I use Backcountry navigator pro for $10 instead of a gps. You can get free USGS topos and sat maps. You can also get Noaa charts. I use a Galaxy Note, but I've seen others in my group use this with iphones. I know my phone uses both the us and russian gps systems. Accuracy is excellent. I'm not as sure with the iphone, but have heard no complaints. Battery life is decent, but just don't keep it on all the time. I also use my phone as a Kindle, so I don't have to take a book. I make up for it with batteries which are weight neutral, but have a volume advantage. It is not waterproof nor as rugged as a GPS, but I keep it in a ziploc and take care of it like I would a nice piece of glass.
For batteries, I use an external battery from Anker.
In summary, this is a cheap system to experiment with. I think it's worth $10 to try and if you like it, you can buy a $30 external battery for longer use.
