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Of Henges, Rock Art & Lasers; An application of Laser-Scanning techniques at Stonehenge
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Of Henges, Rock Art & Lasers; An application of Laser-Scanning techniques at Stonehenge

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A presentation given in May 2005 at the McDonald Institute for Archaeological Research, University of Cambridge, on behalf of the Stonehenge Laser Scan project team. The presentation was aimed at ...

A presentation given in May 2005 at the McDonald Institute for Archaeological Research, University of Cambridge, on behalf of the Stonehenge Laser Scan project team. The presentation was aimed at highlighting some of the results and well as the technologies used.

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Of Henges, Rock Art & Lasers; An application of Laser-Scanning techniques at Stonehenge Of Henges, Rock Art & Lasers; An application of Laser-Scanning techniques at Stonehenge Presentation Transcript

  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Of Henges, Rock Art & Lasers… An application of Laser-Scanning techniques at Stonehenge, Wilts. Paul Cripps GIS Specialist, Archaeological Projects, English Heritage Postgraduate Research Student, Archaeological Computing Research Group (ACRG), University of Southampton On behalf of the Project Team
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Presentation Outline  Background  Project Team  Aims & Objectives  Equipment used  Related projects  Methodology 1; Data Capture & Processing  Terrestrial Laser-Scanners and Airborne LiDAR  Large datasets!  Registration  Decimation  Methodology 2; Analysis of Results  Three-dimensional digital surface models (DSM)  Geometric transformation & analysis  Lighting & Shading  Animation  Results  The Henge  Stone 53 carvings  Stone 3 carvings  Stone 4 carvings  LiDAR  Conclusions  Review of aims & objectives  Potential for Rock Art research
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Background View slide
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Introduction  Project initiated to assess usefulness of commercial laser-scanning techniques to an archaeological unit  Brought together an archaeological unit and commercial laser-scanning companies  Used a range of techniques on nearby sites for evaluation purposes View slide
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The Project Team and those involved  Wessex Archaeology:  Chris Brayne (IT Manager)  Thomas Goskar (Multimedia Development Officer)  Paul Cripps (Systems Development Officer)  Archaeoptics:  Alistair Carty (Director)  Dave Vickers (Technician)  3D Laser Mapping  Dr. Graham Hunter (Managing Director)  English Heritage:  Paul Cripps (GIS Specialist, Stonehenge & Avebury World Heritage Site GIS) With special access to the stones granted by English Heritage
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Aims & Objectives 1. To assess usefulness of terrestrial (& airborne) laser- scanning techniques as survey tools 2. To record the known inscribed rock art at Stonehenge 3. To assess the potential for future work at Stonehenge
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Equipment used  Riegl Z360 ‘Time-of-Flight’ scanner  Basically a super-TST  360° horizontal scanning range  90° vertical scanning range  Relatively low resolution  Minolta VI-900 ‘Triangulating’ scanner  Behaves more like a camera, recording position of an emitted stripe of laser light  Very high 170µ resolution (0.17mm)
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Related Work at Stonehenge  English Heritage Aerial Survey Team  Looked at the Environment Agency LiDAR dataset in collaboration with Cambridge University (Colin Shell)  Mini-project to assess use of LiDAR as a prospection tool for archaeological features, similar to aerial photograph transcription  English Heritage’s Stonehenge 3D  Collaboration with Intel & IBM  Used photogrammetric model of the stones as 3D source data (English Heritage Metric Survey Team)  Terrain model derived from OS LandForm data
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Historical Context
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stonehenge  Located on Salisbury Plain  Part of the Stonehenge & Avebury World Heritage Site  Multi-phase henge  Earliest phase Middle Neolithic: bank and ditch with wooden posts inserted into bank in the Aubrey Holes  Second phase Late Neolithic: remodelling of ditches, wooden posts within henge, Aubrey Holes partially silted up  Third phase Late Neolithic/ Early Bronze Age: Bluestone and Sarsen megaliths added
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stonehenge  Part of a complex archaeological landscape  Surrounded by approx. 700 individual monuments; barrows, cursuses, enclosures, henges, field systems, etc  Archaeology from all periods from prehistory to modern airfield
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The Prehistoric Carvings  First observed in 1953 (Atkinson, 1953; Crawford, 1954; Atkinson, 1979 pp43-4)  Found on the Sarsen stones  Sarsens thought to have been brought from Marlborough Downs, nr. Avebury  Likely to have been carved after erection of the stones based on distribution  Thought to represent Bronze Age axes and daggers  Axe almost identical to carvings found in nearby Bush Barrow, deposited wrapped in cloth  Other sarsens near Avebury show axe grinding marks
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Previous work on the carvings  Robert Newall took casts and rubbings of carvings on stones 3 & 4  43 casts (mainly by Newall) stored in Salisbury Museum  1967 Atkinson took latex mould of part of Stone 53, subsequently stereo-photographed and used to produce a 0.5mm contour plot (Atkinson, 1968)  Photogrammetric survey (Bryan & Clowes, 1997) led to renewed interest in carvings, with a project outline (1999) by Burton, Pitts & Wheatley (never initiated)
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Methodology; Data Capture & Processing
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Scanning Technologies  Two systems used on site:  Time-of-flight scanner  Triangulating scanner  Data from a third system incorporated  Airborne LiDAR, produced by the Environment Agency, provided by English Heritage
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Time-of-Flight scanners  Fire a laser beam, measure time taken for beam to return  Records bearing from scanner  Uses speed of light constant c to calculate range  Also records other properties of reflected laser beam (eg intensity) http://www.i3mainz.fh-mainz.de/publicat/cipa2001/cipa2001.pdf
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Triangulating Scanners  Fire a laser beam from a known point  Observe laser beam from a known displacement  Use triangulation principles to calculate x,y,z location on target relative to scanner  Much higher resolutions  Digital camera can also be used to capture photographic information http://www.i3mainz.fh-mainz.de/publicat/cipa2001/cipa2001.pdf
