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I'm starting with a DEM, and looking to identify every point that can be directly "seen" from any point in a particular polygon on an overlaid layer defining an area on that DEM - like any other viewshed analysis, but with the additional stipulation that the observer points can only "see" at certain downward angles.

To help motivate what I'm looking for, my real goal is to identify run-out zones for avalanches. It's easy to identify "slideable slopes": just everything that's, say, 30-45 degrees. But if I want to identify the areas which snow would slide from the slideable areas into, the above quasi-viewshed starts to look like what I'm after.

My exploring the surface and hydrology tools (and help files for the tools) in Arc hasn't turned up anything that I think will do the trick. Any ideas - either in Arc or otherwise?

Thanks in advance for any insight.

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Viewsheds have only a tenuous association with slopes. Typically people look at contiguous areas of high slopes when identifying possible avalanche locations; those can then be propagated downward along flowlines. –  whuber Mar 19 '13 at 7:41
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2 Answers

Please have a look at Using Viewshed and Observer Points for visibility analysis

In the lower part of the article you will see that there are several parameters that you can use. The parameter that you need are VERT1 & VERT2

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To use these parameters in your ViewShed Analysis, your input point featureclass should have fields with these names. The ViewShed geoprocessing tool will then run based on the values in those fields.

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In GRASS GIS, you can use r.los (GRASS 6) or r.viewshed (GRASS 7) which also outputs the view angle. Then simply filter by this view angle. Also the observer's height over terrain can be defined.

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