Page 55 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

34 Maritime Archaeology: A Technical Handbook, Second Edition
a GPS or by placing buoys at the start and finish of the first and last lanes of the survey. The start and finish buoys can be placed using a GPS at the locations, and these will serve the boat handler with a simple visual guide. In a simple operation, if the exact timings are obtained, these can be used to determine boat speed. If a chart recorder is used for the output of data, a fiducial mark can be made on the trace so that the output can be accu- rately correlated with position In a more complex operation a data logger can record the position of the vessel together with all the other remote sensing information, which can be used later in post processing (see Sections V.A and VI.D).
C. SEXTANT SURVEY
Although the sounding sextant is now virtually obsolete, it was the main instrument used in hydrographic surveying from the 19th century up to the advent of the GPS. The principles of the instrument are described in detail in the Admiralty Manual of Hydrographic Surveying. (Hydrographer of the Navy, 1965, Volume 2, Chapter 3). The sextant is accurate, portable, rela- tively inexpensive, and simple to use. Two instruments are generally needed to measure the angles included between three points (Figure 3.7). In this example, three land features, A, B, and C, can be observed from point O. The angles AOB and BOC are measured onboard the boat to define the position. This type of survey suffers from two major drawbacks: it requires considerable practice to become competent in using it, and it is only able to measure angles from 70 to 80° (beyond this the instrument can resolve angles up to about 110°, but with great difficulty). Given these two limita- tions, the sextant is very accurate (provided adequate consideration is given to the strength of fix, see next paragraph), and enables a position to be fixed onboard the vessel, rather than having to operate land survey stations. With practice, it can be operated quite easily in relatively rough seas; the skill lies in keeping the sextant horizontal while the boat is in motion.
One important theoretical issue relating to the use of the sextant is the strength of a fix or resection. This is a complex matter and is described in detail in a number of surveying handbooks, including, and possibly the best, Admiralty Manual of Hydrographic Surveying (Hydrographer of the Navy, 1965, Volume 1, pp. 357–360). Essentially, the strength of an intersection depends only on the receiving angles at the unknown point. The strength of resection on the chart, however, depends on the angles at which the posi- tion circles cut each other. These in turn depends not only on the observed angles, but also on the positions of the objects observed relative to the unknown point. In general, the sum of the two angles should not be less