Hi Bruce..
Altimeters are assumed to be accurate within 125 feet when set to the local
barometer reading... If the altimeter (including the pitot, and static air
systems when mounted in a plane) is certified for instrument flight rules
(IFR) then it is assumed to be accurate within 75 feet... Now, this is
absolute accuracy, not repeatability, which is what you are interested in when
using it to measure the slope of the local terrain.. I would assume that the
repeatability is in the ball park of 5 feet, depending upon the amount of
damping in the gear train.....
i have no exeprience with barometers designed for use in surveying but I would
assume them to be geared to be far more sensitive to variations of a few
inches... They would not be useable as altimeters in a plane because the
needle would be wagging wildly with that kind of sensitivity... Even the
pulses of air as the propellor blades went by would cause them to dither...
Altimeters have more damping in the gear train....
Altimeters are reliable instruments because we routinely shoot approaches
where the cloud bases are within 200 feet of the ground (or obstructions)...
If you assume a maximum error of 75 feet in the instrument you can be as close
as 125 feet above the ground before breaking out of the cloud....
Cheers ... Denny
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