h a l f b a k e r y
It's as much a hovercraft as a pancake is a waffle.
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No carpenter's toolbox is complete without the trusty spirit level. However, traditional devices may give inaccurate results in certain circumstances: aboard ship, in orbit, or close to a rotating gravitational quadrupole. This will lead to ill fitting doors and out of true wardrobes.
GPS spirit level. This instrument has a GPS receiver at each end. The positions of the two ends in space are established, and the difference in altitudes (from the earth's centre) is computed. This difference is displayed using a line of LEDs.
||Ordinary GPS isn't accurate enough, even post-SA. You need differential positioning systems. (Localizers!)
||Actually, this *would be* a differential positioning system. You don't have to know the absolute height of the two receivers, only their relative height with respect to one another. It's still doutful you could get enough accuracy here, since you'd need about one
mm or less error. You may be able to improve things by exploiting the fact that the receivers are a known (and small) distance apart and can share some receiver hardware, but I'm not sure how much this would buy. You might be able to further improve accuracy if you had a "standard" level plane against which you could calibrate it before each use (e.g., put it on flat ground).
||In any case, even if this could be made to work, you still would have the problem of needing an unobstructed path to the satellites, which would pretty much limit such a device to outdoor use.