I see a few people here using harrys lap timer. It lets you use a better gps via bluetooth with faster resolution so its more accurate. Apparently a lot of cellphones gps' are only accurate to 10 feet and can show you off the course when you're actually on. The better ones are accurate to 3 feet or less. Also a lot of tracks are built into his, and you can add yours if it isnt.
http://www.gps-laptimer.de/Home.html