Page 122 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 101
or that the site is irregular and the measurements need to be reduced to a common datum plane. There are various ways of surveying in three dimen- sions. One simple way is to measure the vertical component in combina- tion with a horizontal survey, and reduce all the slope measurements to a common datum plane by applying Pythagoras’ theorem to the right-angled triangle formed by the slope distance and the height. Other approaches use three-dimensional spatial geometric measurements to calculate the coordinates.
Trilateration can be used to survey in three dimensions under water, however, it requires at least three reference points, usually two ground points and one elevated point, and can be extremely complex. Therefore, it is worth considering if the simpler approach of a two-dimension survey is justified. The accuracy of the vertical component is related to the elevation of the vertical reference point. Whereas ground reference points are typi- cally 2–5 m apart, a vertical reference point is unlikely, except in special cir- cumstances, to be greater than 1–2 m above the ground level. This is because it is difficult to construct a stable reference much higher than this. As a result, the level of accuracy in the vertical plane will usually be consider- ably lower than that in the horizontal plane.
Although the degree of vertical accuracy may be acceptable, it should be noted that, because the coordinates are interrelated, this can also affect the horizontal accuracy. For example, take a simple, poor fix situation where the intersection lies about 1° from the straight line joining the two survey sta- tions. If the distances are 1000 units, then the fix will be offset by about 17 units. If the measured distances are 1001 units, then the distance will be offset 48 units. Compare this with a situation where the angle of intersec- tion is about 20° and is thus a strong fix. In this case an increase from 1000–1001 makes a difference of 342–345 offset (Figure 4.7).
Obviously, when working within a three-dimensional structure like sub- stantial ship remains, there will be considerable vertical structural compo- nents. These will provide opportunities to establish rigid survey points so this will not be a problem. In this situation, the only problems may be in selecting appropriate points that will allow uninterrupted access, with the tapes, to the interior of the structure.
B. THREE-DIMENSIONAL RECTANGULAR COORDINATE SURVEY
An extremely simple and effective technique for surveying a site in three dimensions was used on the excavation of the James Matthews (Baker and Henderson, 1979; Henderson, 1976, 1977b; Figure 4.8). A rigid three-