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3D GIS for cultural heritage restoration: A ‘white box’ workflow
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Original article
3D GIS for cultural heritage restoration: A ‘white box’ workflow Danilo Marco Campanaro ∗ , Giacomo Landeschi 1 , Nicoló Dell’Unto 2 , Anne-Marie Leander Touati 3 Department of Archaeology and Ancient History, Lund University, Helgonavägen 3 Box 192, 221 00 Lund, Sweden
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Article history: Received 13 March 2015 Accepted 15 September 2015 Available online xxx Keywords: Pompeii 3D GIS Digital archaeology Cultural heritage conservation
a b s t r a c t Structures of architectural heritage are constantly exposed to natural and human-made threats that can compromise their cultural and artistic values. This is the case of the Roman city of Pompeii, whose preserving conditions nowadays are so critical to let a group of Unesco-appointed experts consider the possible inscription of the property on the list of “World Heritage in Danger”. In this respect, improving the effectiveness of preservation strategies becomes a crucial task. A great contribution in this direction is given by the combination of digital technologies such as laser scanning, photogrammetry and computer vision-based techniques and 3D geographic information systems (3D GIS), whose integrated use could exponentially increase the effectiveness of conservation strategies of ancient buildings. This paper presents the results of a research developed as part of the Swedish Pompeii Project, a fieldwork initiated from the Swedish Institute in Rome in 2000. Main objectives of this research were (i) to develop a set of integrated digital methods to be extensively adopted by conservation specialists in the practice of preservation management; (ii) to deal with several aspects connected to the preservation of an ancient structure in a ‘fully-3D’ environment; (iii) to take advantage of GIS analytic tools for investigating architectural structures in three-dimensions. © 2015 Elsevier Masson SAS. All rights reserved.
1. Introduction and aims Strategies of documentation have always been object of special concern for preservation professionals, being universally considered a milestone in the restoration process, as stated by restoration charters [1]. The reason for this urge can be easily found in the very nature of the documentation itself as act of analytic in-deepening. Every work of restoration, in fact, is based on an earlier and detailed study whose strength is univocally connected to documentation (e.g. of geometries, levels of decay, crack patterns, materials characterization). The result of this cross-disciplinary process, in the framework of a job of restoration, overcomes the limits of a mere depiction of the actual state of a monument, acting more as prediagnostic/monitoring tool, eventually standing as the key element for an exhaustive investigation of the existing heritage [2].
∗ Corresponding author. Tel.: +393470000901; fax: +46 (0)46 222 42 14. E-mail addresses: danilo [email protected] (D.M. Campanaro), [email protected] (G. Landeschi), nicolo.dell [email protected] (N. Dell’Unto), anne-marie.leander [email protected] (A.-M. Leander Touati). 1 Fax: +46 (0)46 222 42 14. 2 Tel.: +46 (0)46 222 31 86; fax: +46 (0)46 222 42 14. 3 Tel.: +46 (0)46 222 79 34; fax: +46 (0)46 222 42 14.
Concurrently, due to the constant development undergone by digital technologies in the last decades, traditional bi-dimensional way of documenting appeared to be almost inadequate [3]: new data acquisition and processing systems (laser scanner, photogrammetry, GPS) have changed the concept itself of surveying; 3D digital graphics now allow to create and manage photorealistic models of the existing heritage while geographic information systems (GIS), linking information to graphic representation, offer enhanced tools to perform analysis and archive data. Aim of the present research, which has been developed in the framework of the Swedish Pompeii Project (http://www. pompejiprojektet.se/index.php), was: • to set up a novel three-dimensional system for the management of heritage structures by exploiting the combination of 3D visualization and geographic information systems (GIS) analysis; • to provide conservation specialists with an effective work pipeline for managing the architectural degradation through the use of standard software packages. In brief, the obtained results can dramatically contribute to the creation and management of fully usable 3D GIS thematic layers to record and analyse specific aspect related to the state of preservation of a historical building. As a case study, the north part of
http://dx.doi.org/10.1016/j.culher.2015.09.006 1296-2074/© 2015 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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the double atrium house of Caecilius Iucundus (North House of Caecilius Iucundus) in Pompeii has been investigated.
