Height Estimation from Images or Video Footage

This is potentially the domain of the image analyst and, unless the forensic podiatrist is skilled and trained in such image analysis and image systems, could lead to challenges at court and in cross-examination, should one go beyond their area of expertise. Camera angles, positioning relative to the subject, as well as lens type can all affect an image and how it is viewed. For example, some lenses have vignetting at the edges, which is a reduction in brightness towards the periphery of the image compared to that at the centre. Other forms of distortion may also occur. All lenses have optical defects and create images that are not perfect copies of the subjects which they are pointed at. The amount of distortion a lens suffers is largely dependent on the type of lens and its construction. Cost plays a role, but factors such as the focal length are also important. The wider the angle of the lens, the more difficult it is for straight lines not to appear curved. For instance, zoom lenses are more prone to distortion than prime lenses, simply because it is highly demanding to correct for aberrations at every focal length. There are lots of distortions available in lenses, here are some common ones likely to be encountered which can affect height estimation.

Curvilinear Distortion

Curvilinear distortion or radial distortion comes in several different types, but the one seen most commonly is barrel distortion. This is relatively easy to spot when an ultra-wide lens has been used and causes straight lines at the edge of the frame to bow outwards (Figure 7.1). The effect is more obvious on a 'fisheye' lens, where these distortions are left uncorrected by design in order to have the widest possible field of view. Another form of distortion is that of pin-cushion distortion, often seen with the use of long telephoto lenses and causes straight lines to bend inwards. The effect is usually subtle and is not normally noticeable unless photographing rectangular subjects straight on. Some zooms can show signs of what is termed moustache distortion - where the image can show both

Figure 7.1 The image is a left side view (left lateral view) of an individual displaying the feature of 'forward lean'. Note the curvilinear distortion that appears on the right of the screen where the wall shows some barrel distortion which could affect how estimates of heights are undertaken.

barrel and pin-cushion distortion. Moustache distortion is most commonly seen where wide-angle zooms have been used and causes straight lines to appear wavy.

Chromatic Aberration

Lenses focus different wavelengths of light at slightly different distances. This creates colour fringing called chromatic aberration which can be seen as a coloured halo at the edges of subjects in an image. It is more marked with extremely wide-angled lenses.

Although there are some computer software programs available for video analysis that will assist the practitioner in measuring height, the original video/CCTV footage may still contain some of the aberrations mentioned which can make the estimation of height challenging. This will require the forensic practitioner to consider these factors when forming such estimations and is where validation of the equipment used is relevant.

Validation

Validation, in this context, is demonstrating that a method is fit for the specific purpose intended and that the results can be relied upon. The requirement for this is that forensic reports submitted for use in the criminal justice system (in England and Wales, as stated by the forensic science regulator) should have some form of statement of the validation performed to provide those making decisions on the use of any results with a summary of the validation undertaken,⁵⁰-⁵¹'⁵² for example, where height estimation has been performed, has validation been conducted on the specific population group in this case or does it rely upon estimations using a different subgroup or population. The methods employed for validation will vary depending on whether a quantitative or qualitative method is used. Jurisdictions may vary in their requirements. Experts should be familiar with the requirements/guidelines and court procedure rules of the legal system in the relevant jurisdiction, insofar as it relates to the expert's duty and their responsibilities.