By the length of an object we mean the difference of the object's start and end coordinates measured at the same time.  If you would like to measure the length of a fast moving object, the following should be considered. 

In the system of the observer one places many synchronized clocks in the direction of movement. Let you call one of them - practically the one in the origin of the observer's system - reference clock. This clock is stopped when the end point of the fast moving object goes by, while all the other clocks will be stopped when the start point of the observed object passes by.  The length of the object will be given by those two clocks which show the same time after stopping them. The numerically right formula is acquired by using the Lorentz transformation.

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