Page 148 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 127 Once the control points are in place the measurements between the points
can be made.
9. Collecting the Measurements
To get the best results when collecting measurements, the recorders need to be trained in making measurements under water. Common mistakes can be avoided if one is aware of the problems beforehand. Typical problems include:
1. Control points that cannot be easily found; clearly mark the control points with bright markers and big labels
2. Measurements made from the wrong point; uniquely and clearly iden- tify all points
3. Recording forms are unreadable; use standard forms and teach the operators to use them correctly
4. Numbering formats on the forms vary; standardize in meters to three decimal places (000.000) or millimeters (000000)
5. Values read wrongly from tape measure; teach the operators to read the tapes correctly on land then try under water
6. Wrong part of control point used is used for measurement; demon- strate how and where to measure using the same type of control point
7. Offset was not recorded; where an offset is expected add a field on
the recording form
8. Snagged tape measures; ensure the operators swim the length of the
tape if both ends cannot be seen at the same time
It may be necessary to refer back to the recording sheets during processing to help decide whether a measurement should be rejected. A number of factors affect the quality of the measurements: working in low visibility, currents, and deep water. These all tend to produce more frequent mistakes or blunders. So the visibility, current, and any other potential prob- lems should be recorded on the form together with the measurements.
10. Depth Measurements
Care has to be taken when measuring depth because of the effects of tide. To counter these effects it will be necessary to select one control point on the site to use as a depth reference. When a depth measurement is made at any other point, a measurement of the depth at the reference point should be made so that corrections for changes in tide height can be cal- culated. It is possible that the change in tide could be tracked using a depth- recording instrument, so that at the time that the reading was made the