When the iPad first came out, wifi-only pads did not have GPS; either GPS and 3G used the same chipset or it was just a marketing decision, who knows. Whether that’s still true of all modern iPads or not I don’t know, but I’d guess so.
There was a lot of FUD and misinformation spouted on the Internet at the time, including people who’d tethered phones to find that it “worked”, but it didn’t. What people were seeing at the time was coarse location information calculated from ‘known’ visible wireless APs.
This is however an iPad-specific quirk; my first-gen Nexus 7 has full GPS despite being wifi-only, and I have no doubt many others do too.
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You don’t need a data connection for GPS. As Graham says, you can optionally use Assisted GPS to speed up the initial satellite lock; this uses a tiny amount of data, and is optional.
You do however need data to stream map data. So Google Maps, for instance, can use a lot of data unless you cache the area beforehand. Self-contained maps like CoPilot come with their own map data and don’t suffer from this problem.
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You’d have thought you could just Bluetooth to your iPhone and use the GPS on that – but I’m not sure if that is supported or not. Can’t see why it wouldn’t be, other than profits.
You’d need some sort of app which shared that data and exposed it to the BT stack. I’d expect that’s a lot more likely on non-fruit based devices, I can’t for a moment see Apple letting the iPhone do that.
Similarly assuming you do have a means of turning a phone into an external GPS receiver, I’d almost certainly expect that the iPad would only connect to a MFi-certified device.