFFSETB: is that the height of a target object? What this actually represents? Or is it the height (in the landscape) below which the viewshed is not calculated?

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Offset B is the height of the target object.

Imagine that you want to know if you would see the top of a NEW house in the landscape (the house is not there yet). With offset B, you can define that height and it will be added to the base height in order to test the visibility of the roof top. The difference with an extrusion (adding a value to the base height) is that you test the visibility but it does not hide anything behind it.

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