1.2
Change Over Time
An InternAtIonAl JournAl of conservAtIon And
the buIlt envIronment
Fall 2011
EDITOR IN CHIEF
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Frank Matero
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ISSN 2153-053X
Change Over Time AN
INTERNATIONAL OF
AND
THE
BUILT
JOURNAL
CONSERVATION ENVIRONMENT
FALL
2011
1.2
152 Editorial F R A N K M AT E R O
156 Heritage Recording and Information Management in the Digital Age (SMARTdoc-heritage) M A R I O S A N TA N A Q U I N T E R O A N D O N A VILEIKIS
ESSAYS
166 Dedication: Robin Letellier (1944–2007) FRANCOIS LEBL ANC
CONTENTS
168 Metric Condition Records: Does the Capture Method or the Information Need Determine the Performance of 3D Heritage Records? BILL BL AKE
184 HABS Documentation in the Digital Age: Combining Traditional and New 3D Methods of Recording C A T H E R I N E C . L AV O I E
198 Advanced 3D Recording Techniques for the Digital Conservation and Presentation of Heritage Sites and Objects FA B I O R E M O N D I N O
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216 Photography in Heritage Research: In Search of Digital Standards for Image Capture, Image Processing, and Image Delivery JOSEPH E. B. ELLIOTT
198 236 Multispectral Sensors in Combination with Recording Tools for Cultural Heritage Documentation J O S E´ L U I S L E R M A , T A L A L A K A S H E H , N A I F HADDAD, AND MIRIAM CABRELLES
252 Laser Scanning the Past for the Future: Baalbek Temple
216
¨ R N VA N G E N E C H T E N , M A R I O S A N TA N A BJO Q U I N T E R O , A S S A A D S E I F, A N D G H A S S A N G H AT TA S
268 Terrestrial Laser Scanning: Imaging, Quantifying, and Monitoring Microscale Surface Deterioration of Stone at Heritage Sites
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JESSICA KOTTKE, JOHN HINCHMAN, AND F R A N K M AT E R O
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EDITORIAL FRANK MATERO UNIVERSITY OF PENNSYLVANIA
Figure 1. ‘‘The Art of Restoring,’’ Fun Vol 25, June 27, 1877. The satirical warnings of irreversible restoration fictions by nineteenth-century critics led to the importance of documentation and recording as the foundation for all conservation actions. Improved methods of data capture and manipulation have resulted in an information revolution for heritage professionals; however, the challenge remains as how to best apply and use the new technology for informed conservation decisions. (University of Florida George A. Smathers Libraries; http:// ufdc.ufl.edu/UF00078627/00030/)
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What is past is prologue. William Shakespeare, The Tempest
Many built works pass down through time. How they are received by each generation is ultimately a function of what we know and feel about them and what ultimately becomes heritage. Conservation/preservation therefore has always been about transmission and reception. As the second-century grammarian Terentianus Maurus pronounced, Habent sua fata libelli—books always have their histories—and so it is with the physical places we inhabit. What survives, what is forgotten, and what is cared for or destroyed describe the lives buildings and places acquire over time. Such trajectories are dependent on many diverse factors; however, once consciously examined, all built heritage comes under consideration for its ability to communicate to us; to have relevance in ways consistent or new to its original authorship. As stated by the Italian theorist Cesare Brandi, ‘‘restoration [conservation] is the methodological moment in which the work . . . is appreciated in its material form and in its historical and aesthetic duality, with a view to transmitting it to the future.’’1 It is a true historical event, a human action that is part of the process by which a work is transmitted and received. The act of preservation is therefore the actual moment of the conscious contemplation of cultural heritage primarily for its historical value; it is, in a sense, the ‘‘afterlife’’ of any created work. Recording is one way in which those concerned with built heritage attempt to capture physical aspects deemed significant or defining of a thing or place. For the architect this may be the plan, section, and elevations of a building; for the architectural historian it may be a comparative stylistic analysis of the classical orders; for the engineer it may be the gradual movement of a wall or dome over several years; and for the conservator it may involve recording a multitude of decay phenomena. Regardless of intent, recording and its acquisition-hungry sister, documentation, are the cornerstones of conservation practice. Heritage specialists perform documentation and recording based on the belief that by accumulating and producing records of the tangible aspects of the built environment, one can preserve inherent informational and aesthetic values, or at least the potential values, that may be lost through natural degradation and human modification.
