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Forensic Mapping Challenge
By Steve McKinzie

During the 1880s, Sir Francis Galton, a British anthropologist and cousin of Charles Darwin, began his observation of fingerprints as a means of identification. In 1892, he published his book, "Fingerprints", establishing the individuality and permanence of fingerprints. The book included the first classification system for fingerprints. Galton identified the characteristics by which fingerprints can be identified. These same characteristics are still in use today, and are often referred to as Galton's Details. In 1903 the New York State Prison system began the first systematic use of fingerprints in the US for criminals. In 1904 the US Penitentiary in Leavenworth, Kansas used fingerprints. For nearly 100 years the science of fingerprint identification remains solid, but subject to challenge by its application in our judicial system. This ability to challenge data or an opinion is a cornerstone of our judicial system and a building block of freedom.

The science of Forensic Mapping, based on geometry, has a much longer history traced back to the 1600s, but is subject to the same challenges of application. In the early '90s the term Forensic Mapping was coined to the art and science of electronically documenting a point or coordinate where some physical evidence was located, and then using an automated system of graphic generation to create a map. Over the past ten years advancements in computer software have made this process more an art than science - a benefit that often holds the devil's hand. The Forensic Map's intended use often dictates how much science and how much art may be used. Most maps are intended to document some post-event situation. Most of the events are of a critical nature and ultimately end in some form of litigation.

As with any litigation, a plaintiff or defendant may challenge the opponent's evidence and its basis before a judge. This process is often referred to as a Fry or Daubert Challenge. Users of Forensic Mapping technology must be weary of some software products that generate graphics without geometry to support them. For each point in a map, two things should exist - geometry and a graphic attribute. The geometry is usually generated by a theodolite measuring a horizontal and vertical angle combined with a slope distance from an electronic distance-measuring instrument. This format of measurement is commonly referred to as a polar coordinate, (angle & radius). Attached to this numbered point, the technician assigns a graphic attribute code. The graphic attribute serves as an instruction to the software how to display the measured point. The code may be something simple such as a line or as complex as a 3D symbol.

The coding is akin to the art within the system while the geometry is the science. During the legal battle the art is difficult to challenge because of its subjectivity to the technicians' observations. The usual technique is a simple photographic comparison to the map. Is the map a fair and accurate representation of the scene? The easiest challenge to present is testing the user or technician's ability to explain the geometry. Officer, can you tell the court how your system measures polar coordinates, then calculates Cartesian (X, Y, Z) coordinates?

The answer can be simple or may need to be exact. In either case the technician must be prepared to independently validate the map to ensure the software has performed its job correctly. A computer cannot occupy the witness stand and explain what was done. This responsibility rests solely with the technician. Some software packages have the ability to generate graphics as instructed by the technician without any supporting geometry. What is the supporting basis for these graphics and opinions from them? As an example, a 3D view is generated by an automated software feature for a sight distance analysis. You opine from this software feature that sight visibility is restricted. The next step that should also be automated is asking yourself, Can I prove this? Do I have a point of geometry that proves visibility was restricted? Software packages (like MapScenes) that protect measured data and maintain a history of data changes provide invaluable support to validation.

Automated graphics generation is an invaluable tool to this art and science we call Forensic Mapping. The technician however must be vigilant to protect the integrity of a map. Understanding the basic math that provides support for the geometry (part II) and sculpting the graphic attributes (part III) is the responsibility of the technician. While a software product may be capable of generating an animation quality image, the technician must be able to support it. The map's fair and accurate representation of a scene is vital to protecting the next 90 years of Forensic Mapping success.

Part II will examine the basic math behind the transition from field measurements to a finished Cartesian based map.

Part III will examine automated graphic generation and the differences between 2 and 3D graphics; their benefits and menace.

Part II
The Basic Math Behind The Transition From Field Measurements To A Finished Cartesian-based Map.

As the Forensic Mapping team measures each point of geometry, the point is individually numbered. Each numbered point is assigned a graphic attribute by entering a code in the map database. The team, usually a station operator and rod person assess the site-specific area to be mapped two ways. First is an evidence assessment to facilitate the technical analysis of the crash or crime scene. Second is a visualization of the evidence in relationship to the surrounding landscape. A crash for example can be mapped and technically analyzed to determine speed without curbs, sidewalks and stop signs being measured. When complete however, and the analysis turns to witness assessment, or during a time distance analysis, the surrounding landscape becomes much more important. The goal should always be to measure the physical evidence in the order of its life expectancy, return traffic to a normal flow as soon as possible, and document the surrounding terrain and its features all the while protecting the validity of each point measured. Never assume traffic will yield to your presence.

Protecting the integrity of data points and their assigned graphic attributes is facilitated with an electronic field book, available from many sources. Users can choose the right recorder for their intended environment and budget. Even more important than the recorder is the software that performs the capture and creates the map. One of the most popular and user friendly is MicroSurvey's Evidence Recorder Pro. EvR Pro operates on the Windows CE platform and in color if supported by the individual recorder. Today's user can enjoy real time mapping that provides a data and graphic confirmation of a measured point. Another benefit of the EvR system that is often overlooked is the reduced mental workload of the team. Now, the team can spend more of their time on personal safety rather than constantly visualizing the completed map. See Figure 1.

