In a previous swim sensing technology post, I showed some graphs which showed the position of a swimmer in the pool. These were created by swimming up and down the pool with a smartphone strapped to the back. This paper describes the algorithm I used to do this. Basically it uses disturbances in the magnetic field which change as one swims along the pool in order to sense the position. These disturbances are caused by the structure of the pool surroundings, so each pool has a characteristic magnetic field signature (the picture shows a few of them).
There’s also a Computerphile video about it:
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