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Traditionally, documentation and recording for conservation have long been associated with the physical state or condition of an object or site in danger of being lost or damaged and immediately before and after intervention. No doubt this can be attributed to the physical changes most conservation treatments affect, as well as the contributions that documentation can make toward an increased understanding of the past appearance or technology of the object or site. But it has been through the study of existing condition as a record of past change that heritage specialists have seen the value of documentation and recording in developing a more accurate knowledge of alteration and deterioration, especially long-term trends and patterns of anthropogenic change and natural weathering. By studying current condition as the cumulative result of change over time through careful observation and description, one can come closer to managing change as responsible stewardship. The documentation of structures and sites and associated building arts such as sculpture, mosaics, and wall painting presents enormous difficulties, attributable as much to their size and complexity as to the pressures of utility and context. A great many agents and phenomena, ancient and modern, transform sites over time and in different ways. A variety of natural and cultural processes including natural decay and human actions continually alter a building’s fabric and form. Daily maintenance, intermittent repair, use and reuse, neglect, decay, abandonment, and recovery are among the diverse yet plausible processes that translate past actions and events into present conditions. As a result, the physical status of any structure or site is a record of the interaction of many different determinants over the course of its life. For many structures and sites these include (in general order of occurrence): • original design, materials, and construction techniques • subsequent changes through use, human alteration, and natural aging • • • •
micro- and macro-environments and climate disuse and/or abandonment destruction and/or burial rediscovery (excavation in the case of archaeological sites)
• conservation, reuse, display, and maintenance These determinants characterize individual episodes in the shaping of the built environment and as such define the ‘‘life cycle’’ for materials, structures, and sites. Such models, commonly used by archaeologists to explain multivariate change over time have been adopted by relatively few heritage specialists to study and explain the transformation of cultural resources.2 By considering performance, function, deterioration, and intervention in a more holistic and integrated manner—linking design, environment, and human agency—heritage specialists can develop and apply documentation and recording methods focused on cultural as well as etiological concerns. The road to effective stewardship must therefore begin with a conscious understanding of why and when we record. Within recent years, the technology of producing and
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managing more accurate and comprehensive documentation has increased dramatically through the development of newer imaging and computer graphic software including geographic information systems, 3D laser imaging, and the application of nondestructive investigation. These tools provide the key to better management decisions by allowing greater data integration and dissemination and more frequent monitoring and risk assessment, which goes far beyond earlier interests in simply recording a structure or place for posterity or recording the immediate physical status before and after intervention. Nevertheless, all recording, no matter how sophisticated, is deceptively subjective, always performed with a specific objective or from a particular point of view. Contrary to modern scientistic notions, it is the moment at which interpretation begins. The papers presented in this special issue of Change Over Time represent a diverse array of interests, approaches, and practices by an equally diverse group of heritage professionals. It is our hope that in convening such an international gathering of experts on the subjects of digital heritage recording, documentation, and information management, and publishing a selection of the papers presented in this and the forthcoming issue of the journal, we have opened up the conversation to address not just what and how, but why and when we record built heritage.
References 1. Cesare Brandi, ‘‘Theory of Restoration, I,’’ in Historical and Philosophical Issues in the Conservation of Cultural Heritage, eds. Nicholas Stanley Price, Mansfield Kirby Talley, and Alessandra Melucco Vaccaro (Canada: Getty Conservation Institute, 1996), 231. 2. Examples include: Daniel Bluestone, Buildings, Landscapes and Memory: Case Studies in Historic Preservation (New York: Norton, 2011); M. Christine Boyer, The City of Collective Memory (Cambridge and London: MIT Press, 1996); Stewart Brand, How Buildings Learn (New York: Penguin Books, 1994); and Neil Harris, Building Lives (New Haven and London: Yale University Press, 1999).
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METRIC CONDITION RECORDS Does the Capture Method or the Information Need Determine the Performance of 3D Heritage Records?
