Beagle,
Thanks for that, I have been an aerial photographer and sometime unofficial pilot ( a little knowledge is a dangerous thing ) and you've answered a few of my queries.
One remains though, as I used to work at Dunsfold alongside the Instrument Calibration dept; even with their modern counterparts' best efforts, my engineering training says ' how the hell does one think it's 300 knots not 299 or 301 ?!
I agree that's the mid range and ideal for calibration, but even so...
I've also wondered about the U-2 ( TR something ) as it's said the pilot at altitude needs to keep within a 3 knot margin; either it has a much better pitot system than I've ever seen - Harrier mainly - or it's a fairy story.
I come across the same calibration problem with yachties; just because their depth-sounders are now digital, they trust it to the last decimal point then wonder why they've run aground.