Page 199 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

178 Maritime Archaeology: A Technical Handbook, Second Edition F. RECTIFICATION
The photogrammetric process of rectification is an operation requiring a complex photo rectifier. This large and expensive machine is well beyond the budget or accessibility of most excavators. However, provided high accuracy is not required, rectification can be done quite simply under an ordinary enlarger. The process requires tilting the photographic paper so that the distortions created in the photograph are compensated. This can be a time-consuming exercise, particularly as the various photographs will be at different orientations. To facilitate the rectification process, it is useful to construct a simple, adjustable, photographic printing table. This consists of a normal printing table mounted on a frame that can be rotated about its center and tilted in an axis which passes through the axis of rotation. The table should have facilities to measure the angles of rotation and tilt and a method of clamping the table in the required orientation (Figure 6.6).
The adjustment of the table to give a rectified print can be done either on a trial-and-error basis or by calculation. In the former case, a correctly scaled square grid is drawn up and placed on the adjustable printer board. The board is then adjusted until the distorted grid projected from the enlarger coincides with the correctly drawn grid on the printer board. Alter- natively, complex mathematical equations can be used to obtain the recti- fication parameters. A generalized view of a grid square showing the effects of perspective caused by camera tilt is shown in Figure 6.7. In reality, the photographic darkroom is rapidly becoming obsolete and rectification can now be done far more efficiently using a computer (see Section H below).
G. LAYING UP A PHOTOMOSAIC
At the outset, before mounting a photomosaic, the photographs must all be printed at the same scale and arranged in order. It is advisable to print the photographs at a lower contrast than normal as this helps to prevent the joins showing up and improves the quality when the mosaic is re- photographed and printed.
There are two ways of tackling photomosaics: the controlled method or the so-called uncontrolled approach. In the former method, the optical center of each photograph is first determined. The principal point from the adjacent photograph is then transferred onto the photograph (while viewed stereoscopically). A line is drawn through the principal point of the photo- graph and the transferred principal point. This is the flight line (Figure 6.8). The print is cropped, feather-edged (see later), and fixed on the mounting board. The adjacent print with a similar flight line is then trimmed and