Topographical survey allows archaeologists to measure and map the surface of the ground, accurately locating archaeological features, structures, finds or excavation areas. 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.
Topographical survey uses the idea that the surface of the earth is divided into a regular system of measurement, so that each place has a unique position. In archaeology in Britain, the most familiar use of this idea is in Ordnance Survey grid references, where depending on the number of figures in the reference, a site or find-spot can be located to pin-point accuracy.
Archaeologists use surveying instruments, such as a GPS or Total Station, in order to relate what they are measuring to the national system of the OS grid. The position of any point on the ground surface can be calculated by measuring (or ‘triangulating’) from a known point (such as an OS benchmark or corner of a building on an OS map), using trigonometry. In the past, when analogue instruments such as theodolites were used in conjunction with traditional distance-measuring devices such as surveying chains, this was done manually. Only very experienced surveyors could work quickly and accurately. Thankfully, computer technology has now made some aspects of surveying a little less arduous.
Every site, excavation or survey area has its own regular internal system of measurements and co-ordinates, known as the site grid. Using a Total Station, the corners of the site grid are accurately positioned by measuring off fixed features in the surrounding area such as buildings, or OS bench-marks and ‘trig points’. If these permanent points are not available, a GPS can be used to locate the site grid corners. Within the site grid, measurements can be taken with a Total Station or measuring tapes. Earthworks, excavation trenches and find-spots can be measured and mapped in detail using simple methods such as off-setting with tapes or a plane-table. For larger areas, a Total Station or GPS work best. The measurement points (which often number many hundreds or thousands) each have a reading for East and North on the site grid, and there should be regular levelling measurements). These are either used to compile a drawn plan of the site (a traditional but still important way of presenting data) or by using a computer programme to produce a digital plot.