Page 141 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

120 Maritime Archaeology: A Technical Handbook, Second Edition
Before measurement can begin, each point or feature to be surveyed must be tagged with a unique code or number and, if one operator is doing the survey, the tag must be robust enough to hold the end of the tape measure firmly in place (a strong nail or hook is ideal). A pre-prepared writing slate should list, in a logical manner, all of the combinations of mea- surements required to complete the survey. The distances are then mea- sured under water. If two operators work together, then it is necessary for one to check that the tape is not snagged and possibly hold the writing slate when it is not being used. However, very little time is saved using two oper- ators, particularly in clear water when the operator can see that the tape is straight. On return to the surface, these data are entered in the computer. In practicing this technique, it has been demonstrated that if both the recording slate and the computer entry layout are based on the triangular format (Figure 4.17), then there is less chance for data error and the results are more understandable. Obviously, where communications are available there is the option to record the measurements at the surface and check for reliable fixes.
It may be seen, therefore, that this system has a number of extremely useful features in comparison with other surveying techniques. Any system that provides a rigorous statistical treatment of errors has immediate advan- tages over standard trilateration and the other relatively primitive under- water surveying methods. A practical field trial of the system in Thailand (Atkinson et al., 1989) is worth mentioning. Major features of an excavated hull structure (in 27 m of water) were marked with numbered tags. A total of 15 stations covered an area of 14 ¥ 3 m. All the ranges of the matrix were recorded. The measurements were initially taken by ten divers, but the numbers were later limited to four as inconsistencies among observers became apparent. This problem was compounded by having a number of tapes in use, some of which had both metric and imperial units, adding reading problems caused by the effect of depth. High residual values from the output of these data indicated which ranges were suspect and needed to be re-measured. Re-measurement was continued until shortage of time
A
B
C
D
E
A
27.05
16.55
12.37
15.95
B
7.36
19.76
11.32
C
9.85
22.13
D
14.31
E
Figure 4.17 Triangular format for direct distance measurements. Using a template like this, it is easy to ensure that all necessary measurements are taken.