Quick link to a great blog called Flip. One of the workflows down in the Concepts of The Day list on the side here has a link to this blog as well. Anyways he has a great post about Tracking Arcs....not just any arcs either. He compares how arcs are really choppy in lots of real life situations. Plenty of examples too.
The post is titled: Perfect Imperfection
Lots of other great posts so look around and enjoy
Labels: Arcs, Flip, Tracking Arcs