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To metoder for innsamling av terrestriske laserskandata: Rosslyn Chapel kasusstudieL. Wilson1, D.K. Pritchard2, H.C. McGregor1 and D.S. Mitchell1

Lyn Wilson & al: Two Avenues for Terrestrial Laser Scan Data: The Rosslyn Chapel Case Study

KART OG PLAN, Vol. 72, pp. 314–319, P.O.B. 5003, NO-1432 Ås, ISSN 0047-3278

Constructed between 1440 and 1486, Rosslyn Chapel has had a difficult history. It has suffered vandalism and neglectduring the Protestant Reformation, used as a stable for Oliver Cromwell’s horses during the English Civil War, unsym-pathetically renovated during the 1800’s and nearly blown-up by the suffragette movement in 1914. During the 1950’sthe Chapel has suffered a failing roof in combination with an unsympathetic application of a white cementitious paintto the interior – essentially locking in the moisture into the stone.

As part of an ongoing research partnership, Historic Scotland and Glasgow School of Art systematically digitally docu-mented the interior and exterior of the building and grounds over a three-day period. The purpose of the project was toimplement a full 3D terrestrial laser survey of the existing structure to document its current physical condition and tohelp guide ongoing conservation techniques. Of particular concern was the structural integrity of the main vault duringthe conservation of the existing roof. Fourteen cross-sections were cut through the point cloud data to determine the ex-act thickness of the vault.

As part of the presentation material for the new Chapel interpretive centre, the terrestrial scan data was used to gen-erate a 3D model and subsequent 3-minute animation, depicting the evolution of the Chapel from its construction to theProtestant Reformation. In addition, close-range scanning systems were used to generate sub-millimetre scans.

Key words: Rosslyn Chapel, 3D skanning

Lyn Wilson, BSc, MA, PhD, FSA, Scottish Ten Project Manager, Historic Scotland. Project Manager, Centre for DigitalDocumentation & Visualisation. Email: lyn.wilson@scotland.gsi.gov.uk

1. IntroductionRosslyn Chapel stands in a serene locationon the outskirts of Edinburgh, Scotland.Constructed between 1440 and 1486 and fur-nished with flying buttresses and ornatesculptural carvings covering virtually everysurface, it is widely hailed as a medieval ar-chitectural masterpiece (Figures 1a and 1b).The Chapel has been the focal point of manyreligious and political events since its foun-dation, while being the subject of numerousmyths and legends relating to the KnightsTemplar and Freemasonry.

In the 1950s, the Chapel suffered from anunsympathetic renovation of the roof usingwhite cementitious paint on the interior,which locked moisture into the stonework,caused the roof to fail and accelerated decay

of the carvings. A temporary steel and alumi-nium structure was erected over the Chapelin 1997 to protect the monument from rainand to allow the stone to slowly dry out. In2009, an extensive conservation programmewas embarked upon, which aimed to restorethe Chapel to its former glory and saw theremoval of the temporary protective roof in2010.

As part of the conservation programme,Historic Scotland and Glasgow School of Artwere requested to carry out a 3D survey ofthe Chapel and its surrounding environmentin 2009. The aim of this survey was to tho-roughly digitally document the Chapel’s cur-rent physical condition, inform the ongoingconservation process and provide 3D contentfor interpretation.

1. Historic Scotland, Edinburgh, Scotland2. Digital Design Studio, Glasgow School of Art and CyArk Europe, Glasgow, Scotland

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2. MethodologyA joint team from Historic Scotland andGlasgow School of Art systematically digital-ly documented the interior and exterior ofthe building and grounds over a three-dayperiod. A combination of terrestrial time-of-flight (Leica ScanStation 2) and phase-shiftlaser scanners (Leica HDS 6100) were usedas appropriate: time-of-flight terrestrial la-ser scanners (TLS) to survey the exterior andenvironmental context, phase-shift TLS tosurvey the interior spaces and roof details.For scans on the roof itself, the phase-shiftscanner was set in place and operated remot-ely via wi-fi (Figure 2). At least 30% overlapbetween adjacent scans was ensured. Indivi-dual scans were registered using both HDS

targets and cloud-to-cloud feature recogniti-on techniques. Interior and exterior scanswere joined using targets. This resulted in aunified registration with a mean average er-ror of ± 8mm. The total number of scan posi-tions was 142 and the total number of surveypoints collected was in excess of 4 billion.

360° panoramic high resolution HDR pho-tography was taken at all scan positionsusing a Nikon D3X set upon a Nodal Ninjamount, offset to the same height as the lasersensor. Additional texture photography wastaken to provide further information for mo-del textures.

In addition to the TLS survey, high-resolu-tion sub-millimetre scanning of particularsculptures was undertaken to provide an un-

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Figure 1a. Rosslyn Chapelwith exterior temporary metalroof structure in place from1997–2010.

Figure 1b. Interior sculpturaldetail at Rosslyn Chapel.

L. Wilson, D.K. Pritchard, H.C. McGregor and D.S. Mitchell

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paralleled record of their surface geometry.Both an articulated arm scan system (Per-ceptron Romer v5) and a structured lightscanner (Artec MHT) were used for this pro-cess.

