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

When two corresponding reference points have different measurements that are asymmetrical. If you perform a quick cross check and the measurements are unequal, you must check the dimension charts to find out if the measuring points are symmetrical or asymmetrical. Underbody dimension charts will show specifications for both corresponding points when the measurements are unequal.

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. Height dimensions are calculated 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 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.

A centerline is an imaginary vertical plane that is perpendicular to the length of the datum plane through the center of the vehicle. The centerline is any line drawn through the center of the vehicles datum plane.

Form and pierce is a manufacturing process that forms an area in the metal structure to a desired three-dimensional measurement. The area is pierced with a hole or slot for dimensional gauging and may also be used for structural attachment of component pieces.

Repair equipment for measuring the structural repair must be capable of measuring multiple points of length, height, and width at the same time. Also you must measure 2/3 of the vehicle body to obtain accurate comparisons to the vehicle structure.

Universal measuring systems combine the equipment and techniques used to make underbody and upperbody three dimensional measurements. The universal measuring system provides accurate measurements of length, width and height measurements at the same time.