The Traffic Accident Reconstruction Origin -Article-

Aerial Photography: Applications In Accident Reconstruction

Introduction (by Bob Wyman)

Pick up almost any magazine and you will find an aerial photograph. Beautiful landscapes, scenic coastlines, incredible views of everything from honeymoon vacation spots to "tough trucks" climbing up mountain ranges. Aerials are used to illustrate advertisements, editorials and feature articles. But, a technical side of aerial photography also exists.

Aerial Photography has many applications for the traffic accident reconstructionist. It is far more than just an attractive exhibit to impress jurors or a client. Properly scaled vertical aerial photography can actually be a crucial part of a successful investigation.

A major investigation usually begins this way. You roll up to the scene to find chaos. You observe physical evidence that will help understand the collision. Meanwhile Fire- Rescue is attending victims, while principals, witnesses and onlookers wander about your scene. As the investigator your most important job is to prioritize your time. On the list are the following tasks: secure the scene, identify involved parties, facilitate EMS access, locate witnesses, and eventually document evidence. Usually, by the time you finally get to the evidence it has been disturbed, swept away, or washed down. Your once-fresh evidence has changed. Your scene is losing its integrity.

Here's an alternate response to this scenario. This time, you think ahead, grabbing a can of orange traffic marking spray paint along with your clipboard. You evaluate the scene and head toward the EMS activity, spray-painting the skid mark end point and outlining the debris pattern on the way. After fulfilling your duties with EMS and the victims, you spray the rest-location of the four tires of that vehicle. On your trip back across the intersection to speak to witnesses, you mark more of the evidence and the final rest position of the other vehicles.

The point, of course, is to put evidence documentation at the beginning of your on-scene task list instead of at the end. In this manner, photos and measurements may be completed later or even during the next day or two without any loss of information. Documenting evidence in paint is the groundwork for calling in air support for an aerial photographic mission.

Vertical Aerial Photographs (by Ross Cobb)

An Oblique Aerial of Fisher Island Near Miami

Aerial photographs that are the subject of this paper are defined as vertical photos, or those that are taken looking straight down from an aircraft. These photos usually require a special camera mounting apparatus as well as sophisticated aiming and triggering mechanisms. Photos taken out of an airplane window or the door of a helicopter are oblique aerials, and they are taken at such a severe angle to the ground that distortions and perspective effects make the resulting images difficult or impossible to use in a reconstruction environment. Interestingly, even the most carefully held "vertical" or banking shot out of a helicopter is still far less accurate than a controlled vertical using a camera mount designed for this purpose.

To take and use meaningful vertical aerial photographs, "targets" must be added to the scene. These markings will assist the pilot, aerial photographer and photo lab technician in producing an accurate print. Targets usually consist of "X" designs, squares, triangles or other geometric shapes to help the pilot find the site and establish the photo image boundaries desired. A distance of 100' between targets is useful, especially when the final photo product will be scaled at 1"=10' on the print. In these cases the darkroom technician simply enlarges the image until the targets are 10" apart; the resulting print will be at ten-scale. Placing these targets with a measuring wheel will generally result in accuracy to within a few inches.

The camera mount, mentioned previously, is an important element in establishing a vertical angle to the ground. It also isolates vibrations and allows the camera to be rotated (in the mount) so that proper site orientation may be accommodated. It is probable that wind conditions will force the aircraft to fly over the site on a specific heading. To keep roadways aligned with the rectangular shape of the photo, the camera must be rotated so the street pattern aligns with the rectangular photo frame. This is more of a cosmetic enhancement than a technical requirement, but a photo enlargement aligned to a roadway grid is easier for most people to view and understand.

Some types of aircraft are particularly well suited for aerial photography. Among one of the best is a Cessna 177. This aircraft has a strutless wing for an unobstructed view from the cabin. It also offers an excellent compromise between low speed flight for vertical aerials and respectable cruise speeds.

