The determination between the candidate cameras depended on matching the geometric characteristics embedded in the ground photos to the ground features which could also be identified on aerial photographs. The procedure was as follows. The angular spacing of those features was first determined. This procedure is illustrated in Figure A1, one of the pictures so used. In the photograph, the various features were measured from an origin at the left. The ratio of the overall image width to the measurements (1) through (9) could then be used to calculate the respective angle for each camera system by:
Where: W = width of the print being measured,
FoV = camera field of view,
SF = scale factor of print to original negative.
The measurements made were then plotted on a transparent medium. The height of one or more features was also determined by shadow length methods, and this datum used to confirm the locus of the ground camera's exposure station.
The angles measured in Figure A1 were drawn on a transparency (Figure A2) and then were slid over an aerial photograph and moved until a good visual fit of the various lines was made with picture elements originally measured on the ground photo but as seen from the air.
The shadow height measurements provided an independent check on the visual fits obtained using the transparent overlays. An overlay was generated for each of the candidate cameras. Some of these plots seemed to fit the aerial photographs, but when the shadow heights information was used to compute the exposure station, a large discrepancy occurred. Only one camera - the Voighlander Bessa - provided consistent fits in which both the overlay and the camera station measurements agreed.