Page 127 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

106 Maritime Archaeology: A Technical Handbook, Second Edition
In the center of the platen is a pivot on which a circular protractor turns. This is the second part, the horizontal circle onto which a holder is mounted. Onto this is mounted the third part, which consists of a sighting tube with cross wires at each end attached to a vertical protractor. The center of the axis of the sighting tube and vertical protractor are arranged to lie verti- cally above the center of axis of the horizontal circle. After leveling the platen, the operator sights the object under investigation through the sight- ing tube, aligning the cross wires exactly on the object, and the horizontal and vertical angles are then read. The bearings taken from the horizontal circle may be made relative to a fixed point by taking a bearing from a ref- erence point, thus any subsequent survey from this measuring station can be correlated to this fixed point.
The tripod or stand carrying the theodolite has to be heavy and solid so that it will not be easily dislodged. The theodolite should be carefully calibrated, in air, to ensure that there are no systematic errors in the read- ings of the protractors; in particular, the sighting tube should always be viewed from the same end of the tube. The leveling of the tube should also be checked to ensure that the tube is horizontal. With the vertical marker on zero, an object is viewed in coincidence with the cross wires in one direction through the tube. The sighting device is then rotated through 180° in the horizontal plane and the object should still appear in coinci- dence with the cross wires. The underwater theodolite is a simple and effective instrument for predisturbance and site surveying work, but it is only effective in clear, calm water. Its accuracy is limited because the angle measurements do not benefit from the sophistication of the land-based instruments.
The instrument can be used in a number of different ways to determine three-dimensional coordinates. One option is to set up the theodolite in a particular position. The distance and horizontal and vertical bearings are measured to objects on the site, and these measurements can then be used to calculate the horizontal and vertical components of the coordinates of the objects on the site relative to an arbitrary grid. To properly define the grid, it will be necessary to select a datum point and define its three- dimensional coordinates. In this way, the height of the theodolite above the ground need not be known. Should the theodolite be moved, it is simply necessary to coordinate, at the next station, a few common objects so that the new survey can be matched to the old.
Alternatively, the theodolite can be used as a level. In this case an under- water surveying staff is used. The theodolite is set with the tube horizontal and the operator indicates to the staff operator where the coincidence or level occurs. Horizontal distances can then be taken from the staff to the theodolite.