Surveying by Lidar

Time Team's surveyor, Henry Chapman, is used to dealing with masses of data gathered from his exhaustive surveys, but at Warton Marsh he was faced with a problem. The wetland is constantly changing and being eroded by the tide, and as a bog it can be difficult to travel across to carry out a full survey. A changing landscape and poor access couldn't stop our Henry, though.

'The problem is that a survey transcribed from aerial photographs some years ago is not really going to match up with the landscape as it is today,' he says. 'The bog land just changes so much.'

So how did he get his data? 'Simple, we called in an aircraft equipped with Lidar and had it done in a jiffy.'

Lidar, or Light Detection and Ranging, is a surveying system that uses a laser range finder to scan the ground and gather data. It's a bit like Henry's Electronic Distance Measuring theodolite (EDM), except that it's attached to an aircraft and can take many more measurements much more quickly.

'It's an amazing piece of kit,' continues Henry. 'Whereas I would measure single points one at a time with my EDM, the Lidar-carrying aircraft takes thousands of measurements every second as it flies low over the ground. Measurements are taken at two-metre intervals across a 600-metre swathe. The aircraft does several sweeps over the terrain and then produces this mass of data, which can be crunched down into an incredibly accurate 3DS topographical survey. It's great. It also means that I only have to plot the trenches and finds on this programme.'

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