Once a maritime archaeological site has been located a pre-disturbance survey is the next step. The aim of a pre-disturbance survey is to record the site as it exists, and collect as much information as possible. This includes, but is not limited to, factors such as the extent (dimensions) of the site, water depth, visibility, physical features (both natural and archaeological) and potential difficulties for future work on the site (such as excavation or monitoring) (Green 2004:87-88). A pre-disturbance survey should accurately and efficiently collect as much information as possible. No matter what type of survey method is employed, the basic principles of accurate position fixing and data recording are consistent on any project (Tuttle 2011:114-115). One of the main products of pre-disturbance survey is a site plan, and there are a number of methods that can be used to generate one. The decision as to what technique to use will depend on a number of factors, such as site condition and the environment (Feulner and Arnold 2005:285; Green 2004:87). Two ‘low tech’ (but very effective) methods used in the creation of maritime archaeological site plans will be discussed below. These techniques, known as baseline offset (Figure 1) and trilateration surveys, involve the use of datum points, divers and tape measures to plot the location of site features and develop a site plan.
Figure 1. Recording offset measurements (Flinders University 2013)
A quick ‘mud map’ or sketch provides an overview of the site, and shows its main topographical and archaeological features. Mud maps are extremely useful, but a survey will provide the location of site features in relation to known survey points—which are needed for an accurate site plan (Bowens 2009:117). Control points need to be established on the site. These form known points and are given arbitrary coordinates so they can be plotted to scale on a site plan. Perhaps the easiest method of establishing control points is to set up a baseline (Bowens 2009:118). Unknown points (site features) can then be plotted in relation to the control points. A baseline should be straight, extend past the ends of the site, and be positioned to one side or through the middle of the site itself (Tuttle 2011:126). Once a baseline has been established the next step is to start recording the location of site features. When recording site features, archaeologists take offset measurements at right angles (or 90 degrees) from the baseline (Bowens 2009:120; Burke and Smith 2004:97; Feulner and Arnold 2005:288; Tuttle 2011:126). Two archaeologists are required for the baseline offset technique; one holds the terminal (or ‘dummy’) end of the tape to the feature while the other records measurements on the baseline. The baseline offset technique is relatively quick to set up and does not require very much equipment, so it is frequently used to record the location of site features. It is primarily used to acquire horizontal measurements, but can also be used to record vertical profiles as well. The accuracy of this method is largely dependent on the archaeologist’s ability to ensure his/her measurements are recorded 90 degrees to the baseline. The smallest measurement observed when swinging the offset tape in an arc over the baseline denotes when the tape is at a right angle relative to it (Figure 2) (Burke and Smith 2004: 98).
Figure 2. Establishing a right angle on a baseline (Bowens 2009:121).
Trilateration is another low tech method similar to the baseline offset technique, and can also be used to record site features. Measurements are taken from two control points along the baseline to the feature that is being recorded (Figure 3); this works best if the angle between the two tapes is between 30 and 120 degrees (Bowens 2009:121). This method can use more than two points for measurements. In fact, two points are the minimum; if three or more points are used the results are more accurate (Feulner and Arnold 2055:290). On some sites a single or straight baseline may not be possible. In this case, a network of control or datum points can be established (Tuttle 2011:126). These control points can be assigned arbitrary coordinates (just like the baseline control points) so that they can be used to accurately plot features on the site plan. Trilateration does not require a baseline, but rather known control points. Consequently, if a datum network is employed on the site in question, this technique could be used.
Figure 3. Measuring features through trilateration (Bowens 2009:122).
Baseline offset and trilateration surveys are two of the easiest methods available for recording a site. Both processes can be carried out with limited equipment by a couple of archaeologists. Each method records the site in two dimensions, but there are other forms of survey that record sites in three dimensions. Computer programs make creation of three dimensional plans much easier. The Direct Survey Method (DSM) uses distance and depth measurements to plot site features, and is similar to trilateration. Distances are measured from control points, as are site depths. The DSM computer program takes into account differences in depth and generates a digital site plan (Bowens 2009:127).
A site plan is not the only outcome that is created during a pre-disturbance survey. Site details, such environment, water depth, and visibility are collected. Photographs that detail site features are an essential outcome, and biological surveys—while not standard—are also beneficial. The aim of a pre-disturbance survey is to accurately record the site as it currently exists, and collect as much information as possible to facilitate future research.
A site plan is not the only information that is recorded during a pre-disturbance survey details such about the site environment such as water depth and visibility as mentioned above should be collected, photographs of a site are essential and a biological survey would also be beneficial. The aim being to accurately the record the site as it currently exists collecting as much information as possible to make future work on the site easier.
Bowens, A. (editor)
2009 Underwater Archaeology: The NAS guide to Principles and Practice, Second Edition. Wiley-Blackwell, London.
Burke, H. and C. Smith
2004 The Archaeologist’s Field Handbook. Allen and Unwin, NSW.
Feulner, M. A. and J. Barto Arnold III
2005 Maritime Archaeology. In Handbook of Archaeological Methods, edited by H. D. G. Maschner and C. Chippindale, pp. 270-305. vol. 1. AltaMira, USA.
2013 Methods in Underwater Archaeology Training. Faculty of Education, Humanities and Law. Electronic document, http://www.flinders.edu.au/ehl/archaeology/research-profile/current-projects/abpp/methods-in-underwater-archaeology-training.cfm, accessed 20 September 2013.
2004 Maritime Archaeology: A Technical Handbook, Second Edition. Elsevier Academic Press, USA.
Tuttle, M. C.
2011 Search and documentation of underwater archaeological sites. In The Oxford Handbook of Maritime Archaeology, edited by A. Catsambis, B. Ford and D. L. Hamilton, pp. 114-132. Oxford University Press, New York.