I had a chat with Apple support about this recently. They basically said that the only way to force use of GNSS would be to turn off wifi and cellular data in the iPhone settings. So you'd have to have already configured an offline field map, and your phone would then store the data locally and sync it when you restore access to a cellular or wifi signal (depending on your settings). This brings its own trade-offs, of course, because if you have a team collecting data, they won't be able to see each other's data on the map until everybody syncs, they won't be able to call each other, etc. So it's not ideal (though we do this frequently where cellular signals are just not available!).
I'm also curious whether you might be able to effectively filter out the triangulations by going into the Field Maps settings and reducing the minimum accuracy distance (under Collection Settings) to whatever horizontal accuracy the iPhone is typically getting from satellite sources. I haven't tested this but it might be worthwhile. It also might slow down your field data collection workflow to an unacceptable pace, but it's worth a try.
- Holly
