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How do I calculate height when attempting to do a viewshed analysis in ArcGIS10? I am able to create a viewshed analysis from a poly point file, however I need to calculate for the 80ft to 100ft tower height as well. In 9.3 I was able to directly input the different variables, but in 10 the Environment Settings are different.

I adjusted the Z values under the Environmental Settings, converted everything to meters and then ran the viewshed but it was no different from the original.

Any suggestions?

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  • The environmental settings do not determine point-specific parameters like OFFSETA. (They never have, not even in ArcGIS 9.3.) Please read "controlling the visibility analysis" in the ArcGIS 10 help.
    – whuber
    Apr 26, 2011 at 20:40

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In your Tower point attribute table, you should have one field called OFFSETA which contains the height of the tower. In your case, 80 or 100 feet (or, 24.4 or 30.5 if the DEM is in meters). This is sometimes confusing, because the tower point is being used as the observation point. However, this is the correct way to do the analysis.

Reading your 3rd comment on Dan's answer, it looks like this is what you are doing (which is correct). So, why is there no change in the output? It might be due to the accuracy of the DEM you are using for the elevation raster. By default, OFFSETA is set to 1 (~3 feet if DEM is in meters). Assuming the tower points are all at high points in your DEM, there might NOT be any difference between a 1 meter OFFSETA and a 30 meter OFFSETA. Or, the difference might be so slight, you don't notice it.

A couple of things you could do to test: Use a really big OFFSETA (e. g. 1000') or move a tower location to a spot where you are sure a 100' OFFSETA will make a difference in the output. (e. g. 50' down-slope of a peak).

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  • @Susan (1) You seem to be using OFFSETA in place of OFFSETB. (2) Viewsheds usually do change a little, even with slight changes in OFFSETA or OFFSETB. For some of the subtleties, see the old ESRI forums here, here, here, and here. For more like these, Google the forums.
    – whuber
    Apr 27, 2011 at 18:35
  • @whuber, (1) I believe OFFSETA is the correct field to use as Susan's question appears to be similar to the second scenario you describe in your answer to the first forum question you link to in your comment. This will raise the height of each tower point 80 to 100 feet above ground level before computing the viewshed.
    – Don Meltz
    Apr 27, 2011 at 20:09
  • @whuber, (2) I agree with that. I am only suggesting Susan might not be zooming in and looking very closely at the output. There should be some change, even with a 100' OFFSETA. If every pixel is truly identical between the outputs, then something else is going on.
    – Don Meltz
    Apr 27, 2011 at 20:17
  • (1) Ah, I see there's room for opposite interpretations: are these observation towers or are they objects to be seen within the viewshed? Thank you for the clarification offered in your response to (2).
    – whuber
    Apr 27, 2011 at 20:52

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