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Airborne scanners  Time-of-flight systems attached to an aircraft  Incorporates dGPS for location and uses onboard sensors to detect orientation and aspect  Calculate x,y,z location on target  Transform data to any coordinate system (eg British National Grid)
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Laser-scan datasets - points  Raw data made up of thousands of recorded points (a ‘point cloud’)  Each point has x,y,z locations (plus other attributes)  Very large filesizes (100,000 points captured per second on some systems!)  Difficult to visualise complex datasets http://www.archaeoptics.co.uk/downloads/presentations/m3/6.html
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Laser-scan datasets - processing  Point clouds require processing  Can be seen as a statistical distribution representing probability of a surface occupying a particular space  Possible to fit a surface to the point cloud using best-fit algorithms…  …or force a geometric primitive to fit (makes assumptions)  Surfaces much easier to manipulate; hardware acceleration on graphics cards aimed at gaming is ideally suited to manipulating surface data  Better representation of the real-world situation
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Registration  Multiple datasets must be ‘stitched’ together  Can be accomplished using control points http://www.pobonline.com/CDA/ArticleIn placed in each scan… formation/Article/1,9169,83255,00.html  …or by matching the surfaces within controlled parameters…  … or a combination http://www.research.ibm.com/vgc/pdf/te xalign_TVCG.pdf
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Decimation  Sheer volume of data can be unusable  Many millions of polygons  Process called decimation reduces level-of-detail according to usage requirements  Different levels of detail required for different purposes eg rock art analysis, web dissemination, desktop visualisation, etc
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Decimation – surfaced model
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Decimation - wireframe
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The Real World  Data can be placed in real-world coordinates (eg British National Grid)  Allows multiple datasets to be placed in a common coordinate system (eg airborne LiDAR, close range scans, time-of-flight scans, other DTMs, photogrammetric surveys, geophysical survey data, etc)  Facilitates integration with GIS
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Methodology; Analysis
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 From Points to Digital Surface Model  Triangle mesh produced from point cloud  This digital surface model can be manipulated in a virtual 3D world  Ideally suited to rock art analysis:  Oblique lighting techniques  Dynamic lighting techniques  Geometric analysis http://www.polygon-technology.com
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Lighting techniques; oblique lightning  Low angle light source emphasises carvings  Easy to control, unlike in the real- world; not dependent on external factors
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Lighting techniques; dynamic lighting  A moving light source can highlight otherwise imperceptible surface features  Our eyes highly attuned to detecting subtle changes in light and shadow
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Lighting techniques; dynamic lighting
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Geometric techniques; exaggeration  A digital surface model can be manipulated in 3D space  Vector based transformations X1  Including stretch along z- axis or vertical exaggeration X 10
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Geometric techniques; accessibility shading  Possible to code surface according to accessibility  Use balls of varying sizes to probe surface  More accessible locations receive greater score
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Geometric techniques; range colouring  Possible to code surface according to range from viewpoint…  … or specified plane or point  Range in this image from black to white: 5mm
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Results
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The Henge; time-of-flight scan  Single 360° scan undertaken from central location within Henge  Provides spatial framework for detailed scans
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The Henge; increasing resolution Animation  Gives the impression of a ‘complete’ henge when viewed from scanner location  Animation shows transition between datasets in the same coordinate system
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stone 3 carvings  The lower left part of the outer face of Stone 3 contains the carvings of three axe heads.  These can be seen with the naked eye when close to the stone, and were easily picked up by the scanner.
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stone 4 carvings  The greatest number of carvings on any one stone at Stonehenge is on the outer face of Stone 4  The annotations indicate the locations of carvings
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stone 53 carvings  The famous dagger and axe are clearly visible in the centre of the scan  As is the historical graffiti  And two seams in the sandstone
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stone 53 carvings  A comparison with Newall’s recording shows two previously undiscovered carvings  Very shallow and indistinct compared to known carvings
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Stone 53 carvings
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 LiDAR  Possible to identify and quantify extant archaeology  Some new features identified  Require validation
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 LiDAR - animation
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Conclusions
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Laser-scanning  A very useful survey tool, complements tools already in use  Rapid data acquisition  Ideal for recording surfaces  High resolution, suitable for recording ephemeral carvings  True 3D data ideal for analysis  Erosion monitoring & volumetric analysis
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 The prehistoric carvings  Current records are incomplete; many more carvings than are currently known  The carvings have been considerably eroded since first carved, arguably since recorded in 1950’s  Accurate recordings of morphology of axe & dagger carvings
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 Future work  Evaluation successfully highlighted potential of technique  Systematic survey of all stone surfaces at sub-millimetre accuracy (new scanners capable of 80µ resolution)  Aim: to provide a 3D baseline dataset for management and research purposes
  • McDonald Institute for Archaeological Research, University of Cambridge, May 2005 fin For more information: www.stonehengelaserscan.org With thanks to Tom Goskar and Alistair Carty for their assistance with this presentation