What is Forensic Mapping?
Forensic mapping is a system used to document physical evidence at some critical incident, much like we have documented physical evidence at the scene of highway crashes for years. The system uses an absolute polar coordinate system of measuring in comparison to the base line coordinate system traditionally used. Polar coordinates refers to fixing the location of an evidence point by an angle and radius.
The system utilizes a measuring device called a total station. The system is comprised of four parts; a theodolite, EDMI, Optical Prism, and data collector. The Theodolite measures angles on an azimuth from magnetic north. EDMI stands for electronic distance measuring instrument. The EDMI is a pulsed infrared diode in most cases. Laser total stations are entering the market but require great care in operation. An optical prism is used in conjunction with an infrared total station to reflect the light emitted from the EDMI. The Data collector captures the measurements made by the theodolite and EDMI along with graphic attributes assigned by the technician.
For each point of physical evidence measured the horizontal azimuth from north, and vertical angle from the Theodolite are captured. In addition to that information, the distance from the total station is measured by the EDMI. This geographical information is then combined with graphic attributes.
The accuracy of the system is dependent upon several things. First is the “rod man”. His roll is to not only recognize and assign the graphics to the position measured, but the placement of the prism over the item, (plum) is important as the measurement is recorded from a known height above the position. Second is the theodolite, that is available in varying levels of accuracy. Typically, collision reconstructionists utilizes five to ten second accuracy instruments. As an example, ten seconds of a accuracy at 1000 feet amounts to about .58 inches. The density of air can affect the EDMI. Barometric pressure and temperature affect air density. A 10° Celsius change equates to 10 parts per million change in measurement. For the relatively short distances measured, this 10PPM difference is for the most part not measurable.
To ensure the system is measuring accurately, a reference measurement protocol is recommended. The protocol calls for the first measurement at each site to be recorded to a mechanically measured distance. The mechanical device in this case is a tape measure and the distance is usually fifty feet. At the end of the site examination the same measurement is recorded a second time to ensure no changes in atmospheric conditions have affected the accuracy of measurements
The data is then uploaded to a mapping software program.. The mapping software first reads the geometry for each location measured. The codes that are assigned to the points are associated to a mapping library. The mapping software reads the library and retrieves the instructions for displaying the graphic attributes that produce the map.