Reconstructing an Accident Scene
Written by Daniel C. Brown   


Law enforcement agencies are increasingly using high-precision surveying instruments, such as RTK GPS receivers, to create accurate maps when they are collecting evidence.

RTK is an acronym that stands for “real-time kinematic”. The technology uses sophisticated signal-processing techniques to achieve sub-centimeter accuracy levels. RTK systems consist of a base-station receiver that establishes a point of reference, and one or more rover units. The base station measures errors in the signals from the GPS satellites and transmits error-correction factors to the rovers, normally through a built-in radio modem. The rovers apply these correction factors to their raw GPS measurements to calculate position coordinates with a very high degree of accuracy.

Originally developed for land surveying applications, RTK GPS is proving to be a very useful tool for evidence-recording applications that require high levels of accuracy.

One pioneer in the use of RTK GPS for evidence recording is the Royal Canadian Mounted Police (RCMP). The agency has been using this technology since 2010 to speed up data gathering at traffic-collision sites in the Canadian province of Alberta.

“Once we got past the start-up glitches, we realized that these RTK units are sophisticated pieces of equipment,” said Collision Reconstruction Program Manager Sergeant Sam Hewson, with the RCMP’s K Division Traffic Services. “Once your RTK equipment is communicating well, the advantages become obvious.” K Division covers the province of Alberta, and has 11 full-time reconstructionists, plus 12 or more analysts who assist part-time with collision investigations.

In 2010, RCMP collision analysts and reconstructionists were called to assist with nearly 900 fatal, serious-injury, and high-profile collisions. Of those 900, they reconstructed a total of 586 collisions.

RCMP members who investigate and map a collision first examine the scene and record the locations of important features such as roadway edges, road lines, stop signs, and traffic signals. Second, the reconstructionist examines the evidence itself and records precise locations of skid marks, scuff marks, tire prints, and gouge marks in the pavement made by skidding vehicles. Physical evidence includes metal marks and scars, and the final resting positions of the vehicles involved in the collision.

“We find that with the RTK GPS we can save about 40 to 60 percent of ‘on-scene’ time,” said Hewson. “We can set up quickly and get through the scene quite rapidly. It is a huge savings in time, which is critical to us on the spot.”

evidence forensics reconstruction accident real-time kinematic gps rtk Royal Canadian Mounted Police RCMP collision Allegro Algiz data collectors MapScenes Evidence Recorder software Altus GPS systems

RCMP measurements are used to create a formal drawing of the scene to present in court. The drawing assists the court in visualizing the event. The advent of computer-aided drafting (CAD) programs has changed court presentation. Time and distance issues can be shown. Momentum and vector sum problems can be worked in the CAD program to determine vehicle speeds, and events can even be animated.

RCMP’s mandate includes the enforcement of criminal and provincial laws. In many ways, the RCMP carries out duties similar to those of the Federal Bureau of Investigation and state police in the United States. RCMP investigations assist in the laying of criminal charges appropriate to the circumstances. The end result for the investigator is preparing for and presenting the investigation in criminal court.

A Start with RTK GPS

evidence forensics reconstruction accident real-time kinematic gps rtk Royal Canadian Mounted Police RCMP collision Allegro Algiz data collectors MapScenes Evidence Recorder software Altus GPS systemsIn the early to mid 1990s, the RCMP started acquiring total stations for use in collision investigations. Budgets did not permit every division to have one, but over time, the RCMP upgraded from measuring tapes to total stations.

Two years ago, RCMP began to consider RTK units for collision-investigation purposes. Through a process of writing specifications and putting the purchase out for bids, K Division acquired 12 Altus GPS systems that allowed them to create six RTK sets. The RCMP had not used RTK equipment before, so using it was somewhat of an experiment. The Altus dealer, Butler Survey Supplies, provided training and K Division members began to use them on some of the collisions.

The K Division of RCMP now has 16 sets of Altus GPS systems. (A set includes a base and a rover.) And over the past two years, the RCMP in Saskatchewan (F Division) and Manitoba (D Division) purchased 15 sets of Altus equipment between them, for a national total of 31 Altus sets. “We have progressed to the point where our people prefer using the GPS systems over a total station,” said Hewson.

RCMP uses Allegro and Algiz data collectors with MapScenes Evidence Recorder (EvR) software.

