ACCIDENT Investigation Tips (Part 2) - Using the Coordinate Measuring Method to Mark Evidence

Coordinate Measuring Method: The Coordinate Measuring Method is one of three main measuring methods used by investigators to mark the location of recovered evidence. The other two methods are Triangulation and Total Station. More on these methods later.


The Coordinate Measuring method uses right angles to identify where evidence was found within the scene. The right angles are created when two lines (one vertical and one horizontal) intersect and cross. The place where the lines cross is the Reference Point (RP). The RP looks like a plus sign (+). The vertical line represents north and south distances from the RP. The horizontal line represents east and west of the RP. If an object is located at the same location as the RP it is neither north, south, east or west of the RP.

The Coordinate Method uses the same concepts as a number line in math. In math, 3 and (-3) are on opposite sides of 0. In measuring crime scenes, if 3 and (-3) are on a horizontal line, (-3) would be west of the RP and 3 would be east of the RP.

If 3 and (-3) are on a veritcal line, (-3) would be south of the RP and 3 would be north of the RP.

In this diagram,


the RP is the place where two imaginary lines (shown in red) cross and intersect within the northwest corner of the intersection. One line is vertical and represents north/south. The other line is horizontal and represents the east/west direction. Again, where these lines cross is the RP - aka "Ground Zero." It is the spot investigators use to locate all evidence and objects that will be identified in drawings, scene sketches and photos, and maps. 

The reason investigators use this place as a RP to fix (identify) the location of evidence is because this RP can be re-created easily in the future - if needed. Additionally, this RP is also used to fix the location of traffic control devices, roadway dimension, lane lines, etc. 

In the diagram below a traditional, four-way intersection is displayed. Using the Coordinate Method, an investigator could create mulitple RPs using all four corners of the intersection.

The benefit of using multiple RPs (RP1-NW corner, RP2-SW corner, etc) is it gives the investigator flexibility when fixing the location of evidence that may be closer to one area of the intersection than another.