I got in on this discussion on the Tugnuts a while back. I wrote a piece on using the iPad with a chartplotter app over two years ago. There seems to be never-ending confusion about "do you need an external GPS?"
If you buy any 3G/4G capable iPad (we have the original iPad), it WILL have a built-in GPS chip.
If you buy a wifi-only iPad, it WILL NOT have a built-in GPS chip.
You do NOT have to connect the iPad to a cellular provider for the GPS to work.
If you have a wifi-only iPad, you can add an external GPS unit. Given the cost of w wifi-only iPad PLUS the external GPS unit, it's about the same as buying a 3G/4G capable iPad. It was worth the extra $100 for me at the time to buy an iPad with 3G just to get the GPS chip.
The 3G/4G iPad works great as a chartplotter with the appropriate app (we use Navionics, and have for over 2 years now). NO, you do not need to be in cellular range for it to work. It acquires the satellites faster and is WAY easier to use than our Raymarine E-80. Accuracy seems to be equal. Interestingly enough, my wife's iPhone takes longer to acquire the satellites and is not as accurate as the iPad.
To complicate matters further, the Droid Bionic I use is just as fast and accurate as the iPad.
Any of these devices that have a BUILT-IN GPS chip can be used. If you buy a WIFI ONLY model iPad, it WILL NOT have a built-in GPS chip. That is the only reason you would have to add an external GPS to an iPad.
As Roger stated, you do not have to connect your 3G/4G iPad to Verizon or AT&T for the nav apps to work. We have had our "ancient" original iPad since they first came out and it has never been connected to AT&T... and the Navionics app really works. Really. :wink:
Best wishes,
Jim B.