Page 125 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

104 Maritime Archaeology: A Technical Handbook, Second Edition
accurately leveled so that it formed a horizontal datum 6 ¥ 1m. On this frame, resembling a bed frame, a sliding H-shape was placed so that the ver- tical arms of the H could move along the 6-m bars, and the cross of the H ran between the 6-m bars across the site. From this bar, a plumb-bob was dropped to particular points on the site that were to be measured. The ver- tical distance from the reference plane to the point to be measured gave the vertical or Z coordinate; the distance across the H-bar gave the second, longitudinal or X coordinate (to this had to be added the position of the bar from the start of the survey, because, as the excavation proceeded, the bars were moved along the site and leveled). The distance from the base- line to the H-bar gave the lateral, or Y coordinate. Thus, as the bed frames could be extended and leveled across the site, it was possible to effectively create an artificial reference plane across the whole of the site. Tape mea- sures were attached to the arms of the H and to the bed frame so that the X and Y coordinates could be easily read.
A sketch plan was made of each 6 ¥ 1 m grid frame and the positions of points of interest noted on the plan and numbered. These numbers were then listed, and the recorder made the three-dimensional measurements of the several hundred coordinates in the frame level (Figure 4.9). The draw- back with this technique is that it is extremely time-consuming and only suitable for calm, shallow water sites. It is, however, a highly effective method of accurately recording a site and it can be complemented with pho- tographic recording. This is helpful for adding additional details.
C. ANGULAR MEASUREMENT
The distance-bearing system of measurement can easily be modified for three-dimensional work in clear water. An underwater theodolite is not a particularly difficult instrument to construct. It is based on the land theodo- lite with simplified optics (Figure 4.10). This system, mounted on a solid tripod, consists of three parts. The first part is a table or platen that can be leveled with three adjustable leveling screws. Two small bubble levels are mounted on the platen, one between two of the leveling screws, the other at right angles to this (in the same way as an ordinary terrestrial optical theodolite). Leveling proceeds first by removing the tilt between the two screws that have the bubble level between them. Then the other compo- nent of tilt is removed by adjusting the third screw. Provided the first pair of screws are adjusted by an equal and opposite number of turns, the lev- eling will proceed quickly and a final round of fine adjustments removes the remaining tilt. Thus, the platen is level.