MCDEVITT & ASSOCIATION | Photogrammetry
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Photogrammetry

Photo geometry

Take measurements from photos

Photo geometry is a common process used by investigators who are supplied with photographs of an accident scene, but no measurements. By establishing a mathematical matrix or relationship between numbers of known points in the photograph with points in the real scene, measurements can be extracted. The use of digital cameras has greatly enhanced the process of Photo geometry.

Photo geometry is often applied to the process of crash reconstruction in cases where photographs were taken, but measurements were not.

Many times the only evidence that exists after a collision has been cleaned up are photographs. These photographs can come from police, bystanders, news agencies and insurance adjusters. In many instances dimensional information can be derived from these photographs to provide greater detail for the collision reconstruction.

Clear the roads faster

Recently in an effort to reopen closed roadways more quickly on scene police investigators have used photogrammetry to speed up the process of collecting measurements. It has had very limited success for that application.

MDA reconstructionists have the training and computer software to take an old photograph and convert it to a top-down view, to scale. Essentially, with some reference measurements (4 known points that can be obtained after the photo is taken) the photograph can become the diagram, complete with measurements.

Often this is a procedure that becomes necessary when photographs of an accident scene have been taken by on scene investigators, but no measurements have been recorded.
Photogrammetry allows MDA investigators to measure the scene from the photos, many years after the fact.

How accurate is photogrammetry?

This is a common question; especially when the entire reconstruction is based on the measurements of the scene gained by the process. It has been shown in studies to be very accurate in the hands of a trained individual, with the right camera, software, and training. Our Crashteams members are trained not only to perform these measurements in a highly precise manor, but also to help you identify any unaccuracies or outright errors in the measurements performed by another expert.

Famous Cases

Perhaps the best example is the James Dean crash of September 30 1955. Our Mike Kennedy was asked to reconstruct that crash to answer some key issues; who was driving, impact speeds etc. No proper measurements were taken by the on scene CHP officers, but they did take photographs. Mike used the picture below to rectify and transform it to a top down view.

The wheelbase of the old Ford business coupe was used to establish the mathematical relationship between the photograph and the real scene. The transformed photo was essential to the analysis, and integrated in to the scene diagram in 2D and 3D.
Photogrammetry has been a key process to solving lots of cold cases.

Scale models of the actual vehicles were simply placed on the skid marks after the photograph was rectified. This reveals the exact orientation of the vehicles at impact, and enhances the precision for the speed calculations.

Read the full story abouth James Dean’s fatal accident reconstruction.

What can MDA professionals do to assist you?

MDA professionals have the capability of taking photographs of the crash scene, and converting them to either CAD drawings in any drawing file format, or to a top down image that is to scale and will serve as the foundation of a reconstruction of the event.

We can also review photogrammetry output from other reconstructionists to determine the true accuracy of their output, by testing known distances against distances between the same points in the photograph as the real scene.

For more information call: (843)343-4542