Page 144 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 123
made at every control point. In addition, angle and offset measurements are not to be used in the adjustment. Angle measurements are not included due to their inherent inaccuracy under water. Offset measurements are used in Site Surveyor to position artifacts but are not used to compute the posi- tion of the control network.
Three types of control points can be used in a survey: primary control points, secondary points, and detail points. Primary control points are estab- lished in the planning phase and are the main reference points for the survey. These are the most important points and must remain after exca- vation and should not be placed on artifacts or the structure of the site, but should be fixed to the seabed. Secondary points may be added later to help extend the site or to solve a line-of-sight problem. These points can be left in place after excavation but may be removed, and they can be placed on the structure as long as enough measurements have been made to the sec- ondary point from surrounding primary points so that the secondary point’s position can be reconstructed. Detail points may be added so that the fine detail of the site or structure can be recorded, such as a hull profile. These points are likely to be removed during excavation and are usually placed on the structure rather than on the seabed, as they are usually used to fill in gaps in the coverage of the primary and secondary points.
Artifacts can be used as secondary and detail points but should not be used for the primary control network. The artifacts used as control points should not be able to move and the actual point on the artifact used as the “point” should be obvious.
The optimum network shapes that should be used are circles and ellipses: Circular shapes are useful but generally an elliptical network is required. The ratio between the length and width of the ellipse (major to minor axes) should ideally be less than 2 : 1. Where the site is large, intermediate points can be placed within the site so that long baselines are not used. On some sites these points can be permanent and fixed to the seabed, however, this may not always be possible. Secondary points fixed to structure or big arti- facts can be used to jump the gap between the two sides of the site. Very pointed network shapes should be avoided as the position error will be large for the points away from the main body of the network.
3. Accuracy
A position error is the known accuracy of a position. Position errors are most usually shown as an ellipse drawn around each point. The ellipse indi- cates the probable position of the point, and the point should lie within the area of the ellipse. The smaller the ellipse the more accurate or precise is the position computed for the point. Ideally position error ellipse shapes