BILL BL AKE BILL BL AKE HERITAGE DOCUMENTATION
Figure 1. Undifferentiated and differentiated data. Top: detail from a 1:200 scale orthophotograph of Chatterley Whitfield Colliery, Staffordshire, England, showing undifferentiated data. Bottom: the same segment of the site as recorded by measured survey at a 1:50 scale; this is highly differentiated and has been captured for the detailed planning of the building conservation program. (English Heritage)
168
The interdependency of our questions about the historic environment and the data capture techniques used to answer them is complex. Metric survey is a primary tool that, directed correctly through brief and specification, can provide useful data for the heritage information process. The inherent differences in data quality derived from different techniques are primarily a result of capture by two classes of information: undifferentiated (derived from indirect techniques such as photogrammetry and laser scanning) and differentiated (derived from direct techniques such as measured drawing, total station, and global positioning systems [GPSs]). The value and utility of information is thus dependent on the method chosen for its capture; it may demonstrate selectivity to meet a particular purpose or be an unselective record for future interpretation. An appropriate response to information requirements should be shaped by an understanding of the significance and value of the heritage (as described under Article 16 of the ICOMOS Charter of Venice, May 31, 1964) to be recorded along with the performance of capture techniques.
What Is Heritage Documentation?
Documentation is both a process and a product. Heritage documentation is most effective when it is a continuous process. In this way it best serves conservation by supplying the data necessary for understanding the condition of a site or object and is fundamental for all monitoring and maintenance. Documentation is necessary throughout the conservation process from recording, assessment, and treatment to periodic evaluation. Through the actions of recording, documentation makes tangible and intangible (e.g., song, dance, and ritual performances) heritage available to specialists and the public in a broad array of forms such as metric, narrative, thematic, and societal records. Metric survey is a key aspect of heritage documentation as recognized by ICOMOS in Principles for the Recording of Monuments, Groups of Buildings and Sites (1996) ratified by the 11th ICOMOS General Assembly in Sofia, October 1996: Recording is the capture of information which describes the physical configuration, condition and use of monuments, groups of buildings and sites, at points in time and it is an essential part of the conservation process.
Technologies in Conflict In commissioning heritage records there is a balance to be struck between conservation needs, the level of detail and accuracy required of the documentation, and the significance of the heritage to be recorded. Consideration of these factors informs the selection of the
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recording technique. The balance between selective and objective records needs to be made by a careful choice of technique informed by an appropriate response to significance and information need. The selective nature of recording technologies is variable; for example, a measured drawing has a completely different characteristic than a point cloud collected by a laser scanner. Both sets of data are incomplete and both are dependent on the operator’s viewpoint. They differ immensely in terms of content: the drawing will be a delineation of the draftsman’s view, whereas the point cloud will have no such interpretive information. In all documentation, the practitioner should be able to intelligently answer the questions: ‘‘What and why am I recording, and why have I chosen that specific technique?’’ It may be that an attempt to draw a brick fac¸ade by tracing it is considered inappropriate on the grounds that it is uneconomical and imprecise, but the same fac¸ade recorded with a laser scanner may not produce a more economical outcome. It is not reasonable to assert that a laser scanner is the right tool for the job simply because such a device is available or that the sheer volume of data will provide a ‘‘full digital record’’ or even that the act of scanning equates to ‘‘digital heritage preservation’’!
The Three Key Processes in Documentation The delivery of useful documentation for heritage projects requires negotiating three key processes of measurement, selection, and communication.
