Thanks for everyone’s comments but perhaps I didn’t phrase the question(s) quite right.
Firstly let’s not turn this thread into an argument about pressure settings – that’s not the issue here (I’m sure there are many other threads where that can be done). Regardless of what pressure setting they use they are still asking you to be in the piece of airspace. i.e. 500ft AAL.
The point I am trying to make is that if I were to accept this 500ft AAL clearance then (from a lawyer’s point of view in the unlikely event of being prosecuted) even if the ground is at the same elevation as the airfield then whilst I am going to be 500ft away from the ground I am NOT going to be 500ft away from the top of buildings, pylons etc that I have to fly over. Ignoring instrument errors and pilot accuracy then, albeit pedantic, this is fact and so if one flies directly over anything on the ground then this would be contravening rule 5. Once you then introduce elevation differences the separation may be even less (or more but that’s not the issue here). We have to follow a specific route so I can’t avoid over flying every object on the ground and it also takes us right to the airfield boundaries so I can’t avoid the ATZs without loosing some of the survey due to ‘ATC restrictions’
With regard to being prosecuted, I appreciate unlikely but, I am considering the instance where one is heard to be accepting the clearance and therefore 'must' be breaking the rules rather than simply being reported for low flying by someone on the ground. Therefore I don’t accept the clearance as I don’t want to put my licence on the line. If we have to miss a bit of the survey out it’s not my problem – ‘I’m just the pilot!’
So my questions are therefore:
Do others agree that it is (potentially) a contravention of rule 5?
Do others happily accept this clearance?
Would I do better using the expression 600ft MSD? (and would the military accept this from a civvy aircraft)As this seems to be a military expression can someone confirm the exact definition of Minimum Separation Distance? Does it mean ‘I am flying no closer than 600ft from any object and am not authorised to fly any closer’? If so then perhaps this would be a better statement when making the initial RT call.