Transition Layer,
Herein lies a few of the problems
(a) the spot heights are
(i) not without an error dependent on the accuracy of the source data used
(ii) usually are for ground level (at least in this neck of the woods) - you were going to have a stab at tree height etc. ? What about cultural development not shown ? The only way to get around this either is to use fairly recent OPMs and make local enquiries of public authorities or get out on shank's pony and have a look from the ground with a theodolite over the shoulder. The latter often has to be done with nasty aerodromes .... If access is difficult, I have even resorted to flying the local area in simulated OEI climbs to check the base data.
(b) the spot heights nominated may not be the only significant concerns in the area
(c) sometimes there is a drafting screwup. An operator with which I have an association recently found a letdown chart in which the critical obstruction was located under one of the navaid boxes - ie not shown on the plate
None of this is difficult, although it can be a right pain, but none of it is amenable to pilot decision-making on the run. Conservatism is most appropriate, especially at night, or in low visibility by day.
People may choose to view Centaurus' views as unnecessarily conservative .. but I will continue to be more comfortable in the cabin with that sort of attitude up front ...
You pays your money ... you takes your chances ...
[This message has been edited by john_tullamarine (edited 29 June 2001).]