Abstract: In one embodiment, an apparatus includes a first magnetometer that measures a magnetic induction and direction of a magnetic field. The first magnetometer has a first coordinate frame that includes a first x axis, a first y axis, and a first z axis. The apparatus includes a second magnetometer that measures the magnetic induction and direction of the magnetic field. The second magnetometer has a second coordinate frame that includes a second x axis, a second y axis, and a second z axis. Each of one or more of the second x axis, the second y axis, or the second z axis is has a predetermined orientation relative to the first x axis, the first y axis, or the first z axis.

Abstract: In one embodiment, an apparatus includes a first magnetometer that measures a magnetic field. The apparatus includes a second magnetometer that measures the magnetic field. The first magnetometer includes a first coordinate frame with a first x axis, a first y axis, and a first z axis. The second magnetometer includes a second coordinate frame with a second x axis, a second y axis, and a second z axis. Each of one or more of the second x axis, the second y axis, or the second z axis has a predetermined orientation relative to the first x axis, the first y axis, or the first z axis.

Abstract: In one embodiment, an apparatus includes a first magnetometer that measures a magnetic field. The apparatus includes a second magnetometer that measures the magnetic field. The first and second magnetometer are positioned a predetermined distance apart from each other in a direction. The apparatus includes computer-readable media embodying logic that access a first measurement of the magnetic field by the first magnetometer at a first time and a second measurement of the magnetic field by the second magnetometer at a second time. The logic compares the first measurement with the second measurement and, based at least in part on the comparison, determines that the first measurement approximately coincides with the second measurement. Based at least in part on the coincidence, the logic determines that a device comprising the apparatus has traversed the predetermined distance in the direction that they are positioned apart from each other.