From my time playing Ac, I don't think that is actually possible Before though in AC-CF there was a device (I don't remember what it's called) I think called Fast Replay or something, and you could design your town with that. Of course, it was hacked. I don't think there is a natural way to do it really. Unless this device is out for the 3DS, I don't know. :/
Mine has one bump in it since you cant have perfectly straight rivers, but it's relatively straight and cuts the map in half. I don't like winding rivers