Munnyspinner
the CAA designated PSZ was changed about 10 years ago from a fan shape spreading out from the end of a runway to a triangle with its base extending either side of the centreline and coming to an apex as it extends from the end of the runway. For example LHR has a PSZ designation of both 27L and 27R which extend from a base 350m either side of centreline to an apex at 3.5Km.
Th PSZ is based on modelling of aircraft accidents since WW2 apparently - i.e. the further you get from the end of the runway the less likley you are to crash and, if you do, the more likley that you will be close to the centreline.
On this basis I would agree that within the perimeter there seems to be a very srong arguement that ground is free of non moveable obstacles which could potentially create a more dangerous situation for aircraft in an runway excursion. This would include large gullies and ravines.
Difficult without a picture ... does that triangle originate at the take-off threshold? or the departure end?
If the first, it just avoids the LHR terminals lying between the runways, but doesn't even reach the far end of 27 l/R..
If the second, the MAD quarry/ravine lies outside, as does everything and anything in between the parallel runways.
Innit?