Page 133 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

112 Maritime Archaeology: A Technical Handbook, Second Edition B. THE DISTANCE-ANGLE METHOD
The other similarly simple method of recording a profile is to use a dis- tance-angle system with a circular protractor and a tape (see Figure 4.1). In this case, the protractor is set in the plane of the profile, and the distance and angle are measured to selected points on the profile. A simple example of this system is illustrated in Piercy (1981).
C. MECHANICAL PROFILING DEVICE
Another simple profiling system, described by Leonard and Scheifele (1972), utilizes a series of parallel rods mounted in a frame that can be adjusted to touch the profile to be recorded. The frame is set up on the site, at the point at which the profile is required, and the rods are adjusted to contour the profile. The rods are then clamped and the orientation of the frame holding the rods measured. The system is then brought to the surface and the profile recorded by measuring the extent that the rods project. Although the system is simple to use, the time and effort required to make the frame together with the mechanism for clamping the rods mitigates against its usefulness, especially as there are other simple techniques that are more flexible and easier to implement.
D. LEVELING
1. Hydrostatic Leveling Devices
Hydrostatic leveling devices can be used in situations where three- dimensional measurements are required. The device measures the height or Z coordinate and is, therefore, useful for recording profiles and where relative heights are required.
The most common hydrostatic leveling device is the bubble tube, which is usually a long, clear plastic tube into which air is introduced. Because the air–water interfaces at either end of the tube will be in the same horizon- tal plane, this effect can be utilized to measure levels. It can be a difficult method to use, as the tube is long and unwieldy. There are two ways of using a bubble tube, either in the dynamic or static mode. In both cases, one end of the tube is attached to a reference datum point. In the dynamic system, air is introduced into the top of the tube under a small positive pressure (Figure 4.13). The mobile end is then adjusted in height so that air just begins to bubble out of the datum end of the tube. At this point a vertical