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I need to create a raster layer with cumulative values for 72 viewsheds. Each viewshed is from a different point along a line and has been clipped to a 20km buffer from each viewing point.

When I try to add them all together in raster calculator in QGIS, I am left with a raster that only covers where each viewshed overlaps (see image below with 3 of the rasters covering different areas and the result from the raster calculator in red where they overlap.).

I need the coverage to be over the whole area so it will tell me where only 1 point is visible in an area all the way up to potentially all 72 being visible. Is there a way to do this is QGIS?

QGISsnip

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This can be done, but doesn't need Raster Calculator. You could sum individual visibility rasters, but it isn't necessary as there's an easier way.

You can use the QGIS Plugin Advanced Viewshed Analysis which has a cumulative option. There are other viewshed algorithms (SAGA and GRASS) which might allow this too, but from memory, the plugin is best for this type of analysis.

the options you need are shown below:-

  • select Binary Viewshed
  • check Cumulative (for raster output)
  • Radius 20000 meters

enter image description here

The raster output cells contain the number of viewpoints which are visible at that cell.

If you don't check "cumulative", it'll create a binary viewshed raster for each point. I found this out the hard way with several hundred raster layers opening up :-)

Note : this plugin assumes square cells, so it works best on UTM rasters where the x-pixel and y-pixel sizes are equal.

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