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

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

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

Introduction

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

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

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

Laser Beams

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

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

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

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

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)

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)

Terrestrial Scanners
Leica Scanstations
Getting smaller
Getting faster
Getting more precise
360 ° horizontal scanning range
90 ° vertical scanning range

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

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

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

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

Airborne LiDAR surface model
Digital Surface Model (DSM)
Resolution: 15cm - 2m
Intensity + 3D locations
Massive datasets!!

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

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

Decimation – surfaced model click

Decimation - wireframe click

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

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

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

Lighting techniques; oblique lightning
Low angle light source emphasises carvings
Easy to control, unlike in the real-world; not dependent on external factors

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

Lighting techniques; dynamic lighting click

Spacecraft

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

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

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

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

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

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

SmartNet
Integrates with Total Stations
Accurate position and survey
Flexibility of use and quality of record

Some examples… Lasers : Stonehenge Laser Scanning Spacecraft : Stonehenge Landscape Tour

Stonehenge Landscape Tour

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

On the phone…
Works on Symbian, Windows Mobile, Android, iPhone, etc
GPS location ideal but can use cell tower triangulation

Finding out more…
Links to more info
Wikipedia
SMR online
Aerial photography
Built in to GoogleMaps

Getting there…
Browse list of stops
View map of stops
Directions to stops
Walking, driving, even public transport (where available)

For larger devices…
Mobile Internet enabled netbooks, laptops and tablets can run Google Earth
Support external/internal GPS

And in Google Earth…

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

Laser-scanning at Stonehenge

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

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

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

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

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

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

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)

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

The Henge; time-of-flight scan
Single 360 ° scan undertaken from central location within Henge
Provides spatial framework for detailed scans

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

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.

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

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

Stone 53 carvings
A comparison with Newall’s recording shows two previously undiscovered carvings
Very shallow and indistinct compared to known carvings

Stone 53 carvings click

Cursus Animation

LiDAR - animation click

Other movies and presentations
New Antiquarians
Royal Academy
Salisbury Museum
Sunrise – Sunset
Durrington
LiDAR
Polynomial Texture Map

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

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