Vehicle Crush and Crash Scene Documentation with iWitnessPRO

BACKGROUND
The Washington Association of Technical Accident Investigators (WATAI) invited DeChant Consulting Services (DCS) to photogrammetrically measure several crash tests conducted at the October, 2009 WATAI conference. WATAI also invited DCS to speak at the conference, on the subject of close-range photogrammetry for accident reconstruction.

The crash tests were performed at a test facility (i.e., parking lot) at the “Wild Waves” amusement park in Federal Way, WA, on October 27th, 2009, with W. R. "Rusty" Haight, Director of the Collision Safety Institute, being retained by WATAI to crash several vehicles.

One of the head-on collisions, involving of a 1994 Geo Metro and a 1999 Oldsmobile Alero, will be considered here. Both vehicles were imaged by DCS prior to the frontal impact crash, with a 10MP digital camera, in order to establish their pre-incident dimensions.

The photogrammetry
For photogrammetric measurement purposes, magnetic backed Coded Targets (“codes”) were temporarily placed on the A-Pillars, the hood and on the front fenders, as well as on the ground adjacent to the vehicles. The codes assist in automatic 3D measurement using the iWitnessPRO software system.

It took between 5 and 10 minutes to set the iWitnessPRO codes and image each vehicle, with one of the two events occurring in the rain. Passive Targets were also placed on the front bumper of each vehicle to define its general shape (profile). Only 90 seconds was required to automatically determine the 3D coordinates of these feature points using iWitnessPRO, and the push of only one button (the R++ toolbar button) was needed.

The vehicles were crashed as indicated in Figure 1.

FIGURE 1

Post collision measurements were also made of both vehicles at their respective final rest positions. Although the front bumper of the Metro was separated and had ‘lodged’ itself into the Oldsmobile grill due to the low-speed impact, iWitnessPRO was still able to perform image-based 3D measurement of the partially occluded Alero front bumper (see Figure 2). The metal sub-frame of the Metro was also measured to illustrate how fast and accurate the image-based approach is for recording vehicle crush and comparing post- and pre-crash dimensional data.

iWitness camera parameters dialog box
Click to enlarge - Figure 2

The computer work
As indicated in Figure 3, polylines were measured via the Facet Form Polyline tool functionality within iWitnessPRO. Here the user simply “cursor traces” edges and features of the Alero in two or more images and iWitnessPRO converts the cross-referenced 2D Polyline “trace-work” into an accurate 3D line model representation of the vehicle’s front-end.

iWitness camera parameters dialog box
Click to enlarge - Figure 3

Using Common Points for Network Linking
iWitness and iWitnessPRO utilize a feature called “Import Projects for Linking” (PL) in order to “link” the pre-crash 3D vehicle model into the same coordinate system as that adopted for the post-crash measurement. A minimum of four points common to each measurement, pre- and post-crash, is required to achieve this 3D co-registration. In this study, we simply used four measured points (CP1 – CP4) on the windshield for PL, with the assumption the windshield’s targeted points wouldn’t change much as a result of the actual collision.

Figure 4 shows a close-up of a CP adjacent the A-Pillar, while Figure 5 illustrates the Import Projects for Linking function, the result of which is a coordinate transformation into a common reference coordinate system for both measurements. In this case, the RMS point discrepancy between common points after the transformation was at an accuracy level of better than 0.02 inches.

iWitness camera parameters dialog box
Click to enlarge - Figure 4

iWitness camera parameters dialog box
Click to enlarge - Figure 5

Importing the results to CAD
The iWitnessPRO “linked” projects (Metro and Alero) were then exported to CAD. The results are indicated in Fig 6 and Fig 7.

iWitness camera parameters dialog box
Click to enlarge - Figure 6 & 7


Summary
Although the weather was unfavorable for conducting such an accident reconstruction survey, it is noteworthy that the rain did not adversely impact the accuracy and reliability of the photogrammetric measurement operation. In all, six surveys of three two-car vehicle crashes (pre- and post-impact) were conducted, along with the subsequent final-position diagramming of the "car 5 and car 6" crash event, this being using the iWitness system. The image recording consumed from 5 to 10 minutes for the 20 images, while a total station required >45 minutes to achieve similar results to those obtained with iWitness. Also noteworthy is the fact that for the front-end mapping of the Geo and Olds, it took less than 10 minutes to “scene measure” and less than 2 minutes to automatically model the critical 3D points in iWitnessPRO, to an accuracy of better than 0.04 inches.

iWitness and iWitnessPRO are affordable, proven photogrammetry systems that have been widely adopted by accident reconstruction practitioners and police across the United States and law enforcement as far away as Australia and Iceland.


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