1. Measurement or information capture technique is where the choice of technologies can determine the precision and nature of the data deliverable (for example, drawing, photograph, point cloud, model). 2. Selection of significant information dictates how efficiently the capture method will match the information need. The selection of information from a captured data set can be active at capture or made post-capture by interpretation of an undifferentiated data set. The choice of measurement technique will determine both the degree of information recovery possible and its dependency on the response to significance either during or post-capture. 3. Communication or presentation determines the utility of the captured information set. A poor understanding of the convention or visual language of draftsmanship or cartography can devalue spatial information to the point where it can nullify the value of the documentation process and product. The transmission of information from capture (often by a surveyor) to end user (a conservator or resource manager) requires abstraction by line point, shade, tone, symbol, and model according to convention so that the information is readable, current, and relevant. Getting the balance between capture method and presentation standard is the heart and soul of recording, or in the words of Thomas and Leonard Digges, two of the founders of modern survey practice:
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so the Geometer, how excellent so ever he be, leaning onely to discourse of reason, without practise (yea and that sundrie wayes made) shall fall into manifolde errors or inextricable Laberinthes. Pantometria, 1571. Heritage assets, by their very nature, are vulnerable to a wide variety of stresses, not the least being the very effect of their significance and value. In broadcasting heritage values, touristic and economic pressures on the heritage asset are inevitable. Preventative maintenance inadequacies, material failures, instability, excessive use, visitor damage, excessive loading, and inappropriate interventions are all likely once a structure or landscape has passed from its historic use to a ‘‘heritage’’ role. The documentation needed to inform sound heritage management is dependent on our choices about the nature, utility, and extent of that information in heritage management. Such 3D capture systems as photogrammetry and laser scanning have a variety of performances (e.g., angular measurement precision, distance measurement precision, range precision, light reflectance sensitivity, point density, etc.), and matching the system to the desired information outcome requires a good understanding of both the system and the information-user requirements. Heritage documentation requires information types, which are either differentiated or undifferentiated (Fig. 1). This distinction is important in answering the primary performance question of the information user: should the selection process be active (where the surveyor chooses which points or lines describe the subject) or passive (where the selection of information is made from the data rather than the subject) at capture? The next level of analysis is the ‘‘fit for purpose’’ requirement of the information, which will be influenced by the end use of the captured information. To ensure the information requirement is met, detailed briefs and specifications have been found to be necessary to achieve outcomes that are predictable.
Data Differentiation: Direct and Indirect Capture Methods It is possible to achieve continuous undifferentiated data capture by use of techniques like photography (Figs. 2 and 3), photogrammetry, and laser scanning. The quality of the information can be largely determined by the performance of the capture system. For example, the performance of an orthophotograph is a function of the camera, the subject area captured by each image (photo cover), control, and digital surface model (DSM) generation; all of these can be documented and examined as a part of the process metadata (Fig. 1). Laser scan data has similar process-dependent metadata performance indicators (e.g., area of coverage, control point density, and registration data). The notable difference is that the data are rarely, with the exception of the ‘‘artifact’’ group of scanners, continuous. Laser scanning supplies data that are principally influenced by the choice of scan position and system performance rather than by information selection issues. Data capture by systems like photogrammetry and laser scanning are said to be ‘‘indirect’’ as information selection is not the principal influence on the capture process, whereas direct methods rely on the selection of information at the point of capture. Indirect systems are character-
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Figure 2. Detail from a rectified photograph of a thirteenth-century curtain wall at Tamworth Castle. The image records not only the metric disposition of the material but also surface condition in the form of staining, cracking, friable and hollowed elements, material type by color, and more. (Wessex Archaeology)
ized by the post-capture analysis phases, during which time information can be extracted (e.g., in photogrammetry, the process of line plotting or producing surfaced models from point clouds in laser scanning).
Active and Passive Information Selection Information capture is governed by the primary reasons for undertaking documentation. A passive approach assumes information can be recovered from the captured set at a future date without stating any particular need other than a baseline level of capture. Such approaches are common in ante-disaster records and rely on the robustness of the primary data. In photogrammetry this is well-understood,1 and good examples of successful disaster recovery (e.g., the postďŹ re recovery of lost ceiling details at Windsor Castle in 1992 by
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prefire photogrammetry) show this is effective.2 Active information selection requires an agreement between the user and provider, in the form of a brief and specification, on the function of the captured information and the required constraints on selection performance (e.g., agreement on how many points should be captured to describe a line to be presented at a given scale).
Heritage Documentation Information Needs The specific requirements of heritage documentation are characterized by a number of information performances (e.g., scale of presentation, density of point capture, appropriate selection of lines to describe a plan).3 The principle of ‘‘fit for purpose’’ is often cited as a description of both the scale of output and the level of abstraction required. It is worth noting the requirement for 3D information is not usual in heritage documentation and considerations of condition recording and monitoring in 2D are more common in the acquisition of heritage management information. It is possible to describe heritage information needs as a hierarchy with the inventory as the first level and conservation action and site management as the last. It is recognized that the information requirement is cyclic, and that the conservation cycle has both initial and continuing information needs, at both the asset evaluation and monitoring stages.
