Page 206 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 6: Photogrammetric Techniques 185
over the whole site and processed to produce a series of topographic plans. Each topographic plan was then reduced to a common datum plane by selecting several common points in a pair of plans and applying a reduc- tion formula. In the other common approach, the tower was leveled man- ually so that the plan from each stereo pair was automatically produced in a common datum plane. Although the relative heights between different tower positions may be different, their correction to a common datum was relatively simple.
Another simple method that was used to produce leveled stereo pairs involved a bipod made of a pair of metal tubing legs with a short horizon- tal connecting bar at the top, forming an inverted V-shape with a truncated apex (Figure 6.5). The height of the bar above the ground was set at 1.5 m and the horizontal bar was 750 mm long. Attached to the horizontal bar was a carpenter’s level with a pair of bubble levels, which allowed the bar to be leveled in the horizontal plane. Two Nikonos cameras with 15-mm water- corrected lenses were mounted on the level at exactly 500mm separation. The cameras were accurately aligned so that their optical axes were paral- lel and at right angles to the level. Some form of scale is required, but since the cameras will always be horizontal when the photographs are taken, it is only necessary to have a one-dimensional scale. Because the legs of the bipod were rather flimsy, a string or wire between the bottoms of the legs could be used to set the leg separation at a fixed distance. It should also be noted that a small photo tower can be useful in low light levels, both to ensure constant subject to camera distance, but also to eliminate camera shake and enable ling time exposure shots (Figure 6.10).
During the photography some buoyancy needs to be added to the tower to make it slightly negatively buoyant and thus easy to control. The tower is set above the exposure station with the two legs at right angles to the center line. The level is then adjusted and clamped so that it is horizontal or level in the sideways direction (parallel to the cameras and at right angles to the center line). The operator then slowly rotates the tower in the forward or backward direction, using the bottom of the two legs as a fulcrum, until the bubble level in the forward direction indicates that it is level. The sideways level should be checked to see if it has moved out of alignment. If it is still level the exposures are taken. This system is very simple to use and ensures that the cameras are always in the same plane when the photographs are taken. It has considerable advantages when ana- lyzing the coverage in a stereoscope and avoids the lengthy leveling pro- cedures required with a standard four-legged tower.
With modern stereophotogrammetric techniques, there are a number of ways of calibrating cameras and determining the accuracy of the system, see the following sections. The promise of simple underwater stereo- and