Cross Track

http://ardupilot-mega.googlecode.com/svn/ArduPilotMegaImages/crosstrack.png

Crosstrack correction is intended to guide the plan to an optimal path, as opposed to simply pointing towards the desired waypoint. This is desired when wind blows the plane off course. It can also be useful to keep a plane on a certain line between waypoints for photography.

Crosstrack error calculator (works in Firefox and IE, but not Chrome):

Flash Example