They also vary over time or something, right? So even a more precise measurement device, to get better gravity maps, doesn’t solve the problem… maybe? This is all based on half-remembered other comments.
So I would like to learn more about gravity map matching. I've seen offhand references to it in context of placing an ICBM on target (can't find ref now) but that's it. I imagine it is, (or was for long enough to make info sparse) a classified piece of know-how.
For sufficiently accurate IMUs, like the ones found in ICBMs, you need a gravity map because otherwise the variations in gravity cause your position estimate to drift
That's helpful; so presumably for the same reason that gyrocompasses function (self-aligning vis-a-vis the local gravity vector).
A part of me was subconsciously entertaining that a subsystem of the IMU / INS was actually being used to provide gravimetric navigation (producing a coarse global position update de novo rather than correct for an error-prone open-loop one from an a priori generated gravity map).
a la