Page 61 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

40 Maritime Archaeology: A Technical Handbook, Second Edition
Comparison between a theodolite-based system and a photographic angle measurement system indicates that the theodolite had an accuracy of 1.5¢ and was used on a baseline 2.5 km long with the site about 2.5 km off- shore on a relatively featureless coastline. The resultant positions from tri- angulation (using the theodolite) and photo resection (using standard lens, film, and paper) were 130m apart. It took a survey team a whole day to take the measurements, while the photograph took 1/125 of a second and about an hour of work to plot the data Today, with a GPS, that whole process can be done on the boat in less than a minute with the position accurate to ±2m.
E. DOUBLE THEODOLITE
The theodolite or level is a very attractive instrument to use in survey work because it is easy to use and very accurate. Its application today is related to situations where sub-centimeter accuracy is required. In this area, the GPS is no longer a viable instrument unless one is prepared to hire or buy very expensive equipment. The theodolite cannot be used on a boat, however, it was widely used in the 1960s and 1970s in a double instrument system for locating a site or the position of a search vessel at sea. This was then superseded by electronic systems such as the MiniRanger and then the GPS. In the two-theodolite system, the two instruments are set up at either end of a baseline of known length and the two included angles are mea- sured. Because one side and two angles of a triangle are known, the trian- gle and thus the position of the point to be surveyed can be calculated. A serious problem with this technique in a running fix situation was that of communication. The system relied on the two theodolite operators mea- suring their angles at the same moment in time, and ensuring that they did not miss a mark and get out of step. The theodolites had to be established on shore, requiring a shore landing party and had to maintain a two-way radio, which was an essential part of the system. The two operators zeroed their instruments on each other and then maintained the theodolite cross wires on a marked position on the survey vessel. If the absolute position was required, the distance between the two operators had to be determined. This was not a problem if the operators were on common ground, but if they were separated by water, for example, on different islands, the mea- surement was difficult to establish. Plotting was relatively simple by using either a protractor or a programmable calculator.