Condition Monitoring Concepts in Heritage Asset Management Monitoring refers to the measurement of change on the condition or nature of the asset. The changes may be: 1. Micro (chemical composition of material, moisture migration, etc.) 2. Macro (structural movement, damage, color degradation, geotechnical, etc.) 3. Environmental (microclimate, local climatic, fire risk monitoring, etc.) The monitoring of structural movement typically involves the measurement and analysis of dynamic strains, loads, vibrations and displacements, pressures, and temperatures. The aim is to determine the risk of structural failure and priority in the structural intervention.4 The identification of the points of likely failure (usually through fatigue, stress, or environmental reaction) is essential and requires careful inspection based on experience of the behavior of similar structures and materials. Structural monitoring includes the investigation and mapping of soil and geological conditions pertinent to structural movement as well as examination of the structure itself. The monitoring of material performance is frequently conducted concurrently with the monitoring of structural stability but uses point-sampling methods to determine the action of moisture, oxidation, and salinity and the effect of atmospheric pollutants such as sulphur dioxide on the material. All monitoring systems require four key concepts to operate:
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Figure 3. Kite aerial photograph of the Archer Pavilion, Wrest Park, Bedfordshire, England. Raising the camera to see surfaces hidden from the ground is easy compared to raising a scanner or total station for the same effect. In this case the camera was raised by a kite. (Bill Blake Heritage Documentation, used with permission of English Heritage)
1. The identification of the theoretical model to be tested by measurement 2. A repeatable comparable baseline data set 3. A consistent system of cyclical recording 4. A program of periodic review of the data, the model, and the structure
Properties of Monitoring Systems for Heritage Asset Management The assertion that laser scan data5 can be used to monitor structural movement needs to be evaluated against structural movement–monitoring measurement system requirements, which include:
1. A known tolerance of each measurement cycle and the recording of the environmental conditions at the time of capture 2. Confidence that the tolerance in the measurement system is sufficient to record the anticipated change 3. Operation of cyclical measurement by personnel trained to include the calibration and measurement of environmental statistics with each cycle
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Figure 4. Laser scan data viewed as an elevation. Note the line-of-sight occlusions and that although individual blocks can be discerned there is insufficient information to form an opinion on the condition of the fabric. (Data supplied by permission of Wessex Archaeology and Latimer CAD)
4. A high confidence that the measurement sites are isolated from interference from effects outside the model to be tested by measurement
Performance of Laser Scan Data Typical structural monitoring regimes require the detection of movements of 3–5 mm over a single period or a series of measurement epochs. Taking into consideration the required tolerance to capture the movement, the measurement system should operate at approximately twice the precision of the minimum detection limit as indicated by the Nyquist/ Lange formula6: Q 1 (m / ) where Q is the quality measure, m is the grid spacing, and is the size of the smallest object to be captured. It is clear that despite the speed and volume of capture possible with a typical 3D terrestrial laser scan data set,7 the scan alone does not meet the four requirements for a structural movement monitoring system for a number of reasons:
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Method
Cycle
Precision
Typical application
Tell-tale
Subject to inspection
Requires repeat cycle
Surface crack monitoring: hit and miss ⬍ 3–5 mm
Strain gauge e.g., Demec
Automatic
submillimeter
Load monitoring across fixed points
Precise level e.g., Leica DNA 03/09 with invar staffx
Manual
0.3–0.9 mm
Floor and ceiling deformation to premarked points
Auto-plumb e.g., Leica ZNLxx
Manual
1 mm at 30 m
Point to point vertical alignment
High frequency fixed position IR
Automatic
0.5 mm
Plant movement, dynamic loads
Close range Photogrammetry
Image recapture
5–10 mm
Surface damage mapping; profile extraction;
Automated selfAutomatic calibrating 0.5 Total station networkxx
0.5 mm
real-time deformation monitoring of fixed points
Fixed plane laser range Automatic scanning e.g., Amberg systemxx
1–2 mm to fixed prism 2–3 mm reflectorless
Tunnel profiling
Terrestrial 3D Laser scanner e.g., Leica HDS/ Trimble GS 200xx
Between 2–5 mm and 5–15 mm
Mass capture of unassigned point positions at 10 mm 10 mm spacing
x
Manual
Information supplied by Button, T, Dip Ld. Surv., SCCS Ltd Leica AG product description, 2003.
xx
Table 1. Measurement methods for monitoring structural movement.
