If the reconstruction is to utilize the calculation of crush energy, the vehicle crush must be measured. The concept of crush measurement is simple. The position of a particular point on the exterior surface of the vehicle is measured, both before and after the accident. The crush at that point is simply the vector difference between the two positions. In a crash test, the usual procedure for preparing for crush measurements is to mark the measurement points beforehand, and then position the vehicle so that the area that is to remain undamaged is located a known distance from a reference surface that is roughly parallel to the area to be damaged. The vehicle is positioned the same way before and after the test, and measurements are made between the reference surface and the crush measurement locations. The crush is simply the difference between the sets of measurements.
The accident investigator uses a different procedure. The crush has already occurred, so it is easy to decide where to make the measurements. (Crush is often measured with uniform spacing, but it is shown in subsequent chapters that uniformity is an unnecessary restriction.) However, no reference measurements were made before the accident, so an undamaged vehicle (either real or digitized) must be recruited as a stand-in for the precrash accident vehicle, and an equivalent set of measurements made. Existing vehicle landmarks can be utilized to ensure that the measurement locations are the same, or inch tape can be applied in the same way to both accident and exemplar vehicles to create artificial, temporary landmarks. The desired crush measurement locations are simply marked at the same positions on the inch tape in both cases.
Chapter 10 discusses other methods such as the National Automotive Sampling System (NASS) protocol, full-scale mapping, and three-dimensional methods including the use of total station and scanning methods. Additional topics discussed include loose parts, other crush measurement issues in coplanar crashes, and rollover roof deformation measurements.