Many sites are visible on the ground as a series of ‚humps and bumps’. These earthworks can be very confusing to look at from ground level. An accurate plan of them can reveal the outlines of building foundations, field patterns or other such features that had not previously been recognised. This plan is produced by topographical (‚shape of the ground’) survey.
Measured topographical survey for military purposes began in the late 18th and 19th centuries when the Ordnance Survey (OS) was set up. The early techniques were turned to archaeological use by pioneer archaeologists with a military background such as General Pitt Rivers. In the early 20th century, many of Britain’s ancient monuments were accurately mapped for the first time by archaeological surveyors such as O. G. S. Crawford, the official Ordnance Survey archaeologist.
Every site is given its own regular internal system of co-ordinates known as the site grid. The position of any point on the ground surface can be calculated by measuring (or ‚triangulating’) from known points on this grid, using trigonometry. The distance and angle to these new points must be measured both vertically and horizontally (and the latter in both a north-south and an east-west direction). Mathematics can then be used to calculate their grid co-ordinates. In the past, this was done manually. Only very experienced surveyors could work quickly and accurately. Thankfully, computer technology has made surveying a lot less arduous. Archaeologists today normally use electronic surveying instruments such as GPS or Total Stations. The measured points (which often number many hundreds or thousands) are downloaded into a computer programme and used to produce a digital plot.
It is essential to locate the site grid in relation to the Ordnance Survey national grid system, otherwise future researchers will be unable to compare their findings with yours. This is done by measuring off fixed features in the surrounding area such as buildings, field boundaries or OS ‚trig points’. Since these are shown on large-scale OS maps, they allow the site grid to be overlaid onto such a plan. If suitable points are not available, a GPS can be used to locate the site grid corners.
A Total Station is a modern electronic device that combines the ability to measure a position horizontally and vertically at the same time. It has two parts, a machine mounted on a static tripod, and a ‚target’ prism on a metal staff, which is moved around the site. Total stations developed in civil engineering, but like many other items of equipment, archaeologists realised their usefulness for archaeological purposes.
The tripod-mounted machine part is set up at a ‚base station’, usually a point from which as much as possible of the survey area can be seen. Several base stations may be needed to cover a survey area; it is always best to have at least two so that some measurements can be taken from both to test for accuracy. Co-ordinates for the base station are programmed into the machine (these have to be calculated before the survey begins) as well as the direction to grid north and the heights of both the tripod and staff. The prism is then carried around the site and positioned vertically at recording points. The person with the prism, rather than the person working the machine, is responsible for what gets recorded.
The machine part of the Total Station has a lens rather like a telescopic rifle-sight with cross-hairs which are focused on the prism. The whole instrument swivels horizontally and the lens swivels vertically too. The Total Station is partly based on a principle used in traditional theodolites, where angles are calculated from vertical and horizontal 360-degree scales. It combines these with a device known as an Electronic Distance Measurer or EDM. This sends out a tiny light signal, which bounces back from the prism giving a time interval that is used to calculate distance. The Total Station has an inbuilt microprocessor that automatically collects these angle and distance measurements, calculates the trigonomical equations and converts them into grid co-ordinates. Instead of the surveyor noting these down in a notebook, they are normally send directly to a data-logger mounted on the tripod, which can then be downloaded into a computer to produce a plot. Advanced machines, known as self-tracking Total Stations, can be controlled from the prism and programmed to follow it automatically, making surveying a one-person job.
GPS stands for Global Positioning System: or sometimes people say ‚Global Positioning by Satellite’ instead. A GPS allows you to find your position on the earth’s surface, using a system of grid co-ordinates. The most common way of doing this in Britain is to use the Ordnance Survey national grid system. It is possible to locate a position with an accuracy to within one metre.
GPS is an excellent tool for locating archaeological finds, earthworks or other field evidence without having to lay out a conventional survey grid and measure a position off known features. This is especially useful where there is little fixed topography marked on maps, such as in open moorland or even in large fields. Some professional surveyors use a ‚differential’ GPS, which has two receivers, one fixed and the other mobile. These are the most accurate, but hand-held single-user GPS’s that are almost as accurate are now available at a fairly cheap price. They are about the size of a mobile phone and display the position in co-ordinates on an LED screen. These are easily accessible to archaeologists and are becoming essential survey ‚kit’. Even metal-detectorists are encouraged to purchase and use them to locate their finds more accurately.
GPS developed out of US Navy technology, using coded signals from four or more satellites to calculate the receiver’s relative distance from them, and therefore its position on the earth’s surface. There have to be at least four satellites within signal range (orbiting on our side of the world), for the angles of their signals to be enough for the GPS to be able to calculate its position on the globe. If less than four satellites happen to be available, the ability of the GPS to determine position can fade for short periods. Where ground-to-sky visibility is a problem, such as within buildings or under heavy woodland cover, the GPS signals can be difficult to receive accurately. Research and development efforts are under way to reduce and eliminate these factors. Another factor affecting accuracy has been the tendency of the US Military to deliberately restrict public access to the satellite linkages for security reasons. Until September 11th 2001 this seemed likely to become less of a problem, but new security fears may possibly lead to future restrictions. As an alternative to the US system, a European system called ‚Galileo’ is under development, backed by the EU and a number of European governments.
W prawidłowym przeprowadzeniu każdych badań archeologicznych kluczowe jest dokładne domierzenie lokalizacji pomiarów, zabytków czy wykopów. W przypadku odwiertów pomiary tracą cały sens, jeżeli nie są potem poprawnie nałożone na mapę. Najlepszym i najtrwalszym sposobem jest umiejscowienie ich w obrębie państwowej siatki geodezyjnej.
M. Bowden, Unravelling the Landscape, Tempus, Stroud, 1999.