imergingenious

Accelerometers measure change in velocity (values can be integrated and combined with gyro data to measure change in position ). Relatively low error but error compounds over time.
Gyroscopes meaure change in orientation. Relatively low error but it compounds when integrated over time and error drastically effects absolute position calculations.
Compass measures fixed orientation relative to the earths magnetic field, which is weak and can be masked by nearby magnets and metals. Relatively high error but doesn’t increase with time and can be improved with calibration. Can be used to minimize/stop long term gyro error buildup through a computational feedback loop.
GPS measures absolute position. Similar to compass, error is relatively high but doesn’t increase with time and can be used to manage smooth and accurate position tracking when incorporated into a feedback loop with the differential position data coming from accelerometer and gyroscope. Requires line of site with satellites (other tech required for indoors) Error should drastically reduce next year with new satellite network (and associated next gen receiver chips) coming online.
Now, with VIO, camera data is incorporated into this feedback loop for even more smooth and exact output.