GPS modules are so cheap now that its perfectly reasonable to carry two devices with GPS functionality, a personal navigation device that you, like fuji86 suggested, turn on to get your bearings, and a data logger that records your track to an SD card so later on you can view it, etc.

Since most of the PNDs have LCD screens, they eat more power than I like, as to log you have to leave them on. If you want the door to door, then it might make sense to get a 12volt battery and a DC-DC converter, or a 6 volt battery and a voltage regulator to bring the voltage down to 5 volts..with a mini USB to plug into a GPS, phone, etc. (5.1 volts is what most GPSs, the ones that use USB power, require) Then you can plug your GPS into your bike battery's power, and it will run and be charged up at the same time. You can charge the bigger battery up at home periodically.