Careful Melvyn, I sense some maths errors here:
“you might have an airfield at 3360ft and the circuit height is 3860, ie 800 feet above ground.”
In fact, in your example, the supposed circuit height is only 500ft. This serves as a great reminder to work out your heights before you even get in your aeroplane, in a ‘no stress’ situation.
If you set QFE, the altimeter should read fairly close to zero (I use within 50ft). Keep in mind height differences around the airfield, and the time of the reading. This reading will then give you an approximate height above that one airfield.
If you set QNH, the altimeter will give you your height above the sea level. It is up to you to do the maths, and calculate circuit heights etc.
‘Standard – 1013.25 millibars, or 29.92 inches’ is set to provide exactly that. A standard height so that traffic over the coutry can accurately gauge their height with all other airborne traffic. It still won’t tell you your height above terrain, or over the sea. MSA’s are helpful for this though.
If you wander away from the airfield (especially overflying terrain) then there is no real way of accurately ascertaining your height above the ground with the use of the altimeter alone. You need a radar altimeter for that – seldom seen in GA aircraft.
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