1. Point density and point selection. The location of monitoring points is key to the monitoring process. Structural engineers select diagnostic points on the basis of their indicative properties; the points are chosen as markers of components and references against which movements can be detected. Point cloud data are characterized by a high density of points that may or may not be coincident with the diagnostic points required for structural movement monitoring. 2. Variable tolerance of range measurement performance. The precision of monitoring measurement must be predictable. Terrestrial laser scanning devices have a wide range of distance variation, and as such measurements cannot be considered in
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the same way as the discrete point to point measurements used for movement monitoring. 3. Scanner location-dependent data (Fig. 4). The point cloud and the implied precision of its points are factors of not only the scanner performance. The incident angle of the measurement beam to the surface measured is unique to each scan. A change in the position of the scanner will generate changes in the incident angles and thus present a different bias in the range data.8 If the scan data is to be used for movement monitoring, scan positions and orientation need to be precisely controlled.
The Problem of Interpolated Surface Data Detecting small-area deflections across surfaces is extremely difficult. The surface information derived from photogrammetry is dependent on imagery with sufficient textural information to achieve good results. Laser scan data is ideal for surface generation but lacks the data recoverability inherent in photogrammetry.
Unknown Performance of Repeatable Data Sets The cycle of information capture for structural movement monitoring may extend over a number of years. For this reason relatively simple devices (e.g., Demec gauge, tell-tale, etc.) with predictable performance are widely used (Table 1). Laser scanners, despite their highly efficient rates of capture, are subject to development, and designs have not been standardized as yet. Variations in reflectance and environmental conditions at the time of measurement can be ameliorated in simple systems (by dint of measuring unique points repeatedly rather than many points indiscriminately), but the impact of these factors on the performance of laser scan data is less well known. For example, a simple constraint like the minimum range of predictable distance propagation may well limit the application of certain types of laser scanners. A comparison between the performance of current, principally 2D techniques and laser scanning shows how the problem of displacement mapping is typically addressed by instrumentation. The importance of discrete point measurement over a fixed period of time (either by automation or manual methods) is fundamental to displacement monitoring in structures.
Heritage Documentation in 3D: Properties of Heritage Assets and Their Records It is important to recognize that the rich and unique qualities of heritage places that must be captured in the information sets needed to manage them are complex and sometimes subjective. The historical, societal, artistic, and associative values of a heritage place may not be recordable as metric data alone but, nonetheless, the captured data will fail in its utility if it does not respect these values at the presentation stage of the data cycle. The expectations of heritage stakeholders will be that the values associated with a heritage place will be present in some form in the quality of the captured record. Heritage records need to satisfy set minimum functions (e.g., metric precision, ade-
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quate information selection, and accurate depiction) to be effective. It is normal for heritage documentation to comprise a variety of information types both metrically and nonmetrically. Some aspects of heritage places can only be shared by means of visual media analysis in 3D such as illustrations to show the spatial development of spaces and objects; for example, rock art in caves or the recording of tunnel systems. In some cases the process of fitting theory to the 3D data by modeling brings new insights to its understanding; in others the ability to section and reveal hidden details is a benefit of 3D information. For example, the modeling of the 1,800 components of the Ironbridge at Coalbrookedale, Shropshire, from wire-frame data was supplied by photogrammetry, whereas laser scan and TST data were used to test manufacture and assembly sequences against historical records to decode the method of erection (Fig. 5, middle). The illustration of Landguard Fort, Felixstowe, Suffolk, England (Fig. 5, left), showing the internal disposition of the magazines, shell, cartridge hoists, and gun embrasures was assembled from a CAD model from metric CAD plans and the selected wire-frame was mapped by reflectorless EDM (REDM). The information was used to control the geometry for a guidebook illustration. The transparent view shows both the large scale of the structure and the functional links of the internal layout, features that are difficult to experience from a position outside or inside the fort. The bell frame in the church tower at Attenborough, Norfolk, England (Fig. 5, right), is an element to which few visitors will be granted access. Bell frames are usually a tight fit into the tower and their forms are hard to comprehend, as a clear view is rarely possible. A 3D model derived from REDM wire-frame and measured drawings shows more than photography alone, and is used on site to explain the function of the tower. The process of constructing the CAD model of the Ironbridge (Fig. 5, middle) enabled testing of design and assembly theory. The metric base for the model was primarily photogrammetric with infill from a laser scan and REDM. Making use of a consistent metric basis allowed the detection of variations in the manufacturing of the components for the first time and enabled a new analysis of the production and assembly process of the span.
