Page 56 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 35
Figure 3.7 Sounding sextant fundamentals. This figure shows how the two included angles, AOB and BOC, which are the angles between Woodman Lighthouse (A) and the Obelisk (B) and the Obelisk and the 193-m high chimney (C) at the Alcoa Aluminum Refinery (all promi- nent features), are used to locate the position of the survey vessel.
than 60°. The central object should be either closest to, or much farther away from the surveyor than the other two objects. Other special consid- erations include: if the objects are in a straight line, the angle between any two objects should not be less than 30°; and where the observer is inside the triangle formed by the three objects, the angles should not be less than 60°. Figure 3.8 shows six types of resection. In each case A, B, and C are the known points and D is the point where the resected angles have been observed. In resections 1 and 2 the angles are exactly the same, with good