Kansas, flat as a pancake. Not convex, not concave! I would like to see a serious experiment done, somehow involving an entire continant.
That's an excellent idea! What's stopping you? If you don't know how to survey yet, you might start by checking your local community college or technical college for courses. Maybe locate a retired old-school land surveyor for advice and tutoring. Since mechanical transits are no longer used by professionals, you may be able to buy one for a song, and one that's in decent condition should be more than adequate for this if you know how to use it.
Kansas would be a terrific place to start, too. Most of the state is covered by a grid of rural roads on one-mile spacing. I've suggested mapping Iowa in the "Expedition" thread, but got no comments; Kansas is bigger, population is sparser, especially in the western portion, but still shouldn't pose a problem for provisioning, and the grid is not
quite as complete, but should be an excellent starting point if you'd rather do it than Iowa.
Be sure to keep us updated on your progress.