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Figure 5. Landguard Fort, Felixstowe, Suffolk, England (left); detail of the Ironbridge at Coalbrookdale, Shropshire, England (middle); the bell frame at St. Mary’s Church, Attleborough, Norfolk, England (right). (English Heritage)
Each of these models was prepared for a specific outcome, and the underpinning of 3D data sources met the common constraint of metric performance for a given scale of record. The 3D modeling process has added to the investigative tools by means of ‘‘virtual testing’’ of components for fit and pattern origin. This is a far cry from offering a ‘‘digital surrogate’’ or virtual substitute for an object for analysis but, nonetheless, on the basis of a relatively low density of information, has added to our understanding of these heritage places by prompting questions as to the component manufacture methods, assembly sequence, and repair history to the historical analysts from the modeler.
Managing Information for Heritage Documentation To get the needed information to the right people at the right time requires information capture to be integrated into the conservation process. The control of information quality and its application is the starting point for heritage documentation standards. Managing the capture of information from the historic environment requires two processes to be effective: 1. Preparation and agreement of a brief between the information user and the information supplier for the performance, scope, and content of the deliverables 2. Application of specifications including the definitions of the required products9 Heritage asset management requires integration of the specialist disciplines engaged in the protection, conservation, and development of heritage places. In order for the high cost of 3D records and their products to be validated, cross-disciplinary information needs to be addressed at the conservation planning phase.
Understanding the Value of 3D Records In itself, 3D information is not a goal in heritage documentation; it is a valuable property of many metric data sets and, particularly in photogrammetry, a crucial method of infor-
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mation recovery in the event of loss or damage to heritage assets. The 3D-heritage information is valuable for 1. Data recovery from indirect capture, particularly in ante-disaster records 2. Post-capture analysis where information is of sufficient density to support the required selection needs 3. Proof of data origin; 3D data sets have the potential for automatic capture of crucial metadata, which is invaluable for information recovery 4. Terrain interpretation 5. Clarity of communication of spatial, textural, and surface-biased information (i.e., declaration of surface treatment used in the model, indicator of model parity to measurement set, etc.) The availability of new data types (e.g., the laser-scanned point clouds) has yet to establish a strong value in heritage documentation for structures, despite obvious successes in landscape recording (airborne LIDAR) and for object records such as statuary. The information needs for the management of heritage structures are specific, and for example, the assertion that movement monitoring and condition mapping are possible from laser scan data needs careful evaluation, particularly by those who are charged with these tasks.10 The development of useful 3D products that support the documentation of heritage assets continues, and the acceleration of our understanding and enjoyment of our heritage that rich 3D records enable benefits us all.
Conclusion In looking at the need for condition records, it is possible to describe the information need as threefold: a. Assessment of condition b. Monitoring of condition c. Ante-disaster Each of these information classes requires different levels of data in terms of both intensity and density; for example, initial assessment records need to reflect the extent, heritage value, and areas of vulnerability but need not be at the same scale or point density as an ante-disaster record. Variation in scale, information density, and type will occur according to the project needs and the resources available to acquire them. Traditional 2D records, while adequate in many cases, may well save time and money but may be of limited value if detailed reconstruction or surface monitoring is needed. The distinction between 2D and 3D technologies must be considered carefully as the sustainability of methods is important when cyclic recording is envisaged. For example, a robust 2D method like site photography with 3D TST control has strong advantages in terms of skill accessibility, whereas a 3D method such as laser scanning may generate a
