1) Altitude is a continuous quantity, not a discrete one. You cannot be at 1500 ft, only above it or below it.
I must admit, this has thrown me ten left... how can you not be
at an altitude (i.e. a specific distance above mean sea level)
Does this then mean that any separation based on aircraft at known altitudes is null and void??
I thin I understand where you are coming from but it is
Mean Sea Level - otherwise how can a base of a CTA be
at an altitude?