Types of Survey
Survey is the first stage in establishing what is known and what gaps in knowledge exist about a site, a landscape or a building. Generally the first step is to search the documentary records (books, written papers, maps, diaries etc) to establish what has been recorded in the past. This initial trawl of information then sets up a series of questions for subsequent phases of survey. The aim of the next phase is to enhance or clarify the written record and to fill in the gaps in knowledge.
Field survey should aim to establish the nature and extent of the visible remains. The outcome of field survey is an enhanced suite of interpretations which in turn raise questions for further documentary research, further field survey or intrusive fieldwork. Archaeology has utilised or developed a wide range of techniques to enhance and record what can be seen but the main types of survey with which HS has been involved are:
Methods of landscape survey vary according to the scale of the target landscape and desired level of accuracy. Some surveys, such as the HS-sponsored coastal zone assessment surveys, are designed to give a “snap-shot” of vast areas of landscape under specific conditions. Accurate surveys require instruments which help locate features precisely to the Ordnance Survey national grid. Archaeologists now regularly use electronic, satellite-linked survey equipment, aerial photography, and computer-based instant mapping technology. Developing techniques include air-borne remote sensing technologies, side-scan sonar (for underwater archaeology) and laser-scanning.
Whether focused on a particular archaeological site or building, the range of techniques for survey generally aim to reveal the extent of the site (however defined) and the subtleties of form and relationship between observed features. This more detailed class of survey is not restricted to the land-surface but pertains to maritime archaeology, cave archaeology and urban archaeology. The techniques are generally the same as landscape survey but the density of measurement and the consequent volume of data increases. Whether the surveys employs the traditional techniques of optical instrument survey (e.g. theodolite or EDM) or digital survey (e.g laser scanning) it remains important that the interpretation remains evidence-based and testable.
Even when all of a site’s nature and extent is masked by later soil accumulation it is possible to observe features that survive beneath the present ground surface through techniques of geophysical survey.
- Resistivity measurement – electricity passes more easily through damp than dry soil. Measuring the resistance can help detect features like filled-in ditches (often relatively wet) or walls where the soil covering may be thinner and drier.
- Magnetometry measurement – metal objects or burnt material can cause changes in magnetic fields. Metal detectors are one of many sorts of equipment that show these changes.
- Ground-penetrating radar (GPR) – a 3D picture is built up using special frequency signals which “bounce” off buried remains and the points where different kinds of layers meet.
Geophysical results always need to be checked. This is done by “ground truthing” – digging or boring holes into the site, to relate readings to reality.