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Using the same data, I ran ZTV's in ArcMap 9.2 desktop & QGGIS (using the viewshed plugin & r.viewshed) to compare them.

The ArcMap viewshed is different (more detailed/nuanced) to the QGIS/GRASS ones which were pretty similar.

Any ideas why, & which is more correct/better modelled?

  • What were the precise steps that you performed with each software? – PolyGeo Apr 5 '17 at 12:49
  • DTM = OS terrain50 -QGIS>viewshed plugin. Elev raster = my DTM, my Obs pt=shp with OFFSETA height. No intervis, radius 15000m, obs ht=2m(client, not me) Binary viewshed + earth curvature, precision Normal. -GRASS-haven't got access to that PC currently, but broadly same as QGIS -ArcMap: input surface=myDTM, obs pt=mt obs pt with OFFSETA height, earth curvature, Z=1, cell=50 – user3418765 Apr 5 '17 at 13:02
  • There is an edit button beneath your question that you should use to revise it with these and any additional details. – PolyGeo Apr 5 '17 at 13:15
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The results have to differ because the algorithms in the softwares, you mentioned, are not identical. Fortunatelly, the algorithms do not differ much. Of course, there are differences in the outcome but those are rarely essential for the overall impression of the analysis.

You can take a look at source codes for QGIS and GRASS to figure out how the viewshed is calculated, but unfortunatelly the algorithm used in ArcGIS is not publicly known. If you are intrested in details, about the differences amogst algorithms that can be used to determined viewshed, there is a great article by Pete Fisher covering this topic - http://www.tandfonline.com/doi/abs/10.1080/02693799308901965.

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