No, the plotting is all done in geographic. That's just the shape of TN's border. It's not a straight line in any coordinate system. (For example, have a look

No, the plotting is all done in geographic. That's just the shape of TN's border. It's not a straight line in any coordinate system. (For example, have a look at google maps, which is in geographic. The northern border of the state roughly follows a parallel, but the details are more complicated due to history and local politics.) I didn't look at the code in detail (and my R is quite rusty), but the fact that he's using the geosphere package suggests that the intersection calculation is being done on a spherical shell, rather than cartesian space.