Old age onset:
I forgot to mention that the threshold elevation can play a significant part in determining if an object close to the approach surface is treated as an obstacle; i.e. if the threshhold is lower than the ground on which the object stands, then the object's elevation increases by the difference between threshold and ground height (and vice versa).
Likewise for potential obstacles outside the runway strip that could infringe the transitional surface.
Mister B