Remember - if you have an iPhone newer than the 3G, your phone has a built-in GPS chip that can be used to display your geographic position on apps. If you're out of cell phone range while on the trail, many apps will allow you to download maps (and imagery) beforehand so that cell access isn't necessary. Very handy! If you have an iPad, the GPS chip is not installed and you'll need an external GPS unit like the Bad Elf GPS to locate yourself on a map. For some excellent advice by Adventure Alan on smartphone use on the trail, click HERE.