Redpin is a fingerprint-based indoor localization system designed and built 
to run on mobile phones. The basic principles of our system are based on known 
systems like Place Lab or Radar. However, with Redpin it is possible to 
consider the signal-strength of GSM, Bluetooth, and WiFi access points on a 
mobile phone. Moreover, we devised methods to omit the time-consuming training 
phase and instead incorporate a folksonomy-like approach where the users train 
the system while using it. Finally, this approach also enables the system to 
expeditiously adapt to changes in the environment, caused for example by 
replaced access points.