I am trying to use the Viewshed Analysis plugin in QGIS to create what is sometimes known as an 'inverse viewshed' - essentially querying where I can see points from the ground, rather than where I can see the ground from points. (Same result as a viewshed, and usually the same GIS query, but just answering a different question.)
I have a raster DEM (ELEVATION RASTER), and a points shapefile (OBSERVATION POINTS) which contains a field denoting height above ground for each point.
My issue is that because I want to query which areas of the DEM my points are visible from, rather than the opposite way around, I want to set a viewer height on the ground of 2m (in effect adding 2m height to each sampled Z value point within my DEM).
I know this can be done in ArcGIS using OFFSETA / OFFSETB, but in the plugin in QGIS I only have the option to set the height of the observation points (which I have set to the field showing their height above ground).
Is there a simple way to do this?
Should I create an additional field in my points shapefile adding an additional 2m on to the current height above ground - resulting in an extra 2m on each point height but a 0m height on the ground - or will this give an erroneous result?
Should I create a new DTM 2m higher than the original?