I'm attempting to run a viewshed analysis in QGIS using r.viewshed, but the terrain is hilly and building is wide and low, so I get dramatically different results depending on which corner of the building I use (or any other part of the building). Is there a suggested workflow for generating a viewshed which shows me the visibility across the landscape of any part of the building? Can I generate a viewshed from multiple points?
You can create individual viewsheds for the four corners of the building (or any number of points) using -b flag to generate 0/1 output rasters. After it use the r.mapcalc to calculate the sum of the viewsheds belonging to a point. This way you get a raster where the value is the number of corners/point you can see that raster. If you select from the sum raster where the value equal to the number of points, you get the area which is visible from all points.