Just a quick note on this.Johnny said:Empirically there is no difference. If you measure something as 6 inches (and you've ensured that your ruly is mechanically correct) then we must assume that it is six inches. We can invent all sorts of ideas about what the length really is, but empirically it is six inches and there is no valid reason to say that your inertial frame is the correct one. In other words, if someone else measures it as four inches because of their velocity, then empircally it is four inches for them and six inches for you. Neither of you is more right than the other.
If you're using a tape measure you should inspect the tab on the end, if it has been dropped on that corner the tab may be bent. Thus skewing yor measurement.