Henry Chapman's 'electronic distance measurer': the machine that follows you everywhere

Total station
Time Team surveyor Henry Chapman used the latest 'electronic distance measurer' (EDM) at Nassington. The EDM sits on top of a tripod and is referred to as a 'total station' because it can record any distance, height or angle.

'It's like having a theodolite, level and distance measurer all in one instrument,' says Henry. The EDM can measure points in relation to the total station and this information can be drawn together into a plan on a computer.

Laser-fired
The EDM works by firing a laser, which is then reflected back by a prism attached to a surveying staff. The total station records where the staff is in relation to itself at each reading.

The special thing about Henry's EDM is the fact that it doesn't need two people to operate it (one on the total station and one with the staff). 'It's got sensors and built-in motors, so it follows you everywhere on its own as you walk about the site holding the staff,' says Henry. 'It can work over several kilometres and still be millimetre accurate.'

GPS
The usual global positioning system (GPS) was only used for the outside trenches at Nassington. 'The GPS requires a "line of sight" with the orbiting satellites and we'd got trenches there that were actually inside a building, so its just not suitable,' says Henry.

'The EDM is just the right instrument to tackle a job like this. The data I record with the EDM is stored on the same palm computer that I use with the GPS in the field, so everything is easily joined together into one big survey by the time I download it onto my PC.'

Back to basics
With just about every technical surveying gadget at his fingertips Henry still likes to get back to basics at times. 'I like the basic work with trigonometry, using hand tapes and having to work it all out on paper,' he says. 'The GPS equipment is probably my favourite of all the technical kit, but it does so much for you. Sometimes its nice just to do it the old fashioned way.'

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