For 1990-2009 cars

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

The height dimensions determine the location of the datum based on the equipment being used. Therefore, different datum lines are established for the same vehicle. You can add or subtract an equal amount from all height dimensions and create a new datum line for your usage.