Page 116 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 95
It should be noted that the distance-bearing system has a constant error function, i.e., the position error is a function of angle and distance and although it increases linearly with distance, the angle error is constant irre- spective of the angle, unlike trilateration which depends on placement of the control and the strength of the fix. Thus the distance-bearing system can be a very attractive system, because there is no need to take into consider- ation factors relating to placement of survey points and accuracy or strength of fix. The survey point may be placed in the center of the site or off the site, depending on the situation. When working close to the survey point, if the sighting device stands above the seabed, care must be exercised to ensure that the slope angle of the tape measure to the horizontal is not greater than a few degrees; otherwise offset errors will occur.
C. RECTANGULAR MEASURING SYSTEMS OFFSET SURVEY
A simple form of a rectangular measuring system is the offset survey, where offset distances are measured from a baseline (Figure 4.3). The system requires some method of defining a right angle, usually a rigid right angle cross. This can easily be made out of square, mild steel tubing about
Figure 4.3 Offset recording. Diver is using a tape measure to measure the offset from the baseline running horizontally across the photograph. (Courtesy of Catherina Ingleman- Sundberg, Department of Maritime Archaeology, Western Australian Maritime Museum.)