Page 137 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

116 Maritime Archaeology: A Technical Handbook, Second Edition
there is an appreciable tide, it is possible that the instrument will lose cali- bration and a recalibration at a datum point will be required to ensure that the measurements are related. Choppy sea conditions may also cause prob- lems, because in shallow water the meniscus will oscillate and an average will be needed to define the value of interest.
2. Carpenter’s Level
A carpenter’s level can be used to obtain vertical measurements, either by measuring directly down from the end of the level to the point of inter- est or by using the level to set a reference bar horizontal so that measure- ments can be taken from the bar. A development of the former method was made in Thailand by Geoff Glazier and the author, and this has proved to be particularly useful in low-visibility conditions (Atkinson et al., 1989). A simple form of the system consists of a carpenter’s level and a ruler. Start- ing from a fixed point, the level is adjusted so that the bubble indicates that it is horizontal. A ruler is then used to measure the rise or fall at the other end of the level. The position is marked on the site and the level reset on this mark. Using this system, a number of rises or falls can be recorded across a profile for fixed distances equal to the length of the level.
A modification to this system consists of two bubble levels mounted at right angles in a Perspex block. Two adjustable steel rulers can be moved in and out at right angles to each other through the block in directions par- allel to the levels. The instrument can be used in three ways. In the first way, the horizontal is set at a fixed distance (e.g., 1 m), and then the rise or fall is determined across a profile, regardless of the features (see Figure 4.12c). In the second method, the horizontal and the vertical distances are mea- sured from one feature to another, thus giving precise details on the struc- ture. In the third system, only the vertical component is recorded, so that an effective leveling process is carried out around a site from feature to feature until finally ending up at the start again.
This method is extremely efficient, although it requires meticulous record keeping. In particular, great care is required to note what a rise is and what a fall is. A simple way of working is as follows. If the long arm of the starting point extends away from the start, in other words, the vertical ruler rises from the origin and the horizontal arm extends out to the next point, then this is noted as an outward reading or, in surveying terms, a “fore” reading. In the opposite case, where the horizontal ruler starts at the origin and extends out to where the vertical ruler drops to the next point, then that reading is an inward or “back” reading (see Figure 4.16). If one proceeds around a traverse, finishing at the starting point, then the fore readings should equal the back readings, i.e., the rises should equal the falls.