Recent survey work at Stonehenge - the standing and fallen stones Paul Bryan BSc Geospatial Imaging ManagerRemote Sensing Team, Investigation & Analysis Division Heritage Protection Department English Heritage 37 Tanner Row, York, YO1 6WP
Existing survey material • Within the NMR’s Historic Plans Room a wide variety of survey material already exists for the Stonehenge site • Includes aerial photographs, topographic and elevational surveys: • 1:500 scale topographic survey produced by Plowman Craven & Associates (PCA) in 1974 • 1:50 scale topographic survey of the stone circle produced by MJ Rees & Co in 1989 • 1:10 scale hand measured survey of stones produced by EH Survey team in 1974 • Very little measured survey exists on the standing and fallen stones themselves
Previous detailed survey • As stated in Archaeological Survey and Investigation’s (AS&I) Stonehenge WHS landscape project design: “In 1993, photogrammetric survey provided what is probably the most complete survey of the standing stones to date” • This utilised ‘traditional’ film-based stereo-photography and theodolite survey control approaches on site • Post-processing was carried out using one of the ‘new’ Digital Photogrammetric Workstations (DPW) • Pixel-matching software used to automatically create 3D surface models for each stone at 2cm resolution
Application of modern survey techniques at Stonehenge • Since 2006 low-cost photogrammetric approaches, as used within the Northumberland & Durham Rock Art Project (NADRAP), have been trialled at Stonehenge • These demonstrated potential to automatically capture higher resolution 3D surface data, with mm point spacings, using stereo-imagery captured by both compact and DSLR cameras • Until last year, modern laser scanning systems had not been systematically used at Stonehenge • …so what advantages would they bring?
Laser Scanning Advantages: • Laser scanning is applicable on all 2D and 3D surfaces • Very fast – captures up to 500,000 3D points per second • Depending on scanner can provide very detailed, sub-millimetric accuracy 3D data • Can integrate imaging using on-board CCD or external DSLR • Excellent for objectively recording three dimensional surfaces • Ideal for recording, monitoring, modelling and visualisation Disadvantages: • Produces very large data files, still often difficult to view in full resolution without high-end computers and dedicated software • Hardware expensive – currently between £30K - £110K • Requires sophisticated software to post-process point data • Capture of discrete edges needs high-resolution point data • Usable line-based output still requires manual digitisation Longthorpe Tower, nr Peterborough Images courtesy of Oakes Surveys • But heritage application rapidly maturing!
Previous application of laser scanning at Stonehenge • Ad-hoc application includes work undertaken by Wessex Archaeology and Archaeoptics back in 2002/03 • As noted on the Stonehenge Laser Scans website - http://www.stonehengelaserscan.org/index.html - this investigated the application of close-range scanners, e.g. Minolta VI-900, to record the stone surfaces and provide potential for revealing new carvings • Generated a lot of media interest! • Also carried out trials of terrestrial laser scanning using a Riegl Z360 supplied by 3D Laser Mapping • However as the website states “Many more scans need to be taken to enable us to capture every facet of the monument, and create a complete model of Stonehenge”
Proposed application of laser scanning within AS&I project • The AS&I’s landscape survey project design noted: “the detailed survey of the ‘dagger’ carvings and other graffiti might be improved dramatically and indeed possible unknown artwork detected by using modern scanning technology. The research potential this offers justifies preparation of a laser scan” • Rather than concentrating on just the standing/fallen stones and carvings, this would complement and contribute to AS&I’s recording across the Stonehenge WHS landscape using their own respective survey approaches • Funded through EH’s National Heritage Protection Commissions Programme (NHPCP) NImage supplied through Pan Government Agreement (PGA)
