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
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