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The purpose of making scene measurements is to establish a better understanding of the positioning of relevant accident elements and to enable an accurate sketch to be created. The scene measurements provide valuable data to accident investigators that give a better overall understanding of the accident and aid in the reconstruction process.

There are numerous methods available to accident investigators for making scene measurements. Each method system have unique advantages and disadvantages that the investigators should consider when applying a particular measurement system.

The methodology below details the following measurement techniques:

Orthogonal Coordinate Grid System

Triangulation

Path Coordinate System

Photogrammetry

There are several alternate measurement techniques available using highly sophisticated equipment such as laser scanners, total stations, theodolites and differential GPS. These systems require an experienced user but can save time while collecting high quality data. However, due to the cost they are unlikely to be used for most accident scene investigations and therefore further details will not be described.

A simple method of measuring the accident scene is to record the measurements on an orthogonal coordinate grid by first finding a reference point and then defining a (straight) reference line which will represent the X-axis. The Y-axis measurements are then measured perpendicularly to the reference line.

One or two persons available

Straight and simple road layout is present

Measurements of the accident scene can be facilitated by first establishing a fixed reference point. The point of reference must be fixed and can be street lamps, fixed posts, building wall corners, etc.

After selecting an initial reference point, an axis can be established from the reference point. This axis will form the reference base that all other points of interest will be measured against.

After establishing the X-axis, all additional points of interest can given as measured distances in the Y direction from the defined X-axis (note: for long distances (>3m), it is recommended to use a measurement wheel and for short distances (<=3m), a measurement tape (2 persons) can be used).

Relatively fast and simple measurement method

Requires a simple road layout

With this procedure, two points of reference are selected, where the distance between these points is known. The distance to each point of interest should then be measured from both the two reference points.

The angle between the reference base line and the line to the point of interest should not be obtuse nor too acute. If the distance between reference point A and point B is too long and interest point C is too close to the connecting line AB, then there won’t be an intersection and point C cannot be found with this method. An acute angle and inaccurate measurement causes an offset of point C.

Two persons available

Complex measurement on straight road layout

Measurements of the accident scene can be facilitated by first establishing a fixed reference points. These points of reference must be fixed and can be street lamps, fixed posts, building wall corners, etc. At least two points of reference are required to make accurate measurements. These two reference points will act as base points to which all additional points of interest are related to.

After defining the reference points, the distance between the two should be measured.

After establishing the reference points and determining the separating distance, the location of various evidential elements of interest can be determined. All relevant accident elements (i.e. accident vehicles, collision objects, transient evidence, traces, debris, etc.) should be measured in relation to the original reference points.

The angles between the reference points and the point of interest are used to determine the position of the point of interest in relation to the reference points (see the figure below).

Each point of interest should also be numbered to enable accurate record keeping and identification of each element.

An example of measurements created through triangulation is shown in the figure below.

Figure 1: Triangulation measurements

Very accurate method for measuring as long as the point of interest is within a suitable area

Takes more time than other methods

Two persons required for measuring

Low level of accuracy for obtuse and acute angles

If the accident site is relatively large or complex, it can be divided into a network of triangles. These triangles can be measured according to the same principles as stated in the above triangulation method. The measured points are then used as reference points to measure the next unknown point until all unknowns have been accounted for.

Two persons available

Complex road layout is present with a relatively large area.

Same as above triangulation,but with multiple triangles.

Very accurate measurement method

Time consuming

Two persons required for measurements

A path coordinate system is a measurement method where the x-axis is set to follow a path (e.g. the road edge) and the y-axis values are measured perpendicular to the x-axis. This, however, requires that the path itself is well defined.

One or two persons available

Path can easily described

Measurements of the accident scene can be facilitated by first establishing a fixed reference point. The point of reference must be fixed and can be street lamps, fixed posts, building wall corners, etc.

After selecting an initial reference point, an axis can be established from the reference point. This axis will form the reference base that all other points of interest will be measured against. As stated above, the X-axis is usally chosen to follow the road path.

After establishing the X-axis, all additional points of interest can given as the perpendicularly measured distances in the Y direction from the defined X-axis (note: for long distances (>3m), it is recommended to use a measurement wheel and for short distances (<=3m), a measurement tape (2 persons) can be used).

Very simple method for measuring of complex road path

Long chords required for accurate measurements

Perpendicular measurements may be difficult

Photogrammetry is the practise of producing real world measurements from photographs. There are several photogrammetric techniques available. 2-D photogrammetry methods require only one photograph and can provide adequate accuracy for nominally flat surfaces. Even when no measurements have been taken at the time of an accident, it is possible to obtain adequate dimensional data to rectify photographs by going to the scene and taking measurements of painted lines or other long-lasting roadway features that appear in the original photographs.

One or two persons available

Measurement of the reference distances only

Measurements of the accident scene can be facilitated by first establishing a fixed reference points. These points of reference must be fixed and can be street lamps, fixed posts, building wall corners, etc. At least two points of reference are required to make accurate measurements. These two reference points will act as base points to which all additional points of interest are related to.

After defining the reference points, the distance between the two should be measured.

Based on the scale of the photograph and the known distance between the reference points, the remaining accident site element points can be established by measuring the distances on the photograph itself and scaling the result.

Very simple method for measuring of complex road path

No on-site measurements beyond the reference distances

Measurement points must be clearly visible

Please refer to the main for the Scene Measurements equipment list.

Please refer to the Arrangements section of the section.