Fryer, John 
  
work on the part of the photogrammetrist must be avoided at all times as 
the cross examiner will quickly try to discredit any unscientific 
estimates. In the work this author has presented to courts, reconstruction 
of the stance of the robber was undertaken by carefully positioning the 
feet of an assistant, whose height was similar to that of the robber, and 
measuring (and photographing!) him in that stance. See Figure 6. This 
technique provided upward estimates of 3 - 5 cm for the court to 
consider in conjunction with the actual height determination of the 
robber at the time of image capture. 
5 ERROR ESTIMATION 
Given the above simple physical method of height determination, which 
left the defense little room for questioning and the raising of doubts, the 
only area where the photogrammetrist should expect strong questioning 
will relate to the estimation of likely errors. Courts have been ready to 
accept that the mean of several height determinations should provide a 
better answer than just one determination from one frame. The 
photogrammetrist must behave like the scientist he is and honestly 
evaluate his work and place error bounds around his answer. It is pure 
folly to claim too high an accuracy for the purpose of self-promotion, as 
egos can easily come undone in the tense atmosphere of a court. 
  
The photogrammetrist must realise that while his professional colleagues 
are au fait with normal distribution theory, standard errors and the 99% 
confidence limit, the court will require something simpler. A simple + 
or - bound placed around the height determination is something 
acceptable to the court and whether it is at the one, two or three standard 
error level is something best left to the photogrammetrist to decide, 
based on the merits of each situation and the advice of his attorney. 
  
Figure 6. Estimating an allowance to be 
added to height for the stance of a suspect. 
Most lay-persons believe that a standard error is a an absolute bound of accuracv, rather than an indicator of precision. 
Care should be taken lest the photogrammetrist suddenly finds he must present what amounts to a statistics lecture to 
the court. Such a situation would probably confuse and weaken the findings. Error bounds placed at the 3-sigma or 
99% confidence level are reasonably easy to defend under oath in the witness box. 
6 THE ANHARMONIC RATIO 
A fundamental property of a perspective projection is the anharmonic ratio (sometimes known as the ‘cross ratio’ (for 
example see Wolf, 1983, 252 ). This ratio can be best described by consideration of Figure 7. The cross product of | 
AC.BD divided by AD.BC remains invariant for a particular object through a perspective projection. 
In a recent court case involving the author as an expert witness, the objects in question were an M1 Carbine rifle and a 
Winchester pump-action shotgun. Both weapons had been shortened in both the barrel and the stock and were therefore 
distinctive but not necessarily unique. 
The guns, and the accused, had been seized by police at an incident (see 
Figure 8) which was quite separate to the bank robbery which occurred 
several months earlier and was under prosecution in the court. The 
defense attorney claimed that the weapons in the video scenes of the bank 
robbery were not the same as those confiscated by the police on the later 
occasion. These weapons would provide compelling circumstantial 
evidence that the person arrested by police for the latter offence was the 
gunman wearing the balaclava in the bank robbery (see Figures 9 and 10). 
  
Figure 8. Shotguns seized by police 
  
250 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000.