What is the purpose of the four square block checkerboard painted on the aft fuselage of Boeing's flight test aircraft? It is either break in markings for fire rescue or some sort of optical measurement for speed, distance associated with flight testing, I believe.
the checkerboard mark allows accurate indication of the position of the plane using cameras as the plane flies by. by tracking the movement of the checkerboards between frames of a series of camera shots, the plane's speed, roll rate, acceleration, etc. can be measured.