I once attended a talk in which the speaker presented a system for inferring the orientation of a mobile device in space. This was before smart phones and the

I once attended a talk in which the speaker presented a system for inferring the orientation of a mobile device in space. This was before smart phones and the system used accelerometers, some computer vision, and fancy algorithms which were running on a server because the device didn't have enough computing power. One person in the audience asked: Why don't you simply use a compass? The answer was silence.