All~

A rhumb line is used for nautical navigation. It can be difficult to follow a great circle as the heading of one constantly changes. Thus to go between to nearby points on the world one often uses a rhumbline. Essentially you figure out the initial heading that puts you to toward the point you wish to go to and follow a constant heading, you will take a rhumb line.

Rhumb lines form a spiral to the nearest poll (unless your heading is 0). The advantage to having a constant heading is that it is easy to hold a constant heading in a control loop, wheras controlling on a great circle requires much more math.

http://en.wikipedia.org/wiki/Rhumb_line