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Interesting fact of the day.. a magnetic compass (when properly designed) doesn't just tell you the direction to north and south poles, it also tells you how far north or south you are (latitude), Simply put magnetic dip (the tendency for a magnetic compass to dip down or up) can be measured and used to determine latitude or compass direction.
I suspect the only reason we dont have digital compasses that can do this is simply because GPS is a more accurate and simpler to measure. But it would not be technologically very difficult to do.
@robryk you would need a map of the magnetic fields, a high accurate sensor, and you'd not even approach GPS level accuracy.. but its doable in theory yea.
People use (I don't know how successfully) gravimetric anomalies to supplement inertial navigation underwater (in a similar mode to those early car navigation systems that could never locate you, but would use their knowledge of how the roads look like to correct dead reckoning). I wonder if this could be used in a similar way.
As a scuba diver I can speak on underwater navigation a lot and have tested a lot of systems that approach it in different ways... Not sure of any gravimetric based systems though, doesnt mean they dont exist I just cant speak on that specifically.
@freemo I speculate we can't get enough accuracy in practice due to environmental disturbances e.g. magnets. It's ok to be a few degrees off when pointing north, but not ok if measuring latitude.
@mjambon The wobble itself you can account for by normalizing with a gyro or force sensor. The issue of magnets around you, I mean yea if one is around it will throw it off. But making sure there isnt a magnet within a few feet of it isnt really a huge issue, these fields weaken fast.
You are right however the reason is accuracy. The magnetic field is weak and the resolution of the sensors are limited. Also GPS is hyper accurate. So there is no doubt its an accuracy thing.
@freemo @mjambon It's a little more complicated than that. Magnetic deviation can be caused by anything that produces or alters the magnetic field, including trace ferrite in a nearby hill, nearby power lines, or even the phone in your pocket. Then you also have other issues, such as magnetic variation (the difference between magnetic north and true north), and even additional counterintuitive errors caused by acceleration and turning.
For example, in every airplane cockpit, you'll see something like this (to compensate for local fields caused by electronics):
@LouisIngenthron No doubt it is a real effect, but easily managed in the wild. Any field that effect it are going to more or less within hands reach plus a little extra . So while its totally true it might be effected by an electronic device fairly close to it, or a large chunk of metal, anything even moderately far away (such as a deposit of metal a dozen feet under the earth) isnt likely to have much of an effect.
@freemo Depends on your acceptable margin of error. But if you want the level of precision necessary to measure latitude dip, you definitely have to take such things into account.
@LouisIngenthron Well as I said, its margin of error is well below the alternatives anyway.
@freemo
Mechanical compasses of that sort are more complicated, at least because you need a vertical reference and a gimbal with 2 degrees of freedom.
Also, that's not very accurate if you don't know your longitude: the isolines of vertical strength are amusingly curvy: https://upload.wikimedia.org/wikipedia/commons/4/4c/World_Magnetic_Inclination_2020.pdf
BTW I wonder how many sports trackers use the magnetic field to simplify their motion tracking, and then work very differently sufficiently far north or sufficiently close to the equator. (E.g. for breaststroke in swimming pools oriented east-west in Switzerland keeping a running average of the magnetic field direction at wrist is a more reliable way of counting lengths than what Garmin was doing up until something like 1-2 years ago)