Page 166 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 145
resulting in a positive and relatively large residual. Thus all measurements with residuals over 100mm were ignored.
One interesting issue was that most of the depth readings also had a slight negative residual. This may be due to the program assumption that the pressure sensor depth is calibrated for seawater, and although the HPASS program corrects for salinity in the sound velocity, it does not correct for density in the depth. As the Maas is largely a freshwater river, this may be the explanation for the negative residual.
The deployment of HPASS in The Netherlands and on Pandora further proved the system’s capabilities in poor visibility. In its present form, HPASS presents an interesting alternative solution to conventional under- water surveying techniques. The system has a number of advantages: it is fast; it is accurate; it can be operated in low visibility and relatively deep sites; the speed of operation, once the transponders have been deployed, is at least as fast as a three-tape trilateration system; it gives an accurate and more reliable z-coordinate than the three-tape system; there is a consider- able saving on post-dive processing; and there is reliable assessment of errors.
VII. COMPARISON OF TECHNIQUES
In Chapter 6 photogrammetric surveying techniques will be discussed. There are some interesting comparisons of the efficiency, relative merits, and accuracy to be made between standard tape trilateration, acoustic measuring systems, and photogrammetric techniques (Green and Gainsford, 2003).
Reports of recent underwater surveys using standard tape trilateration, photogrammetric techniques using PhotoModeler (Green et al., 2002; Holt, 2003), and an acoustic system, HPASS (Green and Souter, 2002), raised questions about the efficiency, relative merits and accuracy of the various methods. In 2002, the Department of Maritime Archaeology at the Western Australian Maritime Museum, in conjunction with James Cook and Flinders Universities, conducted a practicum as part of a postgraduate diploma course. This course was designed to teach students the practical aspects of maritime archaeology and one of the student projects was to examine these survey issues. The assignment required the use of Site Surveyor in trilater- ation (on land and under water), PhotoModeler (on land and under water), and HPASS. The results of this work prompted the decision to carry out further studies and collate the results. One system not evaluated at that time was trilateration using control points. After the course was over, the system