Page 138 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 117
Figure 4.16 Back and fore readings. This shows the process from point A to point G with fore and back readings using the modified builders level shown in Figure 4.12(c).
It is possible, using statistical methods, to close the traverse and readers are referred to the standard works on surveying (Hydrographer of the Navy, 1965, 1982).
3. Depth Gauge
Absolute pressure measurement is a useful method for obtaining verti- cal height, particularly because electronic gauges are available which will measure depth to about 0.1 m. At depth, the effect of surface waves or swell produces minimal fluctuation of pressure signals. Additionally, there are systems that allow the downloading of depth information where necessary. An alternative method consists of using a pressure gauge to measure the differential pressure between a reference point and the point to be mea- sured. A standard aneroid differential pressure gauge or two electronic pressure transducers can be used; the tube from one end of the gauge is attached to a reference point, and the relative pressure readings are taken at the gauge. Issues to do with pressure–depth readings are discussed in Section VI.
V. COMPUTER-BASED METHODS A. GENERAL CONSIDERATIONS
Calculating the position of a point—the fundamental objective of survey—is not the only consideration in producing survey results. It is also important to know the precision of the location of the point. All