Evaluation of scale placement and camera angle in footwear impression examination
MetadataShow full item record
Footwear impressions can be found at any type of crime scene. Footwear impressions are valuable pieces of physical evidence when discovered, properly documented and collected. Two-dimensional footwear impressions can be left behind when the outsole of a shoe comes into contact with a substance and then the substance is transferred to a surface, leaving a positive impression on that surface [1-3]. 2-D impressions can also occur when a shoe steps onto a dusty surface and the dust adheres to the tread elements, which take away the dust leaving behind a negative impression [2,3]. Footwear impression evidence may display class characteristics, such as the type, manufacturer, the approximate size and shape and general description of the shoes . They are most commonly documented and collected through photography, specifically examination quality photographs. The value of the photographs taken depends on the quality of the images and requires the correct placement of the scale and angle of the camera. In order to ensure an accurate representation of the evidence, the camera should be parallel to the impression and a scale must be present in the same plane as the impression. Taking photographs at any other angle can distort the object in the photograph, resulting in incorrect measurements, which can hinder the comparison process between the photographed object and the physical object. Digital imaging applications and software can be used to calibrate images, meaning the scale is used to adjust, or calibrate, the photographed image into a 1:1 image or actual size. Forensic photography guidelines provided by SWGTREAD stress the importance of scale placement and the position of the camera lens, but research on how exact these two steps must be executed to accurately determine the physical size or dimensions of the shoe remains limited [3,4]. The goal of this project was to determine how the positioning of a scale in relationship to the footwear impression can potentially lead to possible distortion of the actual size of the photographed impression. This project seeks to evaluate the extent to which the depth of the scale and impression can vary before the size of the photographed impression becomes measurably distorted as well as evaluate the tolerable range of angle tilt of the scale allowed before the size of the photographed impression becomes distorted. A left counterfeit Nike Air Jordan’s shoe from the Jordan Melo line, US adult size 7.5 was used to make several test impressions and then the best impression with the highest quality was chosen to be photographed. Examination quality photographs of the test impression with the scale in the same plane were taken, followed by an incremental increase in the height/distance (0-66.9 mm) of the scale from the ground and lastly, with an incremental increase in the angle (0-20 degrees) of the camera horizontally, vertically left and right. The photographs were then calibrated using Adobe® Photoshop® and actual size images were printed out. Based on the tread design, ten areas present on the outsole were examined through measurement. The results indicated that if the scale was placed at a height within 5.7 mm of the height of the impression, then there would be no change in estimated shoe size. However, if the scale was placed a distance or height of 7.6 mm or greater from the bottom of the impression, then the image would result in a change of half a shoe size or more. Also, if the angle of the camera was greater than 8 degrees horizontally or greater than 14 degrees vertically to the left there was a change in the apparent shoe size. However, there was no change in the shoe size when the camera angle increased horizontally up to 8 degrees, vertically up to 20 degrees, and vertically up to 14 degrees. It is important to be aware of discrepancies that can exist in size dimensions when the scale placement and camera angle are not in the correct plane and position. The distorted images may result in false measurements, which can lead to inaccurate size interpretations. This would be problematic when comparing a suspect’s shoe to the photographed footwear impression, possibly resulting in false inclusion or exclusion decisions of the questioned footwear impression.