Symmetrical reference points are the 2 corresponding points on the opposite sides of the centerline that have equal specifications in length, width, and height. Use the symmetrical reference points in order to make quick checks in order to determine underbody structural damage.

When the 2 corresponding reference points have different measurements, the points are asymmetrical. If you perform a quick cross check and the measurements are unequal, check the dimension charts in order to find out if the measurements are symmetrical or asymmetrical. The underbody dimension charts show the specifications for both of the corresponding points when the measurements are equal.

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

Height dimensions determine the location of the datum based on the equipment used. Different datum lines are established for the same vehicle. Add or subtract an equal amount from all of the height dimensions. Create a new datum line for your own use.

A centerline is an imaginary vertical plane that is perpendicular to the length of the datum plane through the center of the vehicle.