Excavations at the amphitheatre over the years have uncovered many interesting objects and structures. More excavations are planned and which should produce even more finds and buried structures. There are also maps produced by geophysical surveys, which show buried remains, and old maps showing buildings and walls which were built on top of the amphitheatre long after it fell into disuse. Many of these buildings have long since been knocked down and we only know where they were because of the maps. A number of buildings and walls still remain on the site, some of which have been there hundreds of years and some are relatively modern.
Understanding how all these various bits fit together through time can be a complicated business. It is like trying to assemble the pieces of a jigsaw without having the picture to guide you. Unless they are assembled in the correct place and order it will not be possible for archaeologists to imagine what the amphitheatre looked like and how the area has changed over the years.
To help them fit the pieces together this type of information is drawn onto plans. Each of these plans is like a piece in the jigsaw. To ensure that the pieces of evidence are put together correctly it is vital to have a single plan of the whole area, which contains enough information to guide the pieces into the right place.
This is why at Chester Amphitheatre we undertook a ‘topographic’ survey, in other words a survey of the land around the amphitheatre and all the structures on it to make one plan of everything that is there today. A land surveyor carefully measured every wall, path, fence and building to produce a very detailed map of the area. It is the most detailed plan ever made of this part of Chester. It is so detailed, that had anyone stood in one place for too long, they might have appeared on it! A daunting job, but the latest electronic surveying equipment and computers made everything much easier than it would have been for colleagues in the 1960s, when the first excavations took place, who did not have such aids.
This topographic survey was undertaken with an ‘total station’ (the machine on top of the three yellow legs). The machine measures angles and distances electronically and extremely accurately. At Chester it was set up in 30 different locations to ensure everything within the study area was measured correctly. The surveyor took a pole (on top of which is a prism) to a point he wanted to know the position of on a plan. Wherever this prism went, the ‘total station’ tracked its position automatically. When the surveyor pressed a button on a small computer on his arm the position was recorded electronically.
Electronic notes with each point are used to tell the computer whether that point is on a wall, building or whatever. An electronic map was being drawn point by point each time the surveyor pressed the button. Thousands of these points were recorded. This created an electronic map which all the archaeologists will be able to see on their computers. When they make a find, or discover a wall, they will be able to see exactly where it fits into the wider picture.
Strangely enough, although 21st century surveying technology might be heavily dependant on electronics and computers, the way in which plans are made has changed little since the Roman period. Making detailed plans of the land and what is on it, is simply a matter of measuring angles and distances. Although the machinery used to measure those angles and distances has changed since the Roman period, Roman surveyors would understand exactly what modern-day surveyors were doing at the amphitheatre. But they might question his choice of colours for armour! Instead of using a ‘total station’, Roman surveyors used a type of surveying instrument called a ‘groma’.
This was the principal surveying instrument of the Roman ‘agrimensores’, the land surveyors. The instrument itself was simple in design, with crossed arms resting on a bracket, attached to a vertical staff. The four arms each had a cord with a hanging plumb bob. It was designed to survey straight lines and right angles.
This extremely detailed, computer-based survey of the amphitheatre and its surrounding area was one of the first non-invasive surveys to be completed. As part of the survey, a network of very accurate, 3-dimensional points was established across the site and these are now being used by the excavation team to record the position of the features and finds they uncover. The survey has also enabled the team to record the position of earlier excavation trenches dug in the 1960s, so that ultimately there will be a full and complete record of all the excavations on the site.
The above diagram is just a small example of the type of diagram produced. There are two more detailed full versions available for download:
Next: Cartographic Survey