Perspective Analysis

Perspective Analysis

As used in crime scene analysis, photography , and photogrammetry , perspective analysis involves the use of measurement techniques to determine the relative (and exact) sizes of objects within a photograph, digital image, or video image. Essentially, the process involves measurement of the distance between the camera and target item in the image of a known size, and utilizing those two measurements to calculate the size of other objects in the image.

At a crime scene, there are several important perspective aspects or views for photographic documentation. First, the entire scene is captured from a distance (a known or marked distance) in order to gain an overview of the entire scene before it is disrupted. Next, images are taken from midrange in order to estimate the size, or to document the relationships, of items. Finally, close range photographs are taken of individual items of evidence . For evidence items, one-to-one photography is used when possible. This technique involves taking actual size photographs of specific evidentiary items, and using them to make direct comparisons with the suspect. This technique is most often employed with fingerprints, bare footprints, and shoe prints.

In its most rudimentary form, perspective analysis can be used when examining a photograph containing an object of known size by measuring the image of the known object and developing a ratio of the image size to the sizes of other objects in the photograph. This is a cruder and less accurate form of perspective analysis than that involving the direct use of a scale, or the use of camera distance measurement techniques.

Several pieces of equipment are essential for accurate perspective analysis and object measurement: a ruler or scale, a tripod, and a level, in addition to a multilensed camera (35 mm, digital, instant, video, and other cameras are often utilized at crime scenes). When a ruler or scale is used for actual item measurement, the object should first be photographed alone, and then photographed again with the ruler lying in exactly the same plane as the object, and the camera situated in a plane parallel to both. It is also essential to measure the distance from the focal area of the camera to the object being photographed, in order to calculate the size of the objects being photographed.

The ruler or scale used for perspective measurement must be at least as precise as the camera doing the scene recording. The same scale used at the scene should be used to measure with when printing the photographs to a particular magnification. All rulers and scales should be individually marked so as to be readily identified both in the photographs and later during legal proceedings (court testimony or evidentiary presentation).

see also Architecture and structural analysis; Automobile accidents; Bullet track; Computer modeling; Crime scene reconstruction; Photogrammetry.