Figure 1 a, Map View b, Map & Id's c, Map & Description d, Verify Point Data

Each point to be measured is assessed by the rod person, who functions as the architect of the site (The location of a traffic accident after vehicles and people involved have gone) or scene (The location of a traffic accident while people and vehicles involved are still there). The two-phase assessment is identification and classification. A skid mark (A skid mark is a mark left on the road surface, or on any surface, by a wheel in a skid The term "skid mark" includes all evidence of skidding such as scuffing of a concrete coat surface, even if no rubber is left behind ) for example may be classified by deceleration, yaw, or critical speed mark, each having different evidentiary and technical value. Additionally, each must be measured in a format that will ultimately meet the Collision Analysis or Reconstructionist's needs. Virtually every analysis whether at the technical or reconstruction level is based on some form of distance. The electronic total station is well suited to measuring the distances needed in the forensic mapping system.

The adaptation to total station technology in forensic mapping met the need for reducing time during the at-scene investigation (Examining and recording results of the accident and obtaining additional information at the scene of a traffic accident which may not be available later and which supplements data obtained for the accident report. The information is factual as far as possible. Level 2 of Accident Investigation) and improving accuracy. The precision of measurements needed in collision reconstruction or crime scene analysis is much lower than provided by today's EDM, (electronic distance measuring instrument) or theodolite. In the forensic mapping system, the weakest link is the architect of the map. The rod person / architect not only identifies and classifies each point but ensures the point is accurately mapped by positioning of the optical prism. Utilizing modern EvR software provides a double check in the field for coding and general position.

As a point is captured, three measurements are simultaneous recorded. The EDM measures the slope distance, telling us how far the point is away while the theodolite measures the vertical and horizontal angles. This polar coordinate geometry is usually formatted in feet for distance and degrees, minutes and seconds for angle. Software first performs an interpretation of the polar coordinates and calculates a horizontal distance to the point. The horizontal distance is utilized to calculate the X,Y position followed by yet another calculation for determining the Z or elevation position. While compressing a summarization of the process into one paragraph looks ominous, it's not. The map technician should be prepared to explain what the software did when the polar coordinates were used to calculate the position data used in his choice of CAD program.

The slope distance is measured by the EDM, while accuracy is ensured by the Set operator and rod person. The calculations take into account the components of the system. See Figure 2.

In this real world example below, the map is assigned an elevation, (I datum) of 100 as a datum. The I datum is nothing more than an imaginary plane from which the system will calculate the height of the point being measured. The height of the instrument is measured at 5.55, (HI) and the optical prism, (HR) is 6.0 above the point measured. See Figure 3

The EDM provides the slope distance to the prism, in usually less than two seconds. The newest technology in electronic total stations utilizes a Class I Laser that does not require the reflector under about 100 meters. The station operator must be vigilant in recording the target height. When the Laser is utilized, the target height should be set to zero. Figure 3 displays an example of the calculation applied to determine the horizontal distance from the set or station.

Using another point, we see in Figure 4 how the Cartesian coordinate is calculated. The set was aimed at a point 35° from the horizontal datum, usually magnetic north and the calculated horizontal distance was 50 feet. The X,Y coordinate can easily be calculated by determining the sin and cos of the angle multiplied by the horizontal distance. Remember, we are always referencing the horizontal datum when calculating the X,Y coordinate.

If you are limited to 2D computer drafting, extra care should be utilized as the horizontal distances will be displayed. This can be critically important when analyzing a fall or vault.

Once the X,Y position is determined we must evaluate the elevation of the point measured. This step will use the cos of the vertical angle or observation (91°44'30"). You'll notice the calculation also takes into account the I Datum HR and HI, in the event the set and reflector are different heights.

Very few maps are validated point by point. Usually 10% is sufficient to determine if the software performed its function correctly. Some programs are more adept at assisting the mapping technician in this process. The latest version of MapScenes PRO uses Active Drawing Technology, See Figure 6. From the EvR screen in Figure 1d we can compare the data to ensure accuracy. You'll also notice a history feature in Figure Six: MapScenes tracks any changes to the point and codes.

Fig. 6: Point Editing May Be Accessed By Simply Clicking On A Point Number

In Figure 7 you see the rounded values from the Active Drawing Technology option

Fig. 7: Active Drawing Technology

Finally, the time comes to validate the program's work. It certainly is not a guessing game. This is truly a validation of the software. From the Scene Measurement Menu, the work performed by the MapScenes program is provided for all to see. Figure 8.

You will find all mapping software programs calculate the data virtually the same way. MapScenes is one of the very few programs that make the task easier. We can easily slip into a false sense of security due to the flawless manner in which software programs perform these calculations. You must be ever vigilant however and remember the computer cannot take the witness stand and explain what it did. That responsibility will forever rest with the mapping technician.

President of McKinzie & Associates, Steve is an active reconstructionist specializing in commercial vehicle collision reconstruction and Forensic Mapping.

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