2. Previous work The diffusion of digital technologies has strongly affected the way researchers approach and manage the conservation of ancient buildings. Digital instruments are used in risk management at any level, and their application enhances the possibilities to detect, monitor and possibly restore ancient structures [3–5]. The improvement of site preservation management by means of digital technologies, as instruments to prevent and reduce damages, becomes an essential step to define time and cost effective preservation strategies. A great contribution in this respect has been provided by the introduction of GIS, a technology designed to store, manipulate, analyse and visualize database elements associated with geometric features [6]. The use of GIS for cultural heritage management purposes has grown considerably in the last decades, becoming a very widespread toolkit among preservation specialists. On the other hand, the theoretical debate has defined GIS as the most “natural” visualization platform to connect all different types of data [7–10] as well as one of the most influential technologies to manage and analyse information connected with cultural heritage in general [11–15]. Due to the very spatial nature of the discipline, this technology rapidly opened up new possibilities also in cultural heritage management appearing the most effective environment where to perform both technical and historical-critical assessment and where to analyse and control the entire conservation process in a more diagnostic sense. Furthermore, the evolution of visual technologies in the last two decades led to the introduction of 3D digitizing techniques both as an instrument to educate and engage the public about the past and as a means to study and analyse historical contexts [16,17]. This three-dimensional turn has strongly involved GIS technologies raising up several methodological issues and new potential outcomes in the field of heritage preservation [18,19]. So far, despite the growing body of literature related to this special issue, research on 3D techniques in preservation management has been mostly oriented towards data visualization rather than data analysis. In this respect, novel approaches for using 3D GIS system applied to restoration will be presented here, focusing on the case study of the North House of Caecilius Iucundus in Pompeii. Since a close examination of the general use and relevance of 3D GIS is out of the scope of this paper, being extensively discussed in literature [15,20–22], only those experiences strictly dealing with the use of 3D GIS for preservation purposes and thematic maps management (masonry analysis, architectural stratigraphic units description, decay, weathering forms and cracks patterns assessment, risk evaluation, etc.) will be reviewed. At an early stage, the use of GIS tools to address conservation issues has been mainly based on a bi-dimensional approach [23,24]. Canonical forms of graphic representation (plans, sections, facades) were used as a base to create vectorial maps linked to a database in a kind of vertical use of this technology. This solution, despite the undeniable advantage of making GIS-based analysis tools available to the architectural scale, has shown its limitations especially when dealing with complex geometries, a condition that in the field of cultural heritage tends to be the rule rather than the exception. Some experiments have been carried out introducing 3D models, generated by means of solid modelling techniques or by direct triangulation of point-cloud data coming from advanced 3D acquisition systems, such as laser scanners or photogrammetry. Thematic maps describing different aspects of the building were linked, as a part of the database, to the models. In this case, despite
the use of a three-dimensional approach to the problem, the maps were still bi-dimensional and connected by an external link using models as ‘visualization container’ rather than acting as a 3D feature themselves [25–30]. To overcome these limitations, dedicated platforms have been designed by IT specialists so as to combine traditional bidimensional graphical information and 3D models for preservation. In the case of ad hoc conservation module [31], a plugin has been developed as part of a proprietary software (ad hoc 3D) operating on point-cloud models enabling users to draw map patterns of alteration and decay on ‘3D ortho-images’ (planar images that can give information about point coordinates) and link them to the model; a more relevant effort has been put in the frame of NUBES project [4,32,33] where a web-based platform was used to spatially associate data with a 3D model of any 3D structure. In this case, the mathematical replica of the ancient building was split according to architectural criteria into several sub-elements (spans and pillars for instance). Layer-like annotations, drawn on textures by means of a Javascript-driven Scalable Vector Graphics (SVG) editor were projected on the 3D model. Despite their potentials, these last two examples have shown some limitations, requiring a more robust IT contribution for core part development and being confined to buyer-seller (ad hoc 3D) or strictly academic (NUBES) based linkage. Furthermore, some authors have stressed the necessity of adding a three-dimensional component to traditional graphical documents, combining 3D information and GIS. In his seminal work on the digital 3D survey in archaeology [34], Fiorini et al. described new methods to obtain extremely accurate 3D graphical representation of different archaeological contexts of data, usually ignored by monoscopic photogrammetry and normal analytical documentation (plants, sections and elevations). In this case, despite the advanced findings raised up, some limitations were still present: thematic maps (stratigraphic surfaces) were developed as vector representation of the bounding edges without recording irregularities or local curvature; 3D maps were drawn by means of software outside the GIS environment; 3D GIS technologies were used for communication and visualization purposes only. To overcome these limitations, our work was aimed at defining a new methodology, to be used by conservation specialists with basic GIS knowledge, in order to manage several aspects connected with the preservation of an ancient structure directly in three-dimensions. Such a methodological workflow would provide specialists with a simulation system where 3D models are not mere ‘container’ of hyperlinks to 2D drawings but fully queryable GIS feature dataset. In particular, the proposed method would allow: • to perform different typologies of analysis for studying and assessing the structure degradation; • to build statistics capable to provide a more clear view of the temporal trends and the decay progress of the structures; • to generate new thematic maps (risk maps); • to make diagnoses (e.g. putting together crack patterns with masonry singularities to evaluate the weakness of a structure); • to define and visualize conservation actions and treatments. To define this pipeline a ‘white box’ principle was followed: the term, borrowed from engineering science, is used here to define a system where all the information regarding an heritage asset can be managed by culture heritage specialists using well-known GIS software. Such a definition is clearly inspired by the ‘black box’, a platform requiring a major input from IT professionals that can be only seen in terms of input and output without any knowledge of its internal working.