RCMP members investigating collisions often work under severe time constraints. The majority of serious collisions occur after-hours—in the evenings, nights, and on weekends. Often, heavy volumes of traffic are using the road where the accident occurred. Many times, the officer must shut down the roadway to gather evidence. “We have to get on the site quickly, gather our evidence in the shortest time possible, and leave the scene,” Hewson said.

Sometimes the collision involves multiple vehicles, perhaps several semi-trailers and cars. That makes the investigation more complex and it requires more time. Traffic must, of course, be halted.

If you consider a four-way intersection with two cars that collided, it might take an hour or more to examine the scene and record locations using measuring tapes or a total station. But RCMP has found that once the GPS system is up and running, such a scene can be investigated and recorded in 20 minutes.

RCMP members like the Altus units because they take two batteries that are hot swappable and removable. Without that feature, if a battery dies halfway through an investigation, the RCMP member might have to shut down and recharge the unit. That means switching to a different method of mapping the scene, or coming back later.

With the hot-swappable feature, if one battery goes down, the RCMP member switches immediately to the other one. No time is lost. “We can run on one battery while charging the second and then swap out the battery,” said Hewson. “We don’t have to power off the instrument and restart to change batteries. That saves us significant time.”

With the Altus units, the range between base and rover units is 1.8 to 3.1 miles (3 to 5 km). By changing antenna position, the range between units can be increased to 6.2 miles (10 km). With the addition of external radios, it would be possible to increase the range between units to 21.7 miles (35 km). That can save time by avoiding traverses that are required with a total station. “Recently we mapped a murder scene that would have required seven traverses with a total station,” said Hewson. “But with the GPS system, the operator was able to set up the base unit once and work from there.”

On the Scene

Upon arrival at a collision scene, the RCMP member sets up a base station and links it to a rover unit. The two communicate by radio. Available frequencies that do not interfere with something else are easy to find. Plus or minus one-centimeter accuracy is quite sufficient for collision reconstruction.

If time is extremely limited and the RCMP member must open the road quickly, it may be necessary to develop a traffic-oriented plan to clear the road. The force member may start on one side of the roadway and gather evidence there first so that a shoulder can be cleared, then the first lane of traffic, next the second, and so on, until all lanes are cleared.

The points to collect are the points needed to draw the scene. A CAD program needs those points to reproduce the scene of the collision. A simple straight two-lane road with shoulders requires ten points: the road edge, the fog line, the centerline, the opposite road edge, and the opposite fog line—all at both ends. The drawing will connect the points from end to end.

The locations of signs—stop signs, yield signs, curve signs and so forth—are important and must be recorded. If those features have a bearing on the collision, the RCMP member collects them. Sight lines are important and can be measured.

Other than the roadway features, the collision reconstructionist collects physical evidence at the scene. The positions of vehicles after the collision are vital, as are the marks the vehicles make prior to the collision. Skid marks need to be measured, collected, and recorded. The locations of gouge marks made by a vehicle during the collision must be collected and recorded.

After collecting all of those points, they are downloaded from the data collector into a CAD program. The data points are then used to make a drawing of the scene.

The drawing is only one part of what a collision reconstructionist does. The RCMP member also takes measurements and photographs to illustrate points. The drawing shows an overview of what the event looked like. The courts use this information because they are trying to determine whether there is guilt or innocence by the persons involved.

Speeds are important to determine. How fast was a vehicle traveling before the impact? Could the person have reacted sooner if he had been more alert or sober?

The drawing can assist when using momentum to determine the speeds of one or more vehicles in the event. With some drawing programs, one can import Google Earth photos into the drawing and create a 3D photograph that incorporates the data.

Some CAD programs can animate the collision. You can see the vehicles driving and animate the event with them moving. With some programs you can introduce camera views or a view from overhead of the collision.


Hewson said K Division’s experience in moving from measuring tapes to total stations to RTK systems has been positive. The RTK systems are more expensive than a two-person total station, but the RTK units only require one person to operate. They are easy to learn, and they permit on-site investigations to be completed 40 to 60 percent faster compared with total stations. That minimizes traffic disruptions and inconvenience to the traveling public.

About the Author

Daniel Brown is the owner of TechniComm, a communications business based in Des Plaines, Illinois. He graduated from Iowa State University with a bachelor’s degree in Engineering Journalism with a specialty in Civil Engineering. Brown has written extensively for a number of clients in the construction, heavy equipment and engineering fields.

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