My gear: Gitzo Series 3 6x systematic tripod (no center post), RRS BH55 ballhead with L plate for Canon 7D. All three items have some way of determining if they're level. Spirit levels on the tripod and ballhead, and the 7D has an electronic level. What I've been doing is (1) level the tripod via its spirit level; (2) level the ballhead with its spirit level, and (3) check the level on the 7D. First, the camera is rarely if ever is level after doing 1 and 2. Second, if I rotate the ballhead around the tripod's axis, it too does not remain level, i.e, the ballhead's spirit level moves around and outside of the center circle.
So, it seems I'm doing something fundamentally wrong, as I would have expected that once I level the tripod and ballhead, the camera should be level, and that level should be retained around the axis of the tripod.
I'd greatly appreciate some sage advice on the 'correct' way of leveling the camera, ballhead, and tripod. Obviously, this in no way is harming my photography. It simply bugs me, as I thought this was a no-brainer.
Thank you in advance for your advice.