Page 113 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

92 Maritime Archaeology: A Technical Handbook, Second Edition
operators will, at some point, get out of sequence in their readings. It will then be difficult, if not impossible, to reconstruct these data. The two-operator system has the advantage that it is easier to keep the tape free from snags and it is faster.
When using this system, it is important to note the difference between a back or reciprocal bearing and a forward bearing. If a surveyor takes a tape with the zero attached to a datum point and swims in a northerly direction, then, turning around, sights back along the tape, this will give a back or rec- iprocal bearing or southerly reading. Alternatively, if the compass is used to sight in the direction of travel, then this is a true bearing (from the ref- erence point to the object) and will be north. Thus great care is needed in recording what type of bearings are being used.
Underwater angle-measuring devices, by nature, are of simple construc- tion. They unfortunately suffer from inherent accuracy limitations, particu- larly because it is not possible to incorporate an optical telescope into an underwater angle-measuring device to give the equivalent of the highly accurate terrestrial theodolite. Most underwater angle-measuring devices have accuracies of around ±0.5°. There is a limitation, though. For example, if one had an error of 1° in the measurement, then at 10 m from the mea- suring point the error in position will be 175 mm, and this error will increase in proportion to the distance from the measurement point.
The magnetic compass is one of the most simple underwater angle- measuring devices, but because it has an accuracy of about ±5°, it has only very limited application. Also, magnetic compasses are affected by iron objects, thus making it quite unsuitable for survey work on ships that have large amounts of iron on them. The compass is ideal for broad survey work such as defining the extent of the site or in the general preliminary survey work, but it is not useful for detailed work.
An alternative solution, if an angle-measuring device is required, is to enlarge a 360° protractor to a diameter of about 0.5 to 1 m, which can then, because of the enlarged scale, measure angles at greater accuracy (Figure 4.1). A simple way to do this is either to photograph a good quality plastic 360° protractor using lithographic film, or to place it on a sheet of photo- graphic lithographic film and expose it to light to produce a contact nega- tive. When developed, the negative can be used to print a large-size positive on resin-coated paper. The resultant print of the protractor can then be trimmed and mounted on circular plastic or a metal sheet, making an ideal underwater angle-measuring device. The protractor, mounted horizontally on a reference stake, can be used to take direct bearing measurements, but care is needed to ensure that the protractor does not rotate on the mount- ing and that the mounting is rigid. The tape measure can be attached to the center of the protractor with a short length of wire so that the wire can be