It's not a compass.
It only measures how far it moved, and currently only on one plane. Even once they get it up to three planes, as planned, it still only tells you how far it moved. You need another bit of information, where you started from. No idea how it works when rotating it in place, does it even notice? Though I guess having more of them and doing some calculations tells you if it's rotating. You still need to know which direction you where facing to start with. Not to mention that errors add up over time on these sorts of devices, though perhaps the error is really tiny, it wasn't mentioned.
You can get that missing info from, oh I dunno, a GPS and a real compass I guess. Or a surveying point and it's records.
Still, it's cool technology.