Plotting your path with a cell phone

GPS signals aren't available everywhere -- a parking garage, for example. Microsoft researchers have come up with an app for tracking where you have been, GPS or not. Technology Review reports:

The device would collect the trail data while the user walked indoors, underground, or in other spaces where GPS signals are unavailable or weak--such as multilevel parking garages that can baffle people who forget where they parked.

They have produced a prototype phone called Menlo, which packs a suite of sensors: an accelerometer to detect movement, a side-mounted compass to determine direction, and a barometric pressure sensor to track changes in altitude.

What's new about Menlo is an app called Greenfield, which aims to solve the Hansel and Gretel problem by harnessing the data from the sensors. The goal is to count a user's sequence of steps, gauge direction changes, and even calculate how many floors the user has traversed by stairs or an elevator. The app stores the trail data so that a user can later retrace his path precisely.

Greenfield could be used for new kinds of urban street games, to recover lost items, to find friends at a stadium, or to rescue hikers and mountain climbers.