R4D and I like to joust .. or am I just an incurable cynic in my views ? ... certainly I am of the committed view that EVERY decision made is subject to subsequent legal scrutiny and censure. A cursory review of some legal decisions over the years in Australia alone gives chilling food for thought.
I guess that what I was saying in a round about way was that the interpretation which counts is the judge's after the event. In Australia, for instance, it is often very difficult to get some elements within CASA to commit themselves to an interpretation of their own regulations. Certainly it is my understanding that CASA personnel are very wary of the judge's subsequently holding their interpretation to be invalid.
Apart from general PD reading, I have no legal competence, only considerable wariness.
Mutt,
I would be checking the basis of the charts you use. Those in my ken have ground level spot heights, to which one has to add a vegetation allowance.. and there are some big trees out there. Charts may also include heights of specific cultural objects .. towers and the like.
noflypaul,
Don't worry too much. If you get involved in this sort of thing, you must expect to get headaches ... why should you miss out on the fun ? In fact, doing the number crunching is the easy bit .. getting good obstacle data is the difficulty.
I have very jaundiced views of some commercial services. Prudence dictates that I choose not to discuss such things further in this forum.
Trying to do an independent check is very difficult. First you have to get the provider to define the obstacle set used. Then, when you determine your own, it is likely to be different. Unless you input the same data set, why expect to get the same answer ?
It is all very much dependent on the sources of data. A difficulty facing the larger providers is one of locating local data which is more accurate than that which is readily available ... this takes local contacts and knowledge. Much of the really useful data is not public domain and can be extremely difficult to locate. We all develop our appropriate contacts in the geographical area of operation. I end up with data from sources such as mining companies (often very useful), water supply and other local authorities, and so forth. If needs be ... out in the 4WD and take a look for oneself. Sometimes hire a light aircraft and do an aerial inspection to narrow down the specific areas of interest which might need further investigation.
At the end of the day, you pays your money and you takes your chances.. and, to do a good job, it takes resources and time .. and that costs dollars. The crunch comes when the crunch comes and you have to justify your actions and processes.
[ 27 October 2001: Message edited by: john_tullamarine ]