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When A.L. "Fred" Carter graduated from McGill University in Montreal, Canada, the university did not have computers. Today, retired from Carleton University where he taught and worked in experimental particle physics for 30 years, Carter is legendary for his work stemming from his interest in computers. Those who know bloodstain pattern analysis, likely know BackTrack and Dr. Carter. In 1999, the International Association of Bloodstain Pattern Analysts (IABPA) honored him as a Distinguished Member.
For those who are not analysts and do not know BackTrack — it is software that enables analysts to locate the sources of blood using the laws of physics to study and simulate bloodstain patterns.
The term "directional analysis," which was coined in "The Directional Analysis of Bloodstain Patterns, Theory and Experimental Validation," which appeared in the "Canadian Society of Forensic Science" in 2001, describes the analysis procedure carried out by the BackTrack Suite of programs.
Carter created BackTrack in 1989, and he and his son Brian incorporated Forensic Computing of Ottawa Inc. to market the software in 1991. Carter says he became interested in the analysis of bloodstain patterns in 1982 when two members of the Royal Canadian Mounted Police (RCMP) approached the dean of science at Carleton University in Ottawa with a request for a science-based training course for bloodstain pattern analysts. They were especially interested in how to include the curvature of the flight paths of blood drops when determining the area of origin, he says.
Analyzing blood drops in curved paths
The curvatures of the flight paths depend on unknown quantities, he explains. These are, in order of importance: the speed of the blood droplets, the droplet sizes and air resistance force.
A top-view analysis with virtual strings developed in 1992 by Carter limits the effects of these unknowns. The curved flight paths appear as straight lines in this view and can be treated as virtual strings.
"From the laws of physics for projectile motion in the gravitational field, one can prove lack of knowledge of the three factors results in the height determination of the blood source to be restricted to an upper limit or a maximum height rather than a definite value," Carter says.
The top-view analysis with virtual strings enables the horizontal location of the blood sources to be determined from measured quantities with surprising accuracy.
"A unique fitting routine was developed that fits a mathematical ellipse to the shapes of the individual bloodstains," he says. "The ratio of the width to the length of this ellipse is used to compute the impact angle of the blood droplet."
Commonly in the bloodstain analysis of patterns found on vertical surfaces, he says it is assumed the convergence of the string pattern in 3D space projects directly onto the vertical surface containing the bloodstains, often called the 2D convergence. "The analyst determines the 2D convergence and assumes the source of blood is directly from the 2D convergence," he says. "This procedure does not stand up to rigorous examination because the curvature of the individual flight paths does not project onto the vertical surfaces as straight lines." However, he points out the 2D convergence method only is valid on horizontal surfaces such as floors and ceilings.
BackTrack benefits
Using BackTrack offers a number of advantages over manually stringing bloodstains. Carter says they include:
- The results are obtained by using procedures based on solid science.
- Using a computer saves time.
- The computer produces virtual strings that do not sag and are mathematically positioned correctly.
- The software makes use of the latest digital imaging techniques.
- The computer analysis yields numerical values for the horizontal coordinates of the location of the blood source based on five measured quantities for each stain: width and length of the stains obtained by a unique ellipse fitting routine and three spatial coordinates of the stain's position (X, Y, Z).
All of the above lead to high-quality investigative work recognized by the courts, he says.
Proven reliability
"In my opinion, one of the most important criteria is 'proven reliability,' " Carter says. "The analysis procedure must be based on the known physical laws of projectile motion. The results must depend on measured parameters and not upon those based on assumptions. The analysis procedure must be able to stand up to the most rigorous and critical examination."
Carter says bloodstain pattern analysis software differs from other software because of its intended usage. The forensic analyst uses the software to prepare a report that might have to be presented in court. The professional credibility and reputation of the analyst may ride on the acceptance or rejection of that report. Subsequently, software can directly impact an analyst's reputation.
"It is essential, in my opinion, that BPA software have proven reliability," he says. "This proof should be in the form of published documentation in a peer-reviewed journal. This enables analysts to refer to the published work for validation purposes."
Credible training also is an important factor for the analyst's acceptance in the courts, he adds. In addition, to the basic 40-hour course on bloodstain pattern analysis, it is desirable for the analyst to be able to testify that he or she has received expert training in the use of the software.
BackTrack to the future
Further improvements will likely be in the presentation of the results of bloodstain pattern analysis in court, he says.
Routines have been written for BackTrack that enable exportation of the finished analysis in formats which can be read into 3D programs such as Crime Zone and AutoCAD.
Laboratory work also is underway to extend the capabilities of BackTrack to analyze bloodstains found on surfaces that are neither vertical nor horizontal, he says. Such surfaces would include the case of bloodstains found on tilted picture frames and even on curved surfaces.
BackTrack customers are located in 17 states, the RCMP, the Ontario Provincial Police, most regional police services, and a number of city police services in Canada and in 12 countries.
Rebecca Kanable is a freelance writer specializing in law enforcement topics. She lives in Wisconsin and can be reached via e-mail at kanable@charter.net.
author: By Rebecca Kanable