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strong information gain but at a very high cost of specific training. The simplicity of a 2D information set (such as photographs and drawings) may have greater local benefit than a 3D data set dependent on highly skilled interpretation. The output processes of 3D recording are developing, but to date do not present easily accessible information in the way traditional 2D methods do. An assessment and definition of the use of 3D modeling in the documentation of cultural heritage is needed to prepare standardized performance-based criteria for commissioning 3D work for the presentation, understanding, and conservation of heritage assets. A number of actions by appropriate institutions would help the process of getting 3D heritage recording to work better. Reference Data Sets If a public reference set of common data types with known performances (in terms of scale, condition recording, monitoring value, etc.) could be made available, this would be an adequate starting point in preparing specifications for 3D documentation. The availability of good examples of scans, drawings, and 3D models under the auspices of an international institution such as CIPA or by a national heritage agency (HABS/HAER, English Heritage, etc.) would help in the future procurement of data sets. 3D Glossary An agreed definition of terms used to describe modeled data from 3D capture can help avoid confusion when describing 3D work. Many terms are used to describe model types (e.g., ‘‘visualization,’’ ‘‘high parity,’’ ‘‘reconstruction,’’ and ‘‘architectural design’’) but no clarity over their use or that of metric metadata such as point or vector incidence with either measured data or an actual object has been applied in the heritage sector despite such controls becoming the norm in building information management (BIM) systems in the construction industry. Public Test Results of 3D Models from Known Data Sources Published parallel testing of the metric performance of various modeling strategies as applied to a number of fixed user requirements could produce useful guidance for specifiers and help to define the performance of 3D products in the sector. Contractual Controls for 3D Data Capture and Modeling When information is presented as derived from a model, both the originator of the work and the client would benefit from some reference as to the type of model it is and what can be expected of it in terms of precision. The development and publication of the key contractual instruments (standard brief specification and glossary) are required to successfully procure models, core data sets, and metric data commensurate with thematic, investigative, and metric needs. User and Provider Checklists A tested and proven framework of assessment of condition monitoring information needs from the heritage conservation community is required so that data providers can demon-
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strate that they are able to provide data not just governed by hardware performance but data that are relevant to the conservation process. The diversity of application and the ever-increasing availability of 3D recording technologies are set to increase the need for a better understanding of how to apply them. This requires performance measurement against the information needs of heritage management.
References 1. J. Lebeuf, C. Ouimet, et al., ‘‘National Historic Sites of Canada: A Values-Based Approach to Posterity Recording,’’ in Proceedings CIPA XXI International Symposium (2007). 2. A. Escobar, ‘‘Preparing for Disaster: A New Education Initiative in Museum Emergency Preparedness and Response,’’ Conservation, Getty Conservation Institute (2004). 3. K. Clark, ‘‘Informed Conservation: Understanding Historic Buildings and Their Landscapes for Conservation,’’ English Heritage (2001). 4. M. Forsyth and I. Hume, Structures and Construction in Historic Building Conservation, Vol. 2 (Blackwell Publishing, 2007). 5. M. Santana Quintero and B. Van Genechten, ‘‘Three Dimensional Risk Mapping for Ante-Disaster Recording of Historic Buildings,’’ in Proceedings CIPA XXI International Symposium (2007). 6. D. Barber, ‘‘3D Laser Scanning for Heritage Advice to Users on Laser Scanning in Archaeology and Architecture,’’ Heritage3D/English Heritage (2007): 10. 7. For example, a 10 mm 10 mm array point cloud from a scanner with a range tolerance of 2 mm to 5 mm. 8. W. Bo¨hler and A. Marbs, ‘‘Vergliech von 3D —Scanning und Photogrammetrie zur geometrischen Dokumentation in Denkmalbereich,’’ Institut fur Raumbezogene Informations und Messtechnik Fachhochschule Mainz (2004). 9. B. Blake and P. Bryan, ‘‘Metric Survey Specifications for English Heritage,’’ English Heritage (2000). 10. A. Almagro, ‘‘Traditional Drawings versus New Representation Techniques,’’ in Proceedings CIPA XXI International Symposium (2007).
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Change Over Time Running an ad or special announcement in Change Over Time is a great way to get publication, program, and meeting information out to those in your field. Change Over Time is a semiannual journal focused on publishing original, peer-reviewed research papers and review articles on the history, theory, and praxis of conservation and the built environment. Each issue is dedicated to a particular theme as a method to promote critical discourse on contemporary conservation issues from multiple perspectives both within the field and across disciplines. Forthcoming issues will address topics such as Adaptation, Nostalgia, Interpretation and Display, and The Venice Charter at 50. 2012 Advertising Rates Ads are inserted at the back of each issue and on cover 3 (inside back cover). Only cover 3 positioning is guaranteed. Half Page: $200
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Spring 2012 Digital Heritage, Part 2
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uPcomInG Issues Digital Heritage, Part 2 Spring 2012
Adaptation Fall 2012
Nostalgia Spring 2013
Interpretation and Display Fall 2013
The Venice Charter at 50 Spring 2014
Vandalism Fall 2014
Climate Change and Landscape Spring 2015
Integrity Fall 2015
National Park Service Centenary Spring 2016