Page 112 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 91 II. TWO-DIMENSIONAL SURVEYING TECHNIQUES
Because most site surveys are largely two-dimensional, the more complex techniques of three-dimensional surveys will be described sepa- rately, however, the principles are essentially the same. Most, but not all sites, have a small vertical elevation compared with the horizontal compo- nents. Therefore, in many cases where the vertical elevation is less than 5 to 10% of the horizontal elevation, ignoring the vertical components will have very little effect on the survey. For example, the difference between the “true” horizontal measurement of 10m where the relative vertical height between the two points is 0.1 m is 9.95 m. If this was 100-m with a 1- m vertical height the distance would be 99.995 m, a mere 5 mm difference. If this is all that is involved, measuring vertical components could be irrel- evant. Obviously, this needs to be treated with caution, and the survey system needs to take into account the fact that the vertical component is being ignored. It should also be remembered that all measurements will have errors, both due to mistakes in recording measurements and in the accuracy of the measurement itself.
A. DISTANCE-ANGLE OR RADIAL SURVEY
To use a single tape measure and measure the bearing or angle of the object to a fixed reference point is an example of a simple survey system. This technique works best in clear water where the point to be measured can be seen. If one cannot see the end of the tape, a bearing along the tape can be utilized, but one needs to ensure that the tape is not snagged on an obstruction.
This system can be worked with one or two operators. With one opera- tor, the zero end of the tape is fixed to a central reference point. The oper- ator then swims to the first point, making sure that the tape is kept well above the seabed ensuring that it does not snag the bottom. At the point to be measured, the distance and bearing to the reference point is recorded. If it is not possible to see all the tape because of poor visibility, then it will be necessary to swim along the tape to check for snags. With the two- operator system, the mobile operator takes the zero end of the tape and places it on the position to be measured. The other operator, who is at the reference point, records the angle and bearing on a pre-prepared list on the record sheet, while the mobile operator with a similar list makes notes of the features being recorded. This is easier to manage than the reference operator having the zero end of the tape and the mobile operator the tape reel and recording the distance, because it is almost inevitable that the two