Page 117 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

96 Maritime Archaeology: A Technical Handbook, Second Edition
2 m long with a sighting pin at each end of the cross. As with most of this type of work, good visibility is essential. The rigid square is placed on the baseline so that one of the arms of the cross is parallel to the baseline; the arms at right angles give the direction of the offset on either side of the line. In this simple form, it is a reasonably accurate and efficient system with good visibility. Alternative systems include a right-angled triangle and an optical square. In the first case this is a simple variant of the square; in the latter case, an optical square is a complex system which is, again, only useful where there is clear visibility. In general, the usefulness of such systems relies on two issues: first, the ability of the surveyor to project the line or sighting beyond the local range of visibility, and; second, the accuracy and ease of use of the visual system.
The construction of a hand-held instrument on the lines of a terrestrial optical square requires a mirror mounted at 45° to the axis of the direct sighting and two pins on a bar with a 45° mirror in between. By aligning the two pins along a baseline, the visual position at the right angle to a point at which the instrument is located can be sighted. In this situation, a second assistant can move a marker around until the operator has determined that the baseline and the marker are in coincidence, and thus at right angles. This type of system is also very useful for setting up a rectangular grid on a site.
D. TRILATERATION
Trilateration is, in theory, one of the simplest two-dimensional survey techniques, however at times it can be complicated (Figure 4.4). A single point can be uniquely determined relative to two fixed reference points if, first, the distance between the two reference points is known, and secondly, the distances from these two reference points to an unknown point can be measured (Figure 4.5). This represents the textbook situation where three sides of a triangle are known and, as a result, the triangle is uniquely defined.
In practice, two fixed semipermanent markers or survey points are set up on the site, possibly on the baseline. In some cases, several different survey points have to be selected because of the size of the site. These must be rigid and permanent reference points that can be easily identified with some sort of tag. On a sandy or muddy site, the points can be a series of stakes driven deeply into the seabed so that they are rigid. In rocky condi- tions, pitons or steel pins with a ring can be driven into the rock. The survey may be extended from one area to the next by selecting a new reference point and utilizing one point from the last survey with the new point.