Page 130 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 109
plane is thus random. Provided there is a fixed linear feature such as a baseline, the longitudinal position of the bar can be determined, and it can be set at right angles to this baseline (Figure 4.12a). The orientation of the bar to the horizontal plane can be measured with a spirit level or plumb bob, so that the relationship of the bar to the horizontal plane is known (Figure 4.12b). Measurements of the perpendicular distance from the bar to the feature on the site can be made with a ruler that must be set at right angles to the bar. Either the ruler can be made to slide in a slot, which is mounted on the bar and in turn can move on the bar or, alterna- tively, a square, which can slide along the bar and can be used in conjunc- tion with a ruler to measure the distance (Figure 4.12c). Because the orientation of the bar at the various stations are known, the coordinates can be reduced mathematically to a common datum (see Green and Harper, 1983).
Figure 4.11 Vertical control problems. Note that where there is little vertical elevation the fix is bad. With elevated control points the fix is improved, but constructing elevated control is often difficult. See Figure 6.13 where artificial control has been used.