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Chapter 4: Conventional Survey 125
essential that a depth measurement be made for every point on the site. The flat shape of most survey networks means that it can be difficult to determine the depth of a point from measurements alone. Fortunately, dive computers provide a readily available method of measuring depth to a rea- sonable accuracy.
In general, the site will have some form of obvious symmetry so that the location of the position reference points and thus the coordinate system can be easily established. In other cases the coordinate system will have to be set arbitrarily. In both cases the choice of the coordinate system and the position reference points are worth considering. It is useful to have a system where all of the coordinates in the survey will be positive. Thus the origin can be set well off the site with the site lying in the upper right quadrant of the rectangular coordinate system. Additionally, care needs to be exer- cised when dealing with the Z component. It is important to be sure that the depths are set with the correct sign (down, positive; up, negative).
5. Depth Reference Points
The depth reference point is used as the overall reference when making depth measurements. As seen earlier, the control network can be placed anywhere on a local grid in the same way the depth of the site can be placed at any chosen level. If the equipment is available the depth of the site can be referenced to some other point such as the lowest astronomical tide (LAT), although usually an arbitrary depth is selected. The absolute depth of the site may not be important, however, all of the other points must be positioned in depth relative to one point on the siteā€”the depth reference. The choice of which control point is used is arbitrary, but the first reference point would be a good choice.
6. Specifying Overall Accuracy
The overall accuracy required for the survey should be specified by the archaeological director during the planning phase. The accuracy specified should be realistic and achievable. It is important not to overspecify the accuracy required. The accuracy specified should depend on the accuracy of the tools being used, the time available, the size of the site, and what accuracy is really needed.
Three values have to be specified: the root-mean-square (RMS) or average of the residuals, the maximum position error, and the maximum allowed depth error. Other more advanced figures can also be quoted such as unit variance and reliability values. For a typical underwater survey of a small site, an RMS of residuals of less than 50 mm is easily achievable along



























































































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