Aligned and registered point clouds weretaken into Innovmetic’s Polyworks softwarepackage to produce a mesh. Hole-filling wascarried out for subsequent modelling, but it

was important for us to recognise areaswhich represented voids in the data: our sci-entific approach meant the survey should beas objective as possible and any interventionby the modeller was identified by a differentcolour in the mesh (Figure 3). The mesh wasfinally imported into Autodesk 3D StudioMax to produce photo-realistic models.

3. Results3.1 Survey and ConservationThe 3D survey provided baseline informati-on on the condition of the Chapel prior to theconservation programme. A monitoring pro-gramme has been established for particularexterior stone panels: re-scans will be carri-ed out at two yearly intervals and compared

against the baseline using Polyworks Inspectpackage. Any erosion to the stone surfacewill be quantified and passed to HistoricScotland’s stone conservators to make an as-sessment on the course of action needed. Or-tho-rectified point cloud elevations and planviews were also generated of the Chapel andits grounds (Figures 4 and 5).

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Figure 2. Positioning the pha-se-shift scanner on the roof ofRosslyn Chapel for control viawi-fi.

Figure 3. Mesh of RosslynChapel, obtained from TLSsurvey. Purple areas indicatemodeller intervention, as itwas impossible to obtain scandata from these locations.

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Of particular concern was the structural in-tegrity of the main vault during the conser-vation of the existing roof. Fourteen cross-sections were cut through the point clouddata to determine the exact thickness of the

vault (Figure 6). This information allowedthe on-site engineers and conservators to as-sess safe working practices for the durationof the project.

Figure 4. Point cloud plan viewof Rosslyn Chapel and itsgrounds.

Figure 5. Ortho-rectified pointcloud elevation of Rosslyn Cha-pel showing some internalstructures and vaulted crypt.

Figure 6. Image showing cross-sections cut through the pointcloud to determine precise di-mensions of vault.

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3.2 InterpretationAs part of the presentation material for thenew Chapel interpretive centre, the TLSdata was used to generate a 3D model andsubsequent 3-minute animation, depictingthe evolution of the Chapel from its con-struction to the Protestant Reformation. In

addition, close-range scanning systems wereused to generate sub-millimetre scans of par-ticular sculptures. The pagan ‘Green Man’(Figure 7) and the ‘Apprentice Column’ weretwo of the sculptures digitally documentedusing laser and structured light scan sys-tems.

The point cloud data was developed into amesh then a model, then animated in highdefinition at 25 frames per second to producean interactive interpretive feature known asthe ‘Pathways to the Sculptures’ (Figure 8).Several of these sculptures lie at inaccessiblepoints within the Chapel. The interpretive

content allows visitors to examine the sculp-tures virtually at close-range, without anyrisk to either the visitor or the sculpture it-self. Combined with textual information oneach sculpture, the 3D data is proving to be avaluable resource for explaining Rosslyn’ssculptural legacy.

Figure 7. Meshed data fromsub-millimetre laser scanningof the Green Man.

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Figure 8. Beginning of ‘Path-way to Sculptures’ interpretivecontent for Rosslyn Chapel vi-sitor centre.

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Finally, an Apple iPad application is current-ly in development which will utilise the geo-metrically accurate point cloud informationand the models derived from it to digitallyshowcase Rosslyn Chapel.

4. ConclusionsThe digital 3D survey of Rosslyn Chapel hasproven beneficial for two distinct avenues inaddition to providing a highly accurate sur-vey record. Tangible conservation applicati-ons and the generation of visually engaginginterpretative materials emphasise the ver-satility of geometrically accurate point clouddata within the heritage sector. As Rosslyn

Chapel’s conservation programme continues,our 3D survey has also secured the digitalpreservation of this inspiring monument forfuture generations.

5. Acknowledgements– All images are copyright of the Centre for

Digital Documentation and VisualisationLLP.

– Rosslyn Chapel Trust: Colin Glynne-Percy– Historic Scotland team: Maureen Young,

Alan Simpson, James Hepher, AdamFrost.

– Glasgow School of Art team: AlastairRawlinson, Craig Logan, Jared Benjamin.

NotisNordisk kongressprisDet blir ved Nordisk landmålerkongress deltut nordiske kongresspriser. De deltagende for-eninger kan dele ut en kongresspris til en stu-dent eller yngre landmåler som har utmerketseg. Kongressprisene ble utdelt under festmid-dagen fredag 21. september på Grand Hotel.

Målsettingen med kongressprisen er åoppmuntre de yngre utøverne. Kongresspri-sen er 1000 Euro og diplom.

Årets prisvinnere er: Karin Kolis, Finland: Förverkligandet avvattenskyddsåtgärder genom ägoregleringarEivind H. Ramsjord, Norge: Eiendomsdan-nelse i Bytransformasjon. Se artikler i Kartog Plan nr. 2-2011 og Kart og Plan nr. 4-2011Camilla Backman og Cecilia Rogvall,Sverige: Planering av planeringen. Kommu-ners arbete med prioritering och tidsplane-

ring av detaljplaner. Se KTH:s publiserings-databas DiVA: http://kth.diva-portal.org/smash/record.jsf?pid=diva2:529124

Vinnarane av nordisk kongresspris 2012. Fråvenstre Cecilia Rogvall og Camilla Backmanfrå Sverige, Eivind H. Ramsjord frå Norge ogKarin Kolis frå Finland.