Page 160 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 139
to import data directly into Site Surveyor. Together with Alec Duncan from CMST, HPASSConvert was programmed so that it exported data in a csv (comma delimited) format enabling these data to be directly imported into Site Surveyor. In addition, the HPASSConvert was modified to take advan- tage of the layers option in Site Surveyor, which allowed each day’s survey data to be placed in a unique layer containing the intertransponder dis- tances in a control layer and the distance to fixes in the fix layer. It was then possible to add that day’s intertransponder distances to the previous inter- transponder data and then recalculate by simply turning on all the control layers. This progressive adjustment of the intertransponder distances pro- vided more refined and accurate results while indicating if the transponders had been disturbed since the previous measurement and adjustment. Once the intertransponder distances had been adjusted, the position of the transponders was locked so that they took no further part in the point loca- tion adjustment, instead simply acting as fixed points. The layer option then allowed all but one set of measurements or fixes to be adjusted. This reduced the processing time while keeping all the measurements in a fixed data set. The process is shown in the flowchart in Figure 4.24.
The flowchart shows the process of adjusting the new intertransponder distances, then locking the transponder positions, and then adjusting these data for the day. This turned out to be an extremely useful process, enabling the manipulation of data on a day-by-day basis.
5. General Results
The final results of the Pandora survey were exported from Site Surveyor as a DXF file and imported into ArcView GIS for final graphic presenta- tion (Figure 4.25). A total of 178 points were recorded by 758 measurements using the HPASS system. The final RMS residual error was 35 mm, which is impressive given that the system was operating over measured distances of up to 45m.
On the overall site plan (Figure 4.25), the system showed a small but sig- nificant error in the alignment of the grid system. There were problems with recording features in excavation trenches, as the probe tended to be in an acoustic shadow at the bottom of a hole. The system was modified with a 1-m extension pole that could be attached to the probe and then carefully leveled with a bubble level thus giving a more reliable fix.
It is difficult to represent the overall accuracy of the system in a form that can be easily visualized, particularly on a site where the precise size of the objects is not known and where the positions of objects are not clearly defined. Two examples are illustrated here to attempt to demonstrate the system’s accuracy. On the stern section of the site, while surveying the grid