Height profiling is another interesting technology. Using stereoscopic cameras in the ceiling, these devices track people based on their body makeup. It knows how tall people are, how far their shoulders are from their head, and other size comparison, and it uses this data to understand people's movement patterns. It also keeps profiles on the other people you are walking with, so if you tie your shoelaces (thus changing your height profile), it will assume you're still there because the people around you are still there.
This isn't used to always know where you are, however, it's used for research. Gatwick Airport — which is trialing it — wants to know how long it takes passengers to go through security. This technology gives the airport real-time data about who is moving, how fast, and what is causing a congestion. And it doesn't give specific information about who the passenger is.