Aerial Imaging Aids Forensics

by Robert Galvin

In this magazine I’ve written about how forensics investigators use total stations and lidar to map crash and crime scenes and how surveyors may be able to tap into this market.  Here’s the aerial perspective.

Until just over a year ago, law enforcement agencies used inexpensive, small, drone aircraft—basically unmanned model planes or helicopters—for photographing and videotaping crash scenes. But the Federal Aviation Administration (FAA) banned the use of the drone aircraft to protect the nation’s airspace until it could devise new rules on aircraft use.

Since the ban, law enforcement agencies have clamored for the FAA to reinstate model aircraft use to help them more precisely document crash scenes. In response, President Obama signed a law in February 2012 mandating that the FAA write rules on how it will license police, fire, and other public safety agencies eager to fly lightweight drones at low altitudes.

Meanwhile, crash reconstructionists have incorporated satellite imagery to spatially document crash scenes. They’ve been using the latest satellite imaging options—Google Earth and Microsoft Bing Maps—to build more comprehensive scene perspectives and verify accuracy of mapped evidence. These options, used in conjunction with total stations and crash-scene diagramming software, so far prove effective as agencies await the FAA’s reinstatement of the drone aircraft.

One such agency is the Fatal Detail of the Las Vegas Metropolitan Police Department (LVMPD) Traffic Bureau/Accident Investigation Section. Initially they used full-size helicopters equipped with digital and video cameras, but they could not maintain a 90-degree angle over the top of crash scenes. The helicopters had to keep circling around the crash scene to take photos. Still, the advantage to having aerial photos taken by a helicopter (or something like it) is that crash investigators can visually capture the scene immediately after the crash occurred while everything is still in place, according to detective Bill Redfairn with the LVMPD’s Fatal Detail Agency.

Aerial Photos Show How Crash Occurred

Today, the LVMPD uses Google Earth and Bing in conjunction with ground-based surveying equipment, which provides an added advantage of not having to shut down a street for any extended period. “You want to collect your evidence using your surveying equipment,” explains Redfairn. “You have reference points [from the crash scene] and the evidence itself, then you rely on one of these two satellite imaging services to come back and get the completed diagram.” Many crash diagramming programs will automatically import satellite images so that a crash scene can be drawn directly on the aerial photo. Redfairn uses his smart phone to access Google Earth or Bing while at a crash scene, enabling him to see how the roadway appears and then decide how to document the evidence at the scene and whether to diagram in 2D or 3D. 

The LVMPD’s Fatal Detail uses the Sokkia SRX robotic total station and GRX1 GPS network rover, making it GPS satellite-based and, thus, able to generate any correction signals. The robotic total station enables the operator to control the instrument from a distance, eliminating the need for another person to hold the reflector prism pole.

This also allows detectives to work independently at scenes and to reopen closed roads quicker. They can feed points mapped into The Crash Zone diagramming software, with Bing Maps built in. “If I use an aerial photo, I’ll bring it in via Crash Zone, scale it, and then assign layers,” Redfairn explained. “Once I’m done, then I merge the total station file [of evidence points] on top of the aerial photograph.”

Crash-scene layering is vital. “A typical layer setup for us would be that we’d have one layer with nothing but the aerial photograph on it, then one with the line work for the roadway layout, one layer for the evidence, and one for measurements,” Redfairn said. The ability to turn layers on and off is a major advantage. “If you’re working with two vehicles, sometimes it’s much easier, when, say, you’re doing a momentum problem and you’re working with angles,” Redfairn said. “You can work with one car at a time, so layering is extremely important.”

Advantages of Spatial Images

Like Det. Redfairn, Master Patrol Officer Joe Warren with the Chattanooga, Tennessee, Police Department also turned to Google Earth and Bing Maps after mothballing his drone aircraft. He uses a Topcon GPT 3207 NW total station to plot crash scenes and The Pocket Zone data collection software from Crash Zone, Inc., to build a quick crash diagram on scene. The total station has an extended non-prism range with a refinement of the pulse EDM (electronic distance meter) for measuring distances up to 1,310 feet without a reflector. This distance range proves safe particularly in areas with heavy traffic.

Aerial photography has become more integrated in Warren’s scene documentation. “We can’t always get all the evidence we need at a scene with a total station,” Warren said. “That’s where we marry our Google Earth image of a scene with total-station measurements.” Often, investigators can map evidence on one side of a roadway, but because the road must be kept partially open, it is difficult gathering evidence on the other side. “The aerial photos of the scene allow us to fill in the rest of the diagram for the portion of the roadway or intersection we could not get to using a total station,” Warren said.

Many ground-level photos are taken of crash scenes, but what Warren says he uses to reconstruct crashes is the topographical view of the mapped scene using aerial photos with Google Earth. Because Warren creates 2D diagrams of scenes almost exclusively, this makes aerial photography even more essential. “It verifies what we measured on the ground with our total station—skid marks, length of skid marks, where tire marks ended, and where the vehicles ended up,” he said. Overlaying an aerial photo over the 2D diagram verifies mapped evidence and is particularly useful for prosecutable cases, mainly vehicle homicides.

Comprehensive Scene Perspectives for Jurors

Both Redfairn and Warren agree that aerial photography of crash scenes merged with evidence mapped using a total station is powerful in the courtroom. “If a case is going to prosecution, an aerial photo can help the jury understand the complex sequence of what goes on in a collision,” Redfairn offers. For example, it is typical in Las Vegas for crash scenes to stretch over two or three city blocks. When such a crash series occurs, events unfold on part of the roadway, then continue to unfold farther down. “Aerial imaging is one way to get everything together, depending on your CAD program, and then go back in and look at individual areas and create diagrams for those,” Redfairn said. In this way, he added, “you only build your diagram one time for the entire sequence of events.”

Aerial photos dramatically aid Warren’s 2D diagrams in court to show where crashed vehicles were at a scene. “The aerial photos from Google Earth make the scene real to the jury,” he said.  More importantly, “when the jury sees the aerial photo of the crash scene, it’s more comprehensive.” And, he adds, district attorneys like being able to have the crash scene with overlaid aerial photos on it up during a jury trial. “The DA finds it easier to refer back to it, pointing out different aspects of the case,” Warren said.

Here’s yet another way surveyors with specialized skills can extend their services.

Robert Galvin writes about indoor and outdoor crime scene documentation and technology for surveying and law enforcement publications. Galvin resides in Oregon City, Oregon.

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