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I have a raster image whose values are a digital elevation model of terrain.

I need to know the surface type (topographic characterization) of particular points along transects of this DEM.

How to find it?

(In my application I have two points: a transmitter end and receiver end. I find the highest point between the transmitter end and receiver end: see the figure below. I would like to know whether the highest point is smooth or is a knife edge; that is, to within the resolution of the image it appears to be non-differentiable.)

smooth or knife edge surface calculation

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  • This seems to be at least two different questions: (1) Given a point clicked on a map of a surface, return a topographic characterization (such as "smooth" or "knife edge"--but note there are many other kinds of useful characterization, such as a saddle point). (2) Find the highest point between two given points. Which of these questions did you intend to ask? – whuber Feb 24 '14 at 15:57
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    Thanks @whuber I have only one question 1) Given a point clicked on a map of a surface, return a topographic characterization (such as "smooth" or "knife edge"--but note there are many other kinds of useful characterization, such as a saddle point). highest point between two given points is a OBSTECLE which surface type i want to know. – neeraj bharti Feb 24 '14 at 17:17
  • @ whuber can you tell me how to get topographic characterization of a map surface using arcgis. – neeraj bharti Feb 24 '14 at 17:20
  • I have attempted to edit this post to reflect your comments. In so doing I had to make a number of assumptions concerning what you are looking for, so please check over these edits to make sure they accurately represent your question. – whuber Feb 24 '14 at 17:30
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    @whuber: this is the exact thing i need. – neeraj bharti Feb 25 '14 at 5:01
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A simple indicator could be the slope around your point location (there is a slope function in ArcGIS Spatial analyst). Then you can take the maximum (or the mean) of the slope around your point using the focal statistics tool.

Alternatively, you could use the difference between the minimum elevation around your point (with focal statistics) and the point itself. This looks similar to slope if you divide by the pixel spacing, but the slope function in ArcGIS uses a neighborhood of three by three centered on your pixel, while with the minimum elevation method you capture the sharpest edge.

Note that the slope is the first derivative of the elevation, and the curvature (slope of slope) is the second derivative. In a raster, because of the built-in point spacing, you should always be able to compute the derivative (if you don't have NoData pixels, of course).

Also note that the proposed methods do not take the position of the sharpest edge into account. If you know the line segment between the receptor and the emittor, then you could use some zonal stats to have a "directional" topographic characterisation.

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  • Thanks radouxju for your answer. I have solved it three years ago. – neeraj bharti Nov 11 '17 at 4:58
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    Well, I hope that it will help someone else, then ;-) . Please note that you can answer your own question if you find the solution by yourself, this helps the community. – radouxju Nov 13 '17 at 7:06

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