The vertical profile of base of airspace defined by a FL on Skydemon could be hundreds of feet plus out because SD doesn’t know the QNH.
I wonder how many users are aware of that? Here, yes, probably everybody 
I’ve just got this in an email. Never thought about it beforehand to be honest!
Naturally, but how relevant is this?
How much complex airspace is delimited on the basis of FL rather than amsl or agl?
With complex I mean airspace where tools such as SD come in handy as an aid – as opposed to “known” airspace such as class C above FL100 in lowland Germany?
How much complex airspace is delimited on the basis of FL rather than amsl or agl?
An annoying amount of airspace in the UK is defined this way – although it is getting less as the TA has been raised to 6000ft in most places.
Surely someone navigating close to such airspace will not be using SD as the primary means of ensuring they remain outside?
Aren’t you just saying GPS altitude is not a FL – or have I missed something?
Patrick: You mean C below FL100? Above is not non-complex, above FL100 is trivial. But in general any airspace where you are supposed to set the altimeter to standard is given as FL. That leaves a bit at the far out edges of C/D airspaces and some restricted areas. But in Germany I don’t see any complex airspace structure at all.
JasonC, I think you underestimate the “creativity” of some pilots.
Never thought about it beforehand to be honest!
Me neither because every aircraft I fly has one or more altimeters installed and zero skydemons 
But kidding apart, how would the value of the transition altitude – 3000, 5000 or 18000ft – have any influence on this “problem”?
To be fair, GPS altitude isn’t an exact match for barometric altitude either – there is always some error due to temperature differences. Its just that this error (approx 4% for every 10C away from ISA temp) is much smaller than the ones from pressure.
I’ve had a few hot summer days where skydemon thought I was in the London TMA when I was happily cruising at 2400ft.
IMO its just something you have to be aware of – GPS is v useful for horizontally avoiding airspace, but vertical avoidance needs to use an altimeter.
how would the value of the transition altitude – 3000, 5000 or 18000ft – have any influence on this “problem”?
These laptop nav tools are often used for tactical airspace avoidance. They use GPS altitude for that, which isn’t any good if the CAS is defined in terms of a FL.
And with e.g. Jepp dropping their VFR charts in 2013, more and more people are flying solely with a tablet app. Much of Europe now has no usable VFR charts (e.g. Italy).
Its just that this error (approx 4% for every 10C away from ISA temp) is much smaller than the ones from pressure.
It isn’t if you go high enough. In the summer, it is quite normal to see say 15500ft on the GPS against 15000ft on the altimeter. The GPS is of course right in the sense that if you wanted to fly right up close to the summit of a 15500 mountain you would use GPS altitude, not an altimeter.
how would the value of the transition altitude – 3000, 5000 or 18000ft – have any influence on this “problem”?
Above 18,000ft very few people care about controlled airspace – they are [almost] all under positive radar control. So moving it up there gets rid of the issue. Except it’s not an issue.
But kidding apart, how would the value of the transition altitude – 3000, 5000 or 18000ft – have any influence on this “problem”?
because SD shows everything at altitude… so FL 50 is actually shown as 5000ft
But in real life FL 50 can be below 5000ft
So SD can show that at 4800ft you will be below this CAS starting at FL50 when in fact you may be entering it…
if you put TA at 18000 ft then everything CAS under it has to be expressed as altitude and not FL
And most of us won’t fly VFR above 18000ft
Hope i got that right and explained it clearly
