Page 191 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

170 Maritime Archaeology: A Technical Handbook, Second Edition A. CONTROL
In order to produce a properly scaled photomosaic, each photograph requires some form of two-dimensional control. The choices are threefold: grid lines can be set up, a two-dimensional grid frame can be positioned in each photograph, or a network of control points can be used. There are merits and drawbacks in all the systems and the choice will depend on a number of complex factors.
B. GRID LINE CONTROL
The grid line system consists of a series of parallel lines graduated in fixed intervals set up across the site. The lines are generally colored rope (to reduce halation) black taped at regular intervals. The control can be implemented by driving a series of stakes into the seabed at either end of the site, set at a fixed, predetermined distance from one another, parallel, and at right angles to the baseline. This operation requires careful survey- ing to set up the right angle; various methods for doing this are discussed in Chapter 4, Section II.B.
Another factor to consider is the type of rope used in grid line control. Rope, and particularly synthetic rope, stretches when put under tension; over 20 or 30m it is possible to stretch a rope 1 to 2m without difficulty, i.e., by 10%. Ski rope, plaited rope, or woven synthetic line are ideal as they have a low stretch; are colored, which is useful for anti- halation; and come in convenient diameters. This type of rope still has some stretch in it, so care must be taken not to overtighten the line. Another alternative is wire, but there are some problems with this. It is generally too fine to be resolved in a photograph so special targets are required; and more significantly, it can be dangerous in rough conditions as it can easily injure divers.
Before setting up the grid, it is important to calculate the appropriate lane separation. In order to establish proper control, each photograph needs at least two, or preferably three lines with at least two graduations on each line. The required flying height can then be calculated once the orientation of the camera format to the baseline has been determined. Assuming that the long (36 mm) side of the frame is at right angles to the axis of the site, then (from Figure 6.3), if D is the line separation and d is the length of the graduations, then the height H is given either by:
H=Ê20¥Dˆ or H=Ê20¥dˆ Ë 36 ¯ Ë 24 ¯