I think the biggest problem with a bubble octant on land is going to be two main effects. I don’t know how severe they are. First, It isn’t as good as an artificial horizon device, which is much more precise because it’s just reflections. Second, any massive objects that are near you will actually shift gravitational down away from the center of the earth, resulting in inaccuracies near mountains, etc. The second may seem like a minor thing, but I have read that it was enough that careful surveys of mountains and the gravitational force were required for accurate land artillery use.
However, if I can get good enough at taking shots to get accurate enough to get within about ten miles while on the ground, I figure I’ll be ready to start trying in the air and seeing how it goes.