Monday, 17 December 2012

Getting technical - Light Detection and Ranging (LiDAR) in Petworth Park

As part of the Petworth Park Archaeology Project we have been looking at a range of different images of the park in order to identify archaeological features.  This has included historic maps and aerial photography, and also LiDAR data.

LiDAR is a form of airborne laser scanning which allows us to build up a 3D model of the landscape. Pulses of infra-red light are scanned across the landscape from an aircraft – the pulses reflect off the ground and are received by sensors on the plane which calculate how long it took for the pulses to return.  Using this information, and knowing the precise location of the plane from GPS we can calculate the position in the world where the pulse hit the ground (±15cm) and represent it as a point in three dimensional space - the sensors can collect up to 100,000 of these points every second.

This gives us millions of point measurements across the landscape – like a blanket of snow.  We can then stitch these points together to create an accurate 3D model of the landscape which we can manipulate and interrogate in all sorts of ways to identify and measure the humps and bumps which might represent archaeological features.

The video below shows a form of processing called Polynomial Texture Mapping.  This allows us to place an artificial light source (a virtual sun) above the landscape and move it around to draw out features through the play of light and shadow.

video


Often these features are very difficult to see on the ground because they are so slight, or due to vegetation cover. The dense beams of infrared light used for LiDAR data, however, are able to penetrate through vegetation, allowing us to ‘filter’ out the trees and taller plants to look at the landscape underneath.

An aerial photo (left), ‘unfiltered’ lidar (centre) and ‘filtered’ lidar (right) for Arbour Hill in Petworth Park, possibly the site of Henry VIII’s banqueting house.  By filtering out the vegetation, the lidar allows us to see the square earthwork enclosure and central mound (circled in red)

As part of the Petworth Park Archaeology Project, throughout December and January we will be ‘ground-truthing’ the sites we identify in the LiDAR data, historic maps and aerial images, photographing and recording them.  This will help to inform the later stages of the project in 2013 which will see volunteer Assistant Archaeologists undertaking geophysics and excavation of specific features.

If you’re interested in volunteering with the project, or want to find out more, get in touch with susan.rhodes@nationaltrust.org.uk or call 01798345525, and keep an eye out for more posts in the future.

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