Page 115 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

94 Maritime Archaeology: A Technical Handbook, Second Edition
used to take the readings on the protractor thus giving a greater degree of accuracy. The bearing is taken by noting the angle that the wire makes with the outside of the protractor.
Alternatively, an alidade, consisting of a simple ruler rotating about the center point of the protractor with sighting pins or a tube with cross wires at either end, can be used to sight independently in the direction of the tape. It is also possible to use a nonoptical, underwater theodolite, which is basi- cally the same as the sighting tape alidade, except that it measures both ver- tical and horizontal angles. This will be discussed in Section III.
B. RIGHT ANGLE SURVEY
An alternative and potentially more accurate option to a circular pro- tractor is a rigid T-shaped angle-measuring instrument. The angle is deter- mined by taking a reading on a tape mounted on the cross-part of the T. Because the distance from the origin (at the bottom of the cross) is fixed, the offset along the T gives the tangent of the angle (Figure 4.2). This instru- ment can give a greater accuracy than the protractor, because it is easier to construct on a large scale. It also has the advantage that it can be con- structed very quickly and simply in the field. Other systems include the hydrolite. See Lundin (1973) and Cederlund (1977) for a description of this system.
Figure 4.2 T-shaped angle-measuring device. This is more accurate than the protractor. In this case the angle is determined from the arctan ratio of D/L.