In June 2014, we were asked to carry out a terrestrial LiDAR survey of the Hellfire Caves in Buckinghamshire. These are a network of artifical chalk and flint caverns extending 500 m underground, situated above the village of West Wycombe at the southern edge of the Chiltern Hills near High Wycombe (Figure 1). The Faro Focus X-330 Terrestrial LiDAR Scanner (Figure 2) was used to carry out the survey, along with a Leica Viva GS15 GNSS unit.
This was the first time that the BGS had used this system and it required a completely different way of working. The method developed for use with the Focus X-330 relied on the accurate positional control of a fixed GNSS point outside the cave system and the use of spherical prisms, checkerboard targets and planar surfaces as markers by which to ‘tie’ the scans together. At least four of these markers were visible in every two adjoining scans, meaning that at least eight markers were visible in each separate scan, i.e. four markers visible to both scans two and three and four markers visible to both scans three and four (so scan three sees both sets of markers). This pattern was repeated throughout all 42 separate scan locations.
The terrestrial LiDAR data produced by the oriented laser scan and GNSS survey were processed to develop a digital surface model (DSM) of the entire cave network (Figure 3). The raw data produced by the Scene software consisted of a point cloud comprising 1.8 billion x-y-z points. These data were oriented using the initial GNSS control position and the markers, using a traverse style surveying method and output as an ASCII file, made up of x-y-z intensity and RGB colour values.
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