I'm surprised to see no-one mention the use of a modified bubble scribe for scribing round timbers. I've seen ones that were made up custom by a machine shop for just such a purpose, as well as the joinery produced by such a device, which is impressive. Its basically a log cabin scribe with a x/y bubble level on the axis point. The level can be moved depending upon the position of the scribe. One arm of the scribe is fixed-- like any scribe-- while the other has a trammel that rotates in two axes around an eccentric swivel. I'm probably making it sound way more complicated than it is, but the idea is that you rotate the moving trammel to follow the contour of the round log. Not sure how widely available they are.
