What is an 'obstacle vista diagram'?
Just taking a guess from the context, I think it could be a "view shed", the term used in GIS (Geographic Information Systems) to refer to a visibility analysis. In other words, it is a calculation that determines which areas of a given terrain would be visible from a specific point.

It is used to know, for example, what would be the coverage of a radio transmitter on a given location, or from which areas would a lighthouse not be visible, etc. To calculate such diagram you need a DEM (Digital Elevation Model) of the area.

To give you an example I just loaded a DEM and run a calculation (using Global Mapper) of the areas visible from a 20 meters tall tower in the top of a specific hill, and within a 40 km radius. And this is how the results looks like:


Where the red shading show the areas that are visible from the tower. The rest of the terrain would be hidden behind topographic obstacles.