When your bae puts this much work into a marriage proposal, you gotta say yes. At this point, he's pretty much moving heaven and earth to be with you. That's something you don't need a telescope to see!
What if someone else who had a girlfriend named "Joanna" flew over this pasture? That would be really, really awkward. What could you say? "It's a different Joanna"? There'd be no way to get out of it.
You have to say "yes" if your boyfriend proposes you like this. If he's that committed to drawing in the sand, just imagine how committed he's going to be during your marriage. Very committed, that's how committed. Very.