The heart of this or any aerial system is the camera, its mount and monitoring equipment. Pictured left is one solution to the problem. This photo shows the cameras mounted in front of the right seat of the aircraft. Note the rudder pedals in the background. Two cameras are shown in black on the right side of the photo. One camera is loaded with film and actually takes the vertical. The second camera sends its video output to the monitor so that the pilot-photographer can be sure of framing and timing. Slow flying is a relative term. Putting the aircraft in the right place at the right altitude with the proper heading takes a concentrated effort. Several of these precision photo-passes are required to be sure critical evidence is not obscured by traffic.

Film type, camera format, lens, altitude, scene size and final desired product are several of the interrelated variables that must be considered next. An example of how these factors influence the photographic outcome follows.

• A photo is desired that encompasses about 400' along the principal direction or roadway approach to the accident. We desire a final product printed at 1"=10'.

• With a 400' long area at ten-scale, verification is obtained from the photo lab that poster-sized enlargements can be printed and darkroom personnel can take the time to enlarge a photo to a specific measurement between targets.

• The resulting picture will be on a 30"x40" piece of photographic paper, illustrating 300'x400' of ground area. Many custom labs can also accommodate larger rolls of photographic paper. 48" wide paper with lengths of twelve feet or more are excellent for scenarios such as railroad accidents.

• A variety of photographic equipment and formats may be used for aerial work, from consumer-grade 35mm cameras to U.S. Government-certified survey cameras costing tens of thousands of dollars. "Reading a license plate" is a realistic possibility with these higher-end systems.

• One above-average combination is a 165mm lens on a medium-format camera. The resulting 6cm x 7cm (2 ¼"x 2 ¾") medium format negatives can be enlarged to a greater degree and exhibit better detail than 35mm combinations. The 165mm lens produces outstanding clarity and edge-to-edge sharpness with negligible distortion.

• Optics formulas yield a calculation that indicates a 400' ground area, with 6cm x 7cm medium-format film and a 165mm lens, requires an altitude of just over 1000'. Each ground dimension, lens combination and film (negative) size will, of course, result in a different altitude.

• Targets should be bright and unobstructed by overhanging trees or awnings, since the aircraft will be flying at 125 ft/sec and the "photo opportunity," for a centered shot, is less than 1 second in duration.

• If standards greater than 1% are required, a Certified Land Surveyor may be retained to provide an independent targeting map of the site with millimeter-level accuracy. The photo can then be further enhanced for accuracy through a rectification process, which eliminates all distortions caused by any slight misalignment of the camera (actually, the film) to the ground.

	This first photograph is about what one would expect from a ground level scene collision documentation photo.
	The aerial (reduced in size).
	Scene detail.
	It is difficult if not impossible to appreciate the impact and utility of a 30" x 40" aerial photograph. What follows is an attempt to accomplish just that. Taking this link will allow the reader to view a 2 color line representation of the full sized aerial. Downloading this 190 kb gif image takes 4% of the time it would take to load the full color version.

Using the Aerial for Reconstruction (by Bob Wyman)

The most practical use of a vertical aerial is the ability to measure the scene from the photo. This is possible because the photo has been scaled to known points. With photos enlarged to a usable scale (1"=10' is most often used by reconstructionists) measuring can be accomplished in several ways. At ten-scale, you can easily use your template to measure distances or angles. A second technique is to use a clear overlay on top of the photo. Accident sequence mapping can be easily accomplished by drawing on this overlay. Finally, if CAD is your scene data's ultimate destination, a scaled vertical aerial can be digitized at your drafting table, rather than wandering about in the road.

Beyond all the utility of an aerial, there is its demonstrative value. Add some informative self-adhesive labels, and you've just completed your deposition or courtroom exhibit. To give you an idea of the impact of such a photo, cars will be over one inch long.

Summary

Aerial photos in Accident Reconstruction provide a highly accurate basis for evidence documentation, scene analysis, and reconstruction calculations. They also are demonstrative evidence for courtroom displays, and depositions. Given a bit of forethought aerial photographs can be a viable alternative to standard accident scene practice.

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