I'm looking for a way to calculate the angle of view from a point on a DEM, in particular from the highest point of a terrain relief (corresponding to the red dot in this image below).
To better explain the case: I have a cell with an altitude of 237 m asl. I'd like to know the angle of "free" view from the observation point when it's not blocked from the surrounding relieves, setting a maximum distance over which the view is considered free. Whenever the DEM exceeds that altitude, the view from the observation point is blocked by the higher relieves. Here's a graphical example of what I mean.
I've tried several times with the r.viewshed algorithm in QGIS. It let me choose a maximum distance, but it doesn't calculate the angle of view and it gives other types of outputs.
Is there a way to obtain this information?