Proposed application of laser scanning within AS&I project • Landscape within the ‘triangle’ – using terrestrial laser scanners to capture a point-spacing of at least 10cm • The stone circle and ‘bank & ditch’ – using terrestrial laser scanners to capture a point-spacing of at least 2cm for the landscape and up to 1mm for the stone circle • The standing and fallen stones - using close-range and/or high-resolution terrestrial scanners & digital imaging to capture as many visible faces of the stones at a point-spacing of at least 0.5mm NImage supplied through Pan Government Agreement (PGA)
Laser scanning undertaken by Greenhatch Group • Following pre-qualification and tendering the successful contractor was the Greenhatch Group (based in Little Eaton near Derby) who proposed: • Landscape within the ‘triangle’ • Leica C10 long range laser scanner •The stone circle and ‘bank & ditch’ • Z+F 5006h short range laser scanner equipped with calibrated M-Cam • The standing and fallen stones • new Z+F 5010 high resolution, short range laser scanner • Nikon D3x DSLR • Digital imaging of each stone surface • Stereo-photography for tops of each NImage supplied through Pan Government stone/lintel using ‘Jimmy Jib’ boom Agreement (PGA)
Post processing • Post-processing carried out between 28th February and 20th May 2011 including: • Registration and geo-referencing of ‘raw’ scan data using Leica Cyclone software • Generation of ‘Truviews from registered data using Leica Cyclone Publisher software • Photogrammetric processing of upper surfaces using Photomodeler Scanner software • Meshing and generation of 3D models for each stone using Geomagic Wrap/Studio software
Initial outputs • Digital Surface Model (DSM) of ‘The Triangle’ landscape @ 100mm and Bank and ditch @ 20mm resolution
Initial outputs • Digital Surface Model (DSM) of ‘The Triangle’ landscape @ 100mm and Bank and ditch @ 20mm resolution • 3D model of stone circle @ 1mm resolution
Initial outputs • Digital Surface Model (DSM) of ‘The Triangle’ landscape @ 100mm and Bank and ditch @ 20mm resolution • 3D model of stone circle @ 1mm resolution • Digital surface data for all standing & fallen stones @ 0.5mm resolution
Initial outputs • Digital Surface Model (DSM) of ‘The Triangle’ landscape @ 100mm and Bank and ditch @ 20mm resolution • 3D model of stone circle @ 1mm resolution • Digital surface data for all standing & fallen stones @ 0.5mm resolution • Summary assessment of initial archaeological findings, including landscape and stone surfaces, using Meshlab • increased knowledge on stone working • ‘grooves’ from stone dressing divided into sections -> different teams of builders?
Detailed archaeological analysis of stones (6457) • A detailed archaeological analysis of every visible surface of every stone is due to commence in March • Funded through NHPCP this will inform further research, monitoring & presentation of the monument…. • …and provide an analytical report, with associated stone ‘biography’, graphic record & catalogues, answering the following research questions: • Evidence for the methods employed in building Stonehenge, in particular shaping the stones • Evidence for prehistoric carvings on the stones • Evidence for later alterations to stones and graffiti • Evidence for architecture and sequence at Stonehenge • To be undertaken by ArcHeritage (Sheffield) and due for completion in early May
Filtering the data (6456) • Due to the extensive coverage of the stones by different types of lichen some surface sections are obscured • Aim of this new NHPCP funded project is to investigate a method for digitally filtering out this lichen cover and other anomalies from the existing scan data, in order to maximise the view of the underlying stone surface • Experimental work looking at the combination of data already acquired: • Raw & registered 3D point data • Laser intensity • RGB colour for each scanned point • Separate DSLR imagery • Undertaken by CyArk (USA) and due for completion in early March it will feed into the analysis project (6457)
Some issues to consider • Data density • Difficulty of viewing high resolution datasets without resorting to decimation • Requires high-end computing hardware and specialist viewing software • Limits potential access to high resolution dataset • Archiving • Over 850GB of data so far generated comprising: • Raw scan data (ASCII & proprietary) – E57? • Registered scan data (ASCII & proprietary) • Mesh model data (OBJ) - 3DPDF? • ‘Truview’ data (proprietary) – other options? • Digital image data (RAW & TIFF)
“Surveying Stonehenge” – Geomatics World articleGeomatics World Nov/Dec 2011 & Jan/Feb 2012 www.pvpubs.com
Thank you for listeningGeomatics World Nov/Dec 2011 & Jan/Feb 2012 www.pvpubs.com