I run Onx OffRoad on an 8" Minno Rugged Tablet. As others have said, I always download detailed maps of the areas I intend on wheelin'. The tablet and Onx switches back and forth seamlessly between having service and not. This tablet has 2 tb of storage, so it will store a lot of offline satellite image maps.
The tablet is literally bolted to the cage, it is very secure, and the Rugged tablet is beefy and pretty bullet proof. You can't remove it from the rig without destroying the tablet, so I have no worries about leaving it unattended with the top off.
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I'm a big fan of Onx. I love the flexibility of accessing my maps on any of my devices, and changes are updated automatically. In other words, I can build a map or a route at my desk, and then it automatically gets to the Jeep. And any tracks I build in the Jeep automatically update to all the other devices. On the trail, I can get out and walk around and use my phone to access my live maps and current location. I've actually done that a lot. On my old GPS setup, I had to move stuff back and forth with disks. Comparatively, this thing is crazy convenient.
I don't rely heavily on Onx's built in maps. I spend a lot of time building my own with a combination of public/purchased .gpx files, supplemented heavily with my own tracks. Another nice feature with Onx is that I can easily share those maps with others. I've built pretty detailed maps of Windrock, and other parks, and have passed them to my buddies. I can also set those shared maps to accept changes and tracks from those I share with, further enhancing the accuracy. That is slick.
The Android-based tablet is also a great way to run other apps for engine monitoring, etc, and leave the stereo for handling music and cameras.
I'm pretty happy with my current setup. It's light years beyond my previous, dedicated GPS.