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At Arc Desktop 10.7.1 I am conducting a viewshed analysis using the Viewshed tool. I am trying to understand how the input raster cell size affects the area covered by the output viewshed raster.

This screenshot illustrates my issue. The yellow symbol, lower-right corner, is the single input point, with OFFSETA = 200 m. I conducted two Viewshed analyses, one where the input raster was a 10-m DSM (blue output), and the other where the input raster was a 3-m DSM (red output, superimposed over the blue). The input DSMs were generated from the same lidar source.

The screenshot includes a 3-m DTM hillshade as a viewing reference. It was not used in the analysis.

The blue and red viewsheds radiate across the body of water located in the screenshot center. It is obvious that the viewsheds are influenced by forest along the southeast shoreline (the input DSMs are not shown here). Thus, the general output trend makes sense: the shoreline tree canopy "shades" the water for a distance to the northwest, where the viewshed eventually comes down to the surface.

QUESTION:

I am surprised that the blue viewshed (10-m input DSM) covers such a greater extent than the red viewshed (3-m input DSM). I expected the blue viewshed to cover a slightly larger area simply due to the larger input cells. But how to explain the tremendous output difference shown in the screenshot, particularly within the highlighted area?

I considered that perhaps the viewing angle is dependent on the input cell size. But the ESRI Viewshed documentation simply states, "The visibility of each cell center is determined by comparing the altitude angle to the cell center with the altitude angle to the local horizon." It does not mention a varying viewing angle.

A web search turned up nothing regarding input raster size and the resulting viewshed extent. I thus turn to SE for assistance.

enter image description here

  • Did you explore the actual altitude of your cells? There is certainly some differences in cells altitudes at a same point due the difference in resolution, e.g. some bumps may be diluted in larger cells or vice versa. What seems astonishing in your image, is that they seem to lay in a flat region. Could you check this altitude difference out? – s.k Apr 4 at 22:17
  • Because larger cell result in a smoother surface, e.g. if the whole raster made of 1 cell, you'll see all of it. – FelixIP Apr 4 at 23:37
  • s.k I'm not sure what you're asking. If it helps, I did create the optional agl output raster when I ran the Viewshed tool. Are you asking me to look at the difference between those agl rasters? – Stu Smith Apr 5 at 0:41
  • FelixP I subsequently re-ran the viewshed with a 30-m resolution input raster and, as before, the output viewshed was larger than the 10-m input. All of this has got me wondering...What is the appropriate input cell size? I may ask in a new SE question. – Stu Smith Apr 5 at 0:45
  • 1
    The answer is as small cell size as you computer can handle, because finer resolution better depicts real terrain. – FelixIP Apr 5 at 0:52

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