Page 193 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

172 Maritime Archaeology: A Technical Handbook, Second Edition
pairs of photomosaic photographs can be used in a stereoscope, it is useful (although not essential) to work with 50% overlap which will allow the images to be viewed stereoscopically. This means that every part of a pho- tographic run will always appear in at least two adjacent photographs; or, in two adjacent photographs, the right-hand half of the view appearing in the left-hand photograph also appears in the left-hand side of the right- hand photograph. If the flying height is H, then the short format coverage (24mm) will enclose a ground distance of 1.2H. For a 50% overlap, the separation of each exposure is 0.6 H. Again, for convenience and to ensure proper coverage it is necessary to work to the next smaller integer, and it may be necessary to adjust the flying height taking care not to compromise the framing (see Baker and Green, 1976).
For example, using the long axis at right angles to the baseline, in order to include three lines in each frame, and with a line separation of 1 m, the flying height must be at least 1.7m and the forward ground coverage will be 2.04m. If exposures are taken at 1-m intervals, there will be a 50% overlap, but the slightest tilt will create problems. In this case, it would be preferable to increase the flying height to 2m, for example, which would give a format of 3.6 ¥ 2.4 m, which provides plenty of leeway for errors.
C. GRID FRAME CONTROL
An alternative approach to the photomosaic lines is to use a square grad- uated frame. Giving due consideration to flying height as discussed previ- ously, the height is adjusted to give the appropriate framing in the view. At the same time the grid frame is observed in the viewfinder and used to assist in leveling the camera (the Baker technique discussed earlier is particularly useful here). This system is most efficient with two operators, one to do the photography and the other to turn the grid frame over. Otherwise, it is extremely time-consuming for the camera operator to repeatedly swim down to turn the grid frame over. Once again, close attention needs to be paid to overlap and camera format orientation to ensure adequate cover- age. The work of turning the grid frame over can be made easier by rotat- ing the frame on one of its edges from one side to the other. The progress of the mosaic has to be carefully monitored to ensure that the run does not start to deviate from a straight line. It may be worthwhile setting up a tem- porary baseline that can be used to align the grid frame. As it is possible to estimate distances up to at least three grid squares from the line, the line will not need to be reset at the end of each run.
It is important that the camera be leveled relative to the grid frame, because tilt creates distortions that cause difficulties in matching adjacent prints. There are a variety of methods available to help orientate the camera