Fryer, John 
  
AN OBJECT SPACE TECHNIQUE INDEPENDENT OF LENS DISTORTION FOR FORENSIC 
VIDEOGRAMMETRY. 
John G. Fryer 
Department of Civil, Surveying and Environmental Engineering 
University of Newcastle 
NSW, Australia 2308 
cejgf@cc.newcastle.edu.au 
Working Group V/1 
KEY WORDS: Calibration, Applications, Information Extraction, Performance Analysis 
ABSTRACT 
Video recording is widely used to capture images in locations prone to criminal activity, for example banking 
institutions. A vital piece of forensic evidence can often be the height determination of a bank robber imaged during a 
crime scene incident. Some justice systems are often claimed to weigh heavily in favour of the accused, so the 
presentation of forensic evidence in the form of videogrammetric height determinations must be concise and explicit. A 
jury of non-photogrammetrists must be certain in their own minds that they understand the physical processes involved 
in the height determination of, for example, a masked bank robber. 
This paper presents the results of real investigations, which have been cross-examined, into some video imagery taken 
during a series of bank robberies. The method devised to extract the height information regarding the accused is 
believed to be unique in that all measurements were made in object space and is totally independent of lens distortion. 
Object space is a concept which a non-expert jury can understand, rather than the mathematical modeling associated 
with the electronic imaging and camera/lens calibration required for an image space solution. 
As well as height determination, the mathematical principles of a perspective projection were used in order to confirm 
that two shortened rifles confiscated by police at a crime scene were in fact identical to two such armaments seen on 
video imagery of a prior bank robbery. The anharmonic, or cross, ratio method was used to produce evidence for a jury 
in this case. 
1 INTRODUCTION 
Video cameras used for surveillance tasks are usually fitted with short focal length lenses so that a wide-angled field of 
view can be imaged. The compromise for a short focal length is angular resolution, and, unless an expensive lens 
arrangement is chosen, large radial lens distortion will be present. Since most surveillance cameras are of the cheaper, 
mass-produced variety, the costs are minimised and the lens distortions are acceptable for qualitative inspections, but 
little regard has been paid to any thought of quantitative measurement. Usually only a single camera is installed, but if 
two surveillance units are used, they are often placed at right angles or in other configurations not suited to conventional 
photogrammetric solutions. 
Some simple properties of perspective projections are commonly overlooked by photogrammetrists as they prepare 
information for police, prosecutors or others involved with the presentation of forensic material for court. It is 
relatively simple to be lured into the use of sophisticated iterative computer programs based on the collinearity (or 
coplanarity) equations to solve for the resected location of the video camera/s and then the intersected positions of 
points to be subjected to judicial scrutiny. 
But, will a judge believe that a jury of lay-persons can fully understand the mathematics? Will the scientific evidence 
be given any weight in the jury's deliberations? 
The use of simple surveying techniques, in object space, can illustrate to a jury the physical principles of 
photogrammetry without the complication of high level mathematics and its associated technical jargon. The next 
section of this paper reviews the classic photogrammetrical techniques for height determination from a single image. 
Subsequent sections show how the same result may be obtained by a fundamental consideration of the physical 
  
246 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 
  
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