Crime Scene Measurements Can Now Be Taken From a Single Image

Two researchers have developed a procedure that will enable forensic police to extract measurement data from crime scenes using just a single photograph.  

by / December 1, 2009
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Two researchers, Diego González-Aguilera and Javier Gómez-Lahoz, from the University of Salamanca, have developed a procedure that will enable forensic police to extract measurement data from crime scenes using just a single photograph.

Their work, published this month in the Journal of Forensic Sciences, actually makes it possible to reconstruct a crime scene in 3D, according to a news release issued today. This, says the journal article, offers "a novel approach for documenting, analyzing and visualizing crime scenes."

"We have studied an unprecedented and original line of research in the field of criminology and forensic engineering, which makes it possible to derive metric data from a single image," explained González-Aguilera. He is co-author of the study and a researcher in the Department of Cartography and Soil Engineering at the University of Salamanca (in the University's Ávila offices).

The process starts by capturing an image that must include easily-identifiable details and at least three vanishing points (the convergence point of straight lines projected in one direction) as well as at least one distance in the scene. These data are used to extract the structural components or most important objects in the image "automatically and robustly."

The researchers also calibrate the camera to be able to determine both the internal (focal distance, main point and radial distortion) and external (lens turns and lens viewpoints) parameters.

As the structural features are geometrically related to the features of the scene and the camera itself, it is possible to take measurements and to analyze the dimensions of the scene based on distances, angles and surfaces.

This means that, at any time after having taken a photograph of a crime scene, forensic police could establish that a knife was 32cm away from the victim, for example.

González-Aguilera says it is better to use a single image rather than several as it is often difficult to ensure that a range of photos overlaps well, and there are always parts of the scene or some features of it that cannot be correctly related to the rest.

This technique has been developed within the field of photogrammetry, a procedure used to determine the geometrical properties of an object based on photographic images. "Until now, this discipline required at least two images to be used in order to reconstruct a crime scene, but now we have broken that barrier," said González-Aguilera.

Another basic principle of photogrammetry and computational vision (another discipline also used in this study) is that it is impossible to reconstruct a three-dimensional crime scene based on a single image. However, the new technology also overcomes this limitation.

An IT tool written in VRML format (Virtual Reality Modeling Language) is used in order to visualize a crime scene from any viewpoint in an interactive and three-dimensional way. VRML is a "flexible, scalable and easy-to-use" language, designed specifically for this task say the researchers.




Blake Harris Contributing Editor