Perhaps I should apologise a tad for waxing too lyrical before ...
Based on the STODA/TODA data, one cannot presume to have identified all the interesting obstacles. Rather, the reverse calculation determines the intersections of the various slope surfaces and one ends up with a less restrictive polygon style of surface.
For each STODA surface, the surface will be determined by the relevant critical obstacle .. this may be the same obstacle for one, some, or all surfaces as the case may be.
Often, there will be only one or two significant obstructions (often the perimeter fence or tree line (or presumed vehicle) along the road and it is usually pretty obvious from the analysis if this is the case.
However, one must still ensure that one postulates a composite surface defined by the polygon by putting in a few dummy obstacles along the surfaces. The end result is a set of "obstacles" which may, or may not, identify the actual obstacles, but defines a safe surface upon which to base a quasi-discrete obstacle takeoff analysis. Generally far better than using the base slope data as the aircraft can take advantage of successive lower slope surfaces.
The trig is straight forward but there are a few traps in setting up the analysis .. suits spreadsheet calcs quite nicely.
So far as reacting to NOTAM changes, this approach works a treat as the whole exercise is based solely on RDS information. Just need a PC to hand to do the sums and run the takeoff analysis. Not suited to pilot calcs via general takeoff data which remain tied to matching available aircraft gradient to available declared surfaces ..