Page 129 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

108 Maritime Archaeology: A Technical Handbook, Second Edition D. THREE-DIMENSIONAL LEAST-SQUARES ADJUSTMENT
A three-dimensional least-squares adjustment program is another solu- tion to the problem of surveying a three-dimensional structure and is dis- cussed in Section VI.
E. THREE-DIMENSIONAL TRILATERATION
Three-dimensional trilateration consists of measuring from a number of control points to the point of interest by using tape measures. The system has been widely used on projects such as the Mary Rose and by the Insti- tute of Nautical Archaeology projects in Turkey. In most cases at least three control points must be used in order to obtain three-dimensional informa- tion, and in general four or more control points are used to determine the accuracy of the fix and to assist in the identification of “rouge” measure- ments. Obviously, for good three-dimensional trilateration the geometry of the control points needs to be carefully considered. Vertical control points need to be reasonably high in order to ensure that vertical accuracy is at least as good as horizontal accuracy (see Figure 4.11). This is discussed in more detail in Section VI.
IV. PROFILING
In certain situations, profiles can be particularly useful and may con- tribute vertical or three-dimensional information to a horizontal survey. There are a number of different situations where profile information can be of use; particularly in the case of recording a flattened hull structure, where the curvature of the hull is of interest. There are various simple systems of recording profiles: the offset bar, the distance-angle method, and the mechanical profiling device. Alternatively, leveling methods can be used to measure profiles (see Section IV.D).
A. THE OFFSET BAR
The offset bar is placed across the site at right angles to the axis and pinned to the site so that it cannot move. Its orientation in the horizontal