Photogrammetry Proves Ideal for Winter Crash Scene Mapping

The winter months bring snow to much of the United States. While this delights the school children and the skiers, it often means police officers have many more crashes to investigate than usual in terrible weather conditions. When inclement weather contributes to numerous crashes in a short period of time, investigators must do everything they can to gather all the critical evidence they need as quickly as possible.

Choosing Photogrammetry

This was the case during one December snow storm when Trooper Jeffrey Westrum of the Minnesota State Patrol was called to investigate a fatality crash involving a semi-truck and a minivan. Westrum (now Lieutenant Westrum) has been a member of the Minnesota State Patrol’s Major Accident Investigation Team (MAIT) for more than 14 years.

At the time of this crash, Westrum had a great deal of experience mapping crash scenes by taking manual measurements and using a total station. The MAIT troopers had recently been trained in the use of the iWitness™ ( close-range photogrammetry system for crash scene and forensic mapping, which is what Westrum chose to use for this scene.

Close-range photogrammetry software systems, like iWitness, allow the user to obtain 3D measurements and an accurate 3D model from a series of photographs. The photographs are bundled through special software calculations that solve for the camera’s orientation and perform a triangulation process to compute 3D coordinates from 2D images. The resulting data can then be brought into The Crash Zone or Crime Zone diagramming software for completion. Today, photogrammetry is quickly gaining popularity as a low-cost, fast, and accurate method of digitally recording crash and crime scene evidence.

The Crash Scene

Crash Zone - Initial Impact

Lt. Westrum’s 3D diagram of the initial impact of the mini-van with the semi- tractor.


Trooper Westrum explains “The crash took place during a snowstorm in the late morning hours. The road was icy and snow was falling at a good rate, coupled with strong gusting winds, causing the immediate area of the crash scene to rapidly fill with snow. A tractor/trailer (semi-truck) was traveling westbound, while a minivan was driving in the eastbound direction.”

“The driver of the truck lost control as it rounded a long, sweeping curve. The tractor/trailer was jackknifed across both lanes of traffic where the mini-van struck the aft portion of the tractor’s rear wheels. The force of the impact and momentum of the collision projected the mini-van in its opposite direction of initial travel, thereby striking a railroad signal post at its final rest position. The tractor/trailer combination continued in the same direction of momentum as the mini-van, striking the mini-van against the railroad signal pole. Two people were killed in the crash.”

Time Saved At The Scene And In The Office

Trooper Westrum continued to explain “I had the option of driving back to the MAIT office and getting the total station to map the scene. All other state trooper’s that morning were assisting in different crashes. The fact that it would have taken at least an hour for me to acquire, and then setup the total station, as well as require the support of another state trooper to help map it, helped me decide that using the iWitness photogrammetry method was the best choice to map this scene. On scene, it only took me a few minutes to setup four photogrammetry-scaling distances and approximately 10 minutes to take the required digital pictures of the crash scene for later measurement using the iWitness program.”

Lt. Westrum notes, “Using Photogrammetry for this crash allowed me to document and then clear the scene within 45 to 60 minutes. Also, it was only my time at the crash scene, since I didn’t need to have a second trooper present to assist with taking measurements during this storm when everyone’s time was at a premium.” Back in the warmth of his office, Lt. Westrum used the iWitness program to obtain critical measurements from the photographs of the scene and create an accurate, scaled diagram in less than two hours.

Completing The Diagrams

Once the 3D points of evidence are documented and measured in the photogrammetry program, it takes only a press of a button to export the data from iWitness to a drawing exchange format (DXF) file that can be opened in The Crash Zone. Once in Crash Zone, you can add symbols, labels, surfaces, and other details to complete your diagrams, just as you would if you brought in measurement data from a laser device.

Crash Zone - MN Crash

Final rest position of the mini-van. A custom symbol was created to show the crushed van.

The result of using iWitness with your scene photographs is an accurate 3D model that is ideal for use in The Crash Zone. You can quickly create courtroom-ready 2D diagrams and accurate 3D views of the scene. The Crash Zone’s easy-to-use animation tools can also be applied to create animations of the event. Crash and Crime Zone diagrams and animations have nearly a 20 year track record of being accepted in court.

Lt. Westrum adds: “It is my opinion that there is not one single perfect solution for mapping scene evidence that fits all of our MNSP needs. Having the iWitness photogrammetry system as one of the tools in our ‘toolbox’, allows us to document evidence immediately on scene, versus the time of waiting for a total station to arrive at the scene, setting it up, and measuring one point at a time. Having used iWitness for many crashes, has proven to me the accuracy and relative ease of using it to map crash scenes.”

 This article is used with permission of the original author, Lee DeChant, Principal of DeChant Consulting Services – DCS Inc. in Bellevue, Washington, specializes in close-range photogrammetric consulting and offers photogrammetry software solutions for law enforcement accident reconstruction. He can be reached at © All Rights Reserved

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