Recent Survey Work at Stonehenge

Recent Survey work at Stonehenge

Trevor Pearson Paul Bryan

• 2009 survey in progress

• Watercolour of the mound

• Photo of Greenhatch working

Recent survey work at Stonehenge

- the standing and fallen stones

Paul Bryan BScGeospatial Imaging Manager

Remote Sensing Team, Investigation & Analysis Division Heritage Protection Department

English Heritage37 Tanner Row, York, YO1 6WP

• 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

Existing survey material

• 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

Previous detailed survey

• 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?

Application of modern survey techniques at Stonehenge

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

• But heritage application rapidly maturing!

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

Laser Scanning

Longthorpe Tower, nr PeterboroughImages courtesy of Oakes Surveys

• 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”

Previous application of laser scanning at Stonehenge

• 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)

Proposed application of laser scanning within AS&I project

Image supplied through Pan Government Agreement (PGA)

N

• 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)

Proposed application of laser scanning within AS&I project

• 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 stone/lintel using ‘Jimmy Jib’ boom

Laser scanning undertaken by Greenhatch Group

NImage supplied through Pan Government

Agreement (PGA)

• Site work commenced on 22nd February 2011 and completed by 18th March, as per agreed programme

• Initial assessment of existing survey materials undertaken by Caroline Hardie from Archaeo-Environment Ltd, Greenhatch’s appointed archaeological sub-contractor, who noted that:

“This report will summarise some of the earlier findings, but the advantage of laser scanning is that it is objective and so the methodology used need not be altered to accommodate existing discoveries. We are as keen to explore the gaps in our knowledge as well as the areas where research has already been carried out.”

Site work and initial assessment

Images by James Davies © English Heritage

• 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 ‘Truview's 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

Post processing

• Digital Surface Model (DSM) of ‘The Triangle’ landscape @ 100mm and Bank and ditch @ 20mm resolution

Initial outputs


• 3D model of stone circle @ 1mm resolution

Initial outputs



• Digital surface data for all standing & fallen stones @ 0.5mm resolution

Initial outputs



• 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?

Initial outputs

• 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

Detailed archaeological analysis of stones (6457)

• 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)

Filtering the data (6456)

• 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)

Some issues to consider

“Surveying Stonehenge” – Geomatics World article

Geomatics World Nov/Dec 2011 & Jan/Feb 2012

www.pvpubs.com

Thank you for listening

Geomatics World Nov/Dec 2011 & Jan/Feb 2012

www.pvpubs.com

Technology

Recent Survey Work at Stonehenge