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Laser-beams, spacecraft and archaeology; recent approaches to the recording, analysis and interpretation of cultural heritage

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A talk about archaeological survey techniques given at Cafe Scientifique, Salisbury, 5th January 2010

A talk about archaeological survey techniques given at Cafe Scientifique, Salisbury, 5th January 2010

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  • 1. Laser-beams, spacecraft and archaeology recent approaches to the recording, analysis and interpretation of cultural heritage Paul Cripps Geomatics Manager Wessex Archaeology Postgraduate Research Student, Archaeological Computing Research Group (ACRG), University of Southampton
  • 2. Outline
    • Introduction
      • Not all archaeologists dig holes!
      • Geomatics; nothing to do with geomancy…
      • Geographic Information Systems (GIS); spatial databases and maps
      • Metric Survey; buildings, sites and landscapes
    • Laser beams
      • The Total Station Theodolite
      • Laser scanners
    • Spacecraft
      • Global Positioning System (GPS)
      • Global Navigation Satellite Systems (GNSS)
      • SmartNet from Leica & Ordnance Survey
    • Archaeology
      • Stonehenge laser scanning
      • Stonehenge landscape tour
  • 3. Introduction
  • 4. A broad discipline
    • Archaeology comprises many specialisms and sub-specialisms
    • Not just excavation!
    • Wessex Archaeology provides the full range of services
      • Hence employs specialists of many disciplines
    • Digital techniques help us in our work as archaeologists
    • Archaeology has a long history of adopting and adapting techniques, methods and theories
    • This talk focuses on Geomatics and spatial information within archaeology
      • Finds
      • Geophysics
      • Environmental Science
      • Illustration
      • Cartography
      • Information Technology
      • Publication
      • Archives
      • Photography
      • Metric Survey
      • Osteology
      • Conservation
      • Heritage Consultancy
      • Web & Multimedia
      • Outreach & Education
  • 5. Geomatics
    • Geomatics:
    • Geomatics is fairly new, the term was apparently coined by B. Dubuisson in 1969. It includes the tools and techniques used in land surveying, remote sensing, Geographic Information Systems (GIS), Global Navigation Satellite Systems (GPS, GLONASS, GALILEO, COMPASS), photogrammetry, and related forms of earth mapping. (source: Wikipedia)
    • Central to archaeology
      • Most archaeological data is spatial, especially measured survey data
  • 6. Spatial Information
    • Metric Survey: measured records of sites, monuments, landscapes and buildings
      • Angles, Positions and Distances
      • Laser beams and spacecraft
    • Positioning
      • Location, location, location
    • Geographic Information Systems (GIS)
      • Maps and databases
      • Integration
      • Desktop PCs, laptops, online and on mobile devices
    • A look at how we arrived where we are today and where next
  • 7. Laser Beams
  • 8. Light for measuring
    • Three components:
      • Angles
      • Distances
      • Positions
    • Optical solutions
      • Lenses, prisms and levels
      • Triangulation and Trigonometry
    • Laser measurement
      • Time
    • The kitchen sink
      • Robotics, GPS, cameras, etc
    Plane Table + Theodolite Theodolite + EDM + computer
  • 9. The Total Station
    • Total Station comprises multiple instruments and a computer
    • Theodolite measures angles
    • Electronic Distance Meter (EDM) measures distances using laser beam
    • Data Logger records measurements
    • Remote controlled via radio
    • Robotic
      • 1 person operation
  • 10. 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
  • 11. 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
  • 12. 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)
  • 13. Terrestrial Scanners
    • 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 170 µ resolution (0.17mm)
  • 14. Terrestrial Scanners
    • Leica Scanstations
      • Getting smaller
      • Getting faster
      • Getting more precise
      • 360 ° horizontal scanning range
      • 90 ° vertical scanning range
  • 15. Laser-scan datasets - points
    • Raw data made up of thousands/millions/billions 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
  • 16. 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
  • 17. 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
      • landscape studies
      • buildings studies
    • By means of:
      • Oblique lighting techniques
      • Dynamic lighting techniques
      • Geometric analysis
    http://www.polygon-technology.com
  • 18. Registration
    • Multiple datasets must be ‘stitched’ together
    • Can be accomplished using control points placed in each scan…
    • … or by matching the surfaces within controlled parameters…
    • … or a combination
    http://www.research.ibm.com/vgc/pdf/texalign_TVCG.pdf http://www.pobonline.com/CDA/ArticleInformation/Article/1,9169,83255,00.html
  • 19. Airborne LiDAR surface model
    • Digital Surface Model (DSM)
    • Resolution: 15cm - 2m
    • Intensity + 3D locations
    • Massive datasets!!
  • 20. Airbourne LiDAR in action
    • 133.5 million measurements: x,y,z,i
    • 40 sq km
    • 1m resolution, <20cm z-axis tolerance
    • Used to produce derived products: Slope, Aspect, Hillshades
    • Archaeological features digitised from these maps
  • 21. 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
  • 22. Decimation – surfaced model click
  • 23. Decimation - wireframe click
  • 24. Geometric techniques; exaggeration
    • A digital surface model can be manipulated in 3D space
    • Vector based transformations
    • Including stretch along z-axis or vertical exaggeration
    X 1 X 10
  • 25. 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
  • 26. 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
  • 27. Lighting techniques; oblique lightning
    • Low angle light source emphasises carvings
    • Easy to control, unlike in the real-world; not dependent on external factors
  • 28. 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
  • 29. Lighting techniques; dynamic lighting click
