Vehicle Crush and "Linear Perimeter" Profiling Measurement with iWitnessPRO


Three hundred targets automatically measured in under two minutes


In cooperation with the Seattle Police Department, DCS conducted the measurement of an impounded, damaged 1998 Toyota Camry with iWitnessPRO. The vehicle had sustained extensive front-end damage in a collision with a tree.


Approximately three-hundred adhesive-backed red retrotargets, referred to as Feature Point Targets (FPTs), were affixed around the exterior of the vehicle. The intent of the photogrammetric survey was to illustrate the ease and flexibility of automatic 3D measurement of these FPTs via a color digital SLR camera and iWitnessPRO.


iWitnessPRO utilizes coded target technology suited to color digital cameras. This allows the user to achieve fully automatic, high accuracy 3D measurement of the FPTs. The 3D coordinates of supplementary untargeted or ‘natural’ feature points can then be determined using the traditional iWitness operator-assisted measurement process


Wrecked Camry - automatic measurement in iWitnessPRO


Thirty-six coded targets were randomly placed around the vehicle. It should be noted that in order to automatically solve the exterior orientation of the camera position, at least 5 coded targets should be seen in each of the images. iWitnessPRO also utilizes a special subset of coded targets called Spinner Codes.


As the name implies, spinner codes are turned about the vertical axis and can be spun through 360 degrees. As the photographer moves around the object, the spinner codes are also rotated. In the Camry project, only three spinner codes were used (codes #146 to #148). These were mounted on the roof to enhance the linking of the image network between the two sides of the vehicle. Normally, four or more spinner codes would be recommended in such a survey.


Imaging:
Of the 70 or so digital images acquired using a Nikon D200 digital camera with 17mm lens, only 24 images were used in iWitnessPRO to provide a high-accuracy, high-reliability measurement of the coded targets and FPTs. The onboard camera flash was employed to illuminate the red retroreflective targets, with the background being 1 – 2 aperture stops underexposed, which is generally advisable as it yields a darker background and enhances the automatic detection of red retroreflective targets.


Two precision scale bars were used for absolute scaling (one placed forward of the car, and one place aft of the rear bumper) and three FPTs were placed on the floor in order to define the XY datum plane.


iWitnessPRO on a notebook computer:
As mentioned, only 24 images were selected for the processing depicted in the video link noted below. There is, however, no reason why all 70 images could not be employed, since the fully automatic ‘one-button’ image scanning and photogrammetric measurement processing in iWitnessPRO takes only a few minutes more with the additional 46 images. This project was cut down to 24 images in the video simply to save on internet bandwidth! To see the computational process in iWitnessPRO, please click here to watch the image scanning and FPT measurement process (two and half minute WMV video 5MB).

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