Page 154 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed
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Chapter 4: Conventional Survey 133 A. THE PANDORA
The Pandora wreck site covers an area approximately 50 ¥ 30m on a slightly sloping seabed. The site had a previously established grid system covering the whole area with the grid lines defined with 1.5-m survey poles. The first objective of the HPASS survey was to plot the existing grid to determine the position and accuracy of the coordinate system. The HPASS system was then used to plot some of the major features on the site. As the HPASS system had only just completed field evaluation trials in shallow water, it was uncertain what the performance and operating range of the system would be at a depth of 30 m. The maximum designed survey range of the system was 50 m, so, given that this was at the limit of the range for this site, it was decided to divide the survey into two parts to ensure reli- able coverage.
The stern area was surveyed first and then the bow section. This arrange- ment also allowed the six transponders to be deployed well outside the operating area of the excavation. Thus they would not interfere with the excavation nor be disturbed by hoses and lines. The six transponders were first placed around the perimeter of the site in their tripods (Figure 4.18). Survey work started on February 6 continuing on February 7, 10, and 11. On February 13 two of the transponders in the stern section (T4 and T6) were moved to new positions in the bow area (T7 and T8) and the survey continued on February 13, 14, 16–18, 21, 22, and 24. Each dive lasted 40 minutes and on one day (February 10) the system was used on two sepa- rate dives. During the expedition a total of 217 measurements were made in a total diving time of 520 minutes. Given the delays at the start and end of each dive and the setting up time, the system, on this site, was capable of making about one measurement per minute.
1. Post-Processing
At the end of each day these data were downloaded from the diver-unit (Figure 4.19) into a computer on the expedition vessel in a directory designed to contain all these data for that day. HPASSConvert, the program developed by Alec Duncan at CMST, was then used to process the raw data from the diver unit by converting time delays between sending the signal to a transponder and receiving a reply and calculating temperature and pressure. These data were then output in a format that could be read by Nick Rule’s Web program.
HPASSConvert, in its first version, allowed the operator to manually enter the average temperature for the whole dive and the measured salin- ity. This provided the essential parameters for the program to calculate the





























































































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