Page 147 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

126 Maritime Archaeology: A Technical Handbook, Second Edition
with a maximum position and depth error of less than 150mm. A well- surveyed site will have an RMS of residuals at approximately 20mm and a position error of approximately 100 mm. For large sites or for quickly com- pleted surveys the errors can get quite large. This is only a problem if these large errors are ignored and the survey is quoted as being more accurate than it really is, or worse still, not computed at all.
The scale of the required drawing used to be a significant issue that could be used to determine the required positioning accuracy. As it is now com- monplace to produce the drawings in a computer-assisted design (CAD) program, this is less important as the site plan can easily be produced to any scale. The limiting factor on accuracy is now the accuracy of the tools used to position objects on the site.
7. Line of Sight Problems
In a number of cases the survey will be done around a structure, so there is no clear line of sight from one side of the site to the other. In this case the primary control points should be set around the outside of the site and secondarily, linking points should be set up on the highest points of the structure. If a part of the structure containing a secondary point is moved or recovered, then the secondary point needs to be re-established from the remaining primary points around it. Placing the control points high up around the site has two advantages: first it can be easier to make measure- ments across the site and secondly it adds extra depth information. The adjustment needs this depth information to be able to compute positions in three dimensions on what can be a flat site.
8. Installing Control Points
Once a theoretical control network has been designed, it will have to be set up on the site. It is at this point where the problems associated with sur- veying under water start to affect the quality of the survey. The tape survey is dependent on the diver’s ability to install control points in the right place and to measure the distances between the points with sufficient accuracy.
The first step is to install the control points, which are usually a series of construction tasks involving hammering pitons into rocks or poles into the seabed. Therefore, it is important to ensure that the points are in the right place and that they are secure and absolutely rigid. It is also important to clearly mark each control point so it can be found and uniquely identi- fied, especially in low-visibility conditions. The point should have a label attached on it or nearby along with a bright marker. Short lengths of bright orange tape work well as they are obvious and float up from the seabed.