GPS is notoriously inaccurate when it comes to vertical positioning. And it disappears entirely inside buildings. So pressure sensors are used to help calculate vertical positioning.

The thing is, a pressure sensor decides your altitude based on the pressure of the air, so it must be comparing it to some baseline. The problem with that is that there is no firm baseline pressure: weather, as we all know, affects the air pressure.

That means that pressure is, first of all, a moving target. Secondly, we can never really know our absolute altitude, only relative.

It’s always struck me that there seem to be two critical elements to sensor fusion. There’s the part that can be resolved with math – for instance, compensating a magnetometer reading to account for the tilt as measured by an accelerometer – and then there’s the heuristic part. The latter deals with, for example, deciding that your gyro reading makes no sense and deferring to the compass instead to give you a heading. And while the math in the first part is more or less universal for all players, the heuristics would provide … Read More → "Fusing the Little Details"