Acceleration sensor mapping
Mapping (for example) tunnels with a GPS is not very easy, as the GPS cannot get a satellite lock under ground. One solution to this is to add a gyro sensor (accelerometers) to the GPS, to track relative movements under ground, and use the GPS position to fix that movement to the GPS grid.
Also see about inertial navigation systems which use dead reckoning (e.g. via accelerometers) for positioning.
But it hasn't been cheap to get a GPS with included accelerometers ...
Nowadays (2018) many people have a smartphone which has accelerometer sensors built in. How to use it for position tracking? Also see the personal draft Iphone 4 GPS + IMU.
One relatively cheap accelerometer which came available in 2007 is the Wii handset, the Wiimote.
The Nintendo Wii handset contain accelerometers, and a Bluetooth transmitter. The handset can be used with a Linux machine, so by using a GPS, a Wii handset and a Linux laptop, one should be able to implement a motion tracking OSM data collector in a car driving in tunnels.
I found https://web.archive.org/web/20090416174132/http://www.wiili.org:80/index.php/Motion_analysis analysing the values reported by the Wii accelerometers. The only unknown is if the Wii is sensible enough to detect the movement as the car is moving. The wiimote deliver accelerometer data every time there is a change in the values, and seem to deliver at most 100 values a second (100 Hz).
Instructions to interface it to linux is available from https://web.archive.org/web/20090416031716/http://www.wiili.org:80/index.php/Wiimote .