GPS

From CanyonWiki

GPS stands for Global Positioning System, a system that allows devices on the earth's surface to communicate with a network of geosynchronous satellites to determine the device's location. While cars and cell phones use this technology, a canyoneer is better off with a water-resistant device ruggedized for backcountry. Some significant drawbacks to relying heavily on a GPS are:

- GPS requires line-of-sight triangulation.  Because of the physical restrictions of a slot canyon, there may not be enough available satellites for good triangulation, if at all.  GPS may help locate the trailhead, but it may not tell you which drainage to follow, or let you know the most efficient or effective way to your destination.  
- Data. A information-processing device is only as accurate as its inputs.  Knowing which data set your coordinates is based upon is helpful.  Also, be wary that bad data gets passed around - verify your coordinates with trusted and knowledgeable people, and double check it yourself.  Putting yourself in the wrong place in backcounty could be an invitation to disaster.
- Batteries. Always prepare backups in a watertight location, or be prepared to handle your trip without your device.
- Technology is no substitute for good planning, skills and knowledge.   Good backcountry skills are invaluable.

That said, the tools are pretty slick and getting better (smaller, cheaper, lighter, better resolution) all the time. Recent models can be expected to interface with computers and mapping software to help you better plan and record your explorations. When used intelligently, a GPS can be a very useful tool for supporting your excursions.