Page 62 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 41 F. THEODOLITE AND DISTANCE MEASURING SYSTEM
Where it was impossible or impracticable to establish two stations, a land-based theodolite and distance-measuring system was sometimes used. At the crudest level an optical gun sight was used in conjunction with a theodolite, but the gun sights were hopelessly inaccurate. Other more accu- rate distance-measuring systems could be used. These included the tel- lurometer which used microwaves, and the geodimeter, which used a laser beam. These instruments were used to measure the distance to a reflector based on the vessel.
G. ELECTRONIC POSITION FIXING SYSTEMS
In 1989, I wrote:
By far the most widely used systems for maritime survey work are the elec- tronic distance-measuring systems. These systems are extremely expensive, and can generally only be justified for large-scale and well-funded operations. The unit cost is so high that it is only really practical for a maritime archaeological project to hire the system for the period of the survey, although it may be pos- sible to negotiate with the hire company ways to offset the cost with publicity or a reduced hire rate during a period when the instrument is not required for commercial operations.
This, naturally, is no longer the case.
In this section I discussed various systems that fell into two broad cate- gories: short-range systems (50–80 km) generally using microwaves in the 250–9000 MHz range and measuring distance from a mobile transmitter to a pair of shore stations; and medium-range systems (200–450 km) most com- monly using three or more shore-based transmitters and any number of passive receivers operating in the 1–3MHz or 420–450MHz bands.
MiniRanger was perhaps one of the best known systems at that time for small-scale hydrographic work. The system consists of a master station onboard the vessel and two slave stations on shore. The master station inter- rogates the slave stations in turn and measures the time of flight of a radio signal from the master station to the slave station and back. It then calcu- lates the distance by multiplying half this time by the speed of light to give the distance and displays the result. The accuracy was about ±1 m. In the Decca Navigator system, measurements were made of the phase between the signals, so that if the lock of the system was broken at any point, the survey vessel had to return to a known reference point to recalibrate. Neither system is commercially used today and is more of a reminder of the difficulties experienced in the past.