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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2.1. Pompeii – a 3D revival During autumn 2000, the Swedish Institute in Rome started a field documentation campaign in Insula V 1 in Pompeii with the goal to revisit a Pompeian city-bloc, unearthed mainly in the 1870s, for thorough documentation and study. The choice of a full cityblock as study object was made in order to allow an assessment of a whole social context of ancient city life, not just that pertaining to single large houses. Annual campaigns with the duration of six or twelve weeks each were carried out on site up to 2011. Initially, work was carried out according to traditional building archaeological principles, based on close study of walls, drawings and field photography. The area was divided into four parts, investigated in parallel by four teams. In order to obtain a homogeneous documentation a professional photographer with main task to document all walls, has worked aside the archaeologists since 2006. To facilitate communication and assure access to data in all areas, a web-based presentation of digital photographical montages, orthomosaics, accompanied by written descriptions and commentaries was designed in 2007–2008 and put in open access (http://www. pompejiprojektet.se/insula.php). This digital research platform, characterized by its unique transparency, permitted the first assessment of the history of urbanization and social relations in Insula V 1 to appear only two years later [35]. In the meantime, different typologies of documentation technique were tried in search of a less time and cost demanding workflow. Applied since 2011, laser scanning has proved without comparison the most efficient way to proceed [36]. The implementation of a way to import the orthomosaics from the already existing 2D research platform to the meshed pointcloud opened the possibility to obtain a textured model, which in its turn was imported into a GIS system. Thereby, completely new possibilities of analysis opened for investigation. The above described workflow involving laser scanning and 3D modelling was achieved as the direction of the Swedish Pompeii Project moved in 2010 to Lund University and encountered the technical experience and networks at hand at the Institute of Archaeology and Ancient History. Fieldwork, post-processing and the elaboration of new means of visualization were carried out in collaboration with the Humanistic Laboratory and the Digital Archaeology Lab (DARK) at Lund University and the National Research Council of Italy (Institute of information Technology and science “A. Faedo”) in a conjoint project of digital acquisition of Insula V 1 by means of integrated spatial technologies, such as: phase shift laser scanners and image based 3D modelling techniques [36]. Aim of this work was to acquire a high-resolution model of the entire city-block in order to: • provide a more accurate and all compassing documentation of the structures that characterize the insula;
Fig. 1. This image shows the entire dataset of the insula V 1 Pompeii aligned and meshed.
• use the 3D dataset of information at different level of details to increase the possibilities to study the relations between the different areas that characterize the Pompeian domestic architecture (Fig. 1). The virtual structures so far developed have been used as geometrical reference for the interpretation of several parts of the city-block [37], and have been post-processed and used in relation to the documentation previously realized in the frame of the project. The material developed so far has been employed to design a new data model for the archaeological documentation of the insula V 1, which combine a 3D GIS platform with a large use of textured 3D models of the ancient structures previously acquired by means of laser scanner and image based 3D modelling techniques [18]. For the reason that the maximum transparency should include also the 3D data we experimented the use of WebGL, to visualize the gathered three-dimensional data directly through web browsers, in order to connect this new experimental approach with the classic documentation disseminated during these years through the Internet. The development of such web access for the visualization of the 3D data would provide the opportunity to anyone interested in studying the insula V 1 to access directly the information elaborated by the project team. 3. Methodology In the frame of the Swedish Pompeii Project a geodatabase management system (GDBMS) was already set up with the purpose of providing archaeologists with an advanced instrument for data analysis and interpretation [18,19]. In the proposed case, considering the increasing interest among preservation specialists towards digital processes, a methodological pipeline was implemented in order to build a three-dimensional geographic database that could be used to manage all the thematic maps recording specific aspects
Fig. 2. This image summarizes the general workflow followed to build fully usable thematic maps in a 3D geographic information systems database. The original 3D model is split into facade elements. A horizontal copy of each facade is created and used as a base to draw 2D maps. A link between each 3D facade and its horizontal clone allow to pass attributes from 2D features to 3D multipatches in order to generate 3D thematic maps.
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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Fig. 3. This image shows the ‘Masonry types’ thematic map (b) drawn using the georeferenced texture (a) as a base map. One of the pictures used to map the 3D model of the house of Caecilius Iucundus has been imported as a raster map in ArcMAP, georeferenced (a) using the horizontal copy of the facade as reference and used as a base to draw bi-dimensional thematic maps (b).
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Fig. 4. This image displays the horizontal clone and its reference plane (a) used as base for the elevation map (b) describing the lack of verticality [digital elevation modeling (DEM)]. Once created, the horizontal copy of the original mesh and the reference plane are translated to the origin of the local coordinate system and exported to ArcMAP to be used as a base to create a DEM.
of a heritage asset for preservation purposes. The main aim was to create 3D features that could be superimposed onto the existing textured model and used as fully queryable GIS features. Although the software used to carry out part of the proposed work (ArcScene, ESRI ArcGIS 3