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.
@freemo
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.