Maybe a GPS like this one:
Garmin eTrex Summit
Navigation Features
Waypoints/Icons: 500 with name and graphic symbol
Tracks: Automatic track log; 10 saved tracks let you retrace your path in both directions
Route: 20 reversible routes with up to 50 waypoints (available only by the free update offered on this website)
Trip computer: Current speed, average speed, time of sunrise/sunset, resetable maximum speed, trip timer, and trip distance
Map datums: More than 100
Position format: Lat/Lon, UTM/UPS, Maidenhead, MGRS and other grids
Electronic compass features
Accuracy: ±2 degrees with proper calibration (typical); ±5 degrees in extreme northern and southern latitudes
Resolution: 1 degree
Barometric altimeter features
Accuracy: 10 feet with proper calibration (user and/or automatic calibration)
Resolution: 1 foot
Range: -2,000 to 30,000 feet
Elevation computer: Current elevation, resetable minimum and maximum elevation, ascent/descent rate, total ascent/descent, average and maximum ascent/descent rate
Pressure: Local pressure (mbar/inches HG), 12-hour automatic pressure trend recording
Performance
Receiver: Differential-ready, 12 parallel channel GPS receiver continuously tracks and uses up to 12 satellites to compute and update your position
Acquisition Times:
Warm: approx. 15 seconds
Cold: approx. 45 seconds
AutoLocate™: approx. 5 minutes
Update Rate: 1 second, continuous
Accuracy:
Position: 15 meters (49 feet) RMS
Velocity: 0.1 knot RMS steady state
Dynamics: 6g's
Interfaces: RS232 with NMEA 0183, RTCM 104 DGPS data format and proprietary Garmin
Antenna: Built-in patch
Physical Size: 4.4"H x 2.0"W x 1.2"D (11.2 x 5.1 x 3.0 cm)
Weight: 5.3 ounces (150 g) with batteries
Display: 2.1"H x 1.1"W (5.4 x 2.7 cm) high-contrast LCD with bright backlighting
Case: Waterproof to IEC 529 IPX7 standards
Temperature range: 5°F to 158°F (-15°C to 70°C)
Data storage: Indefinite; no memory battery required
Power Source: 2 AA batteries (not included)
Battery Life: Up to 16 hours (typical use)
Plus all the other neat stuff a GPS can be used for off the bike. If you are a gadget geek, it's the way to go!
__________________
2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153
*estimated