This description of how a "find the nearest" app which is spreading outside Westminster in the UK works is quite interesting:
With SatLav, all the user needs to do is type 'toilet' on his or her mobile phone and send it to a particular number. Satellites, which use technology similar to that of Global Positioning Systems (GPS), will then match the postcode of the phone user with that of the nearest toilet and send the details to the user. Each message would cost the user 25p and the council 9p.
I tried to find the website for the the service, SatLav (clever name!) and came up short. Perhaps that's why there's a bit of confusion on how it works?
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Computer Business Review