The datum line is a base line parallel to the plane of the underbody or frame. All vertical measurements originate from the datum line. The datum line is an imaginary horizontal plane that runs below and parallel to the vehicle. Height dimensions are calculated at right angles from the datum line to a given control or reference point on the underbody.

Height dimensions determine the location of the datum line based on the equipment being used. Therefore, different datum lines are established for the same vehicle. Add or subtract an equal amount from all height dimensions in order to create a new datum line.