Redpin is a Þngerprint-based indoor localization system de- 
signed and built to run on mobile phones. The basic princi- 
ples 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 sys- 
tem to expeditiously adapt to changes in the environment, 
caused for example by replaced access points.