Forensic mapping using a total station 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 or a triangulation system we normally use. Polar coordinate refers to fixing the location of evidence by an angle and radius.
The system is comprised of four parts, a theodolite, EDMI, Optical Prism, and data collector. The theodolite measures angles on an azimuth measured traditionally from magnetic north. EDMI stands for electronic distance measuring instrument. The EDMI in most cases is a pulsed infrared diode. An optical prism is used to reflect the light emitted from the EDMI. The Data collector captures the measurements made by the theodolite and EDMI along with graphic attributes.
For each point of physical evidence measured, captured are the horizontal azimuth from north, and vertical angle from the Theodolite. 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 assigned to the point.
The accuracy of the system is dependent upon several things. First is the "rod man". His role is not only to recognize and assign the graphics to the position, but the placement of the prism over the item is important as the measurement is recorded from a known height above the position. Second is the theodolite.
There are 360 degrees in one revolution, within each degree is 60 minutes and within each minute are 60 seconds. Ten seconds at 1000 feet is about .58". 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 a measurement. For the relatively short distances measured, this 10PPM difference is for the most part not measurable.
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