John_Tullamarine, sorry that it took some time to respond to your “polygonic obstacles” suggestion, but the powers of darkness that control my roster decreed that I be in other places.
Your suggestion does have a lot of merit. I’ve always been extremely reticent in any activity which might be construed as interpolation between STODs, because of the “unconsidered” obstacle that might lie between them. The diagram illustrates my concern –
The diagram shows the TODA with it’s associated OCG, and a 3.3% STOD, essentially two STODs. Obstacle A has been used by the surveyor in establishing the TODA OCG, Obstacle C has been used in establishing the 3.3% STOD. Obstacle D is of no concern to the STODs, but will have to be considered for the 3rd segment acceleration altitude. My concern is for Obstacle B, which has not been considered for either the 3.3% STOD or the TODA OCG, if, however, we were to interpolate between these 2 STODs (as shown by the 4.1% broken line in the example), we will collide with Obstacle B. For this reason, any RTOWs I’ve created using multiple STODs have had an interpolation prohibition between data elements imposed. Your suggestion of the creation of “Phoney” obstacles at OCP intersection points, suitably labeled the “JOHN_T POINT” in your honour, may well overcome the unconsidered Obstacle B. I cannot conceive of any circumstance where Obstacle B protrudes above the polygonic series of OCPs, can you?
But now to the unconsidered Obstacle D, which existence we don’t know of from STODs or the RDS. Do we consider one final obstacle at the lowest published OCP at the Survey Limit? For a 1.6% STOD and a 15000M Survey Limit, that makes a final obstacle of 787.4 ft, with a corresponding acceleration altitude of 1216.1 ft, pretty high! OK, I’m being a bit nit picky here, by that time we’ve passed most nominal acceleration altitudes anyway (400, 800, 1000 feet, whatever!) and are already ‘down and dirty’ looking at discrete obstacle analysis.
All good value stuff John_T, thank you! Still…because I don’t know where the obstacles really are, I still have to keep 1st segment above the Runway and the OCG, be it the STOD Gradient, or the Gradient to the “Phoney” obstacle. Much thinking needed about this new can of worms you’ve handed me.
Now, as I was saying about composite Runway Slope…….
Regards,
Old Smokey