  • 30. Spacecraft
  • 31. Where on Earth are we…?
    • Survey data collected very accurately, but where is it?
    • Traditionally, use of Benchmarks and Trig Points
      • Known locations to survey to/from
      • Carefully triangulated by the Ordnance Survey
    • Or measurements to features shown on maps
    • Essential to relate surveyed measurements to the real-world
  • 32. Whatever did we do before we had…
    • Positioning devices now taken for granted
      • Small, cheap, relatively accurate (3-10m)
      • Increasingly found in mobile telephones
      • Professional systems capable of millimetric precision
    • Very useful for archaeological work
      • eg condition surveys to allow all data to be captured digitally against a location
      • eg on excavations for recording features, finds and trench locations
  • 33. The spacecraft
    • Various constellations of satellites covering the Earth
      • Typically 24-35 satellites in variety of orbits
      • Very accurate time clocks
      • Radio broadcast of signals
    • Global Positioning System
      • US system, the original
    • GLONASS
      • Russian system
    • Galileo
      • EU system, forthcoming
      • Higher power, greater precision
    • COMPASS
      • Chinese system, forthcoming, very similar to Galileo
  • 34. GPS Progress
    • In large scale use since 1990’s
    • Various incremental improvements
      • Differential GPS (dGPS) uses two instruments (Base+Rover) in order to minimise errors and improve accuracy
      • Improvements to mathematical models and transformations
      • Improvements to hardware, particularly antennaes
    • Then came Smartnet…
      • Leica, OS, Trimble
  • 35. SmartNet
    • Real-time kinematic dGPS
    • Millimetric-Centimetric precision
      • Data can still be post-processed to improve accuracy
    • OS Active Base Stations
      • Provide the other end of the baseline to the rover
      • Replaces the benchmark network
    • Survey instrument recieves correction signals via mobile internet
  • 36. SmartNet
    • Set up and survey within minutes
      • Subject to phone coverage…
    • High quality digital record of archaeological sites
      • Ideal for use in GIS and digital mapping applications
    • Dramatically improved workflow
    • Democratised surveying
    • Integrates with Total Stations
  • 37. SmartNet
    • Integrates with Total Stations
    • Accurate position and survey
    • Flexibility of use and quality of record
  • 38. Some examples… Lasers : Stonehenge Laser Scanning Spacecraft : Stonehenge Landscape Tour
  • 39. Stonehenge Landscape Tour
  • 40. Stonehenge Landscape Tour
    • Built using ArcGIS & Google Earth
    • Distributed using Keyhole Markup Language (KML)
      • Accessible to mobile devices (PDAs and SmartPhones) via GoogleMaps Mobile using Mobile Internet
      • Accessible to larger devices (Tablets and NetBooks) via GoogleEarth (using Mobile Internet) and GIS (offline)
    • Shows selected sites around the World Heritage Site
    • Includes links to associated resources
      • Wikipedia
      • Wiltshire SMR online
  • 41. On the phone…
    • Works on Symbian, Windows Mobile, Android, iPhone, etc
    • GPS location ideal but can use cell tower triangulation
  • 42. Finding out more…
    • Links to more info
      • Wikipedia
      • SMR online
    • Aerial photography
      • Built in to GoogleMaps
  • 43. Getting there…
    • Browse list of stops
    • View map of stops
    • Directions to stops
      • Walking, driving, even public transport (where available)
  • 44. For larger devices…
    • Mobile Internet enabled netbooks, laptops and tablets can run Google Earth
    • Support external/internal GPS
  • 45. And in Google Earth…
  • 46. Work in progress
    • Currently ‘proof of concept’
    • More content to be added
      • More full descriptions
      • Images and video
      • More links to external resources
    • More stops to add
    • It is publicly accessible in current form
      • Has been tested by us and other users
      • Feedback
      • www.tinyurl.com/shenge
  • 47. Laser-scanning at Stonehenge
  • 48. 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
  • 49. 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
  • 50. The Project Team and those involved
    • Wessex Archaeology :
      • Chris Brayne ( Head of IT )
      • Thomas Goskar ( Web Manager )
      • Paul Cripps ( Geomatics Manager )
    • 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
  • 51. Aims & Objectives
    • To assess usefulness of terrestrial (& airborne) laser-scanning techniques as survey tools
    • To record the known inscribed rock art at Stonehenge
    • To assess the potential for future work at Stonehenge
  • 52. 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
  • 53. 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
  • 54. 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)
  • 55. The Real World
    • Data can be placed in real-world coordinate systems (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 and 3D packages
  • 56. The Henge; time-of-flight scan
    • Single 360 ° scan undertaken from central location within Henge
    • Provides spatial framework for detailed scans
  • 57. The Henge; increasing resolution
    • Gives the impression of a ‘complete’ henge when viewed from scanner location
    • Animation shows transition between datasets in the same coordinate system
    click
  • 58. 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.
  • 59. 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
  • 60. 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
  • 61. Stone 53 carvings
    • A comparison with Newall’s recording shows two previously undiscovered carvings
    • Very shallow and indistinct compared to known carvings
  • 62. Stone 53 carvings click
  • 63. Cursus Animation
  • 64. LiDAR - animation click
  • 65. Other movies and presentations
    • New Antiquarians
    • Royal Academy
    • Salisbury Museum
    • Sunrise – Sunset
    • Durrington
    • LiDAR
    • Polynomial Texture Map
  • 66. fin With thanks to Tom Goskar and Alistair Carty for their assistance with this presentation For more information: www.stonehengelaserscan.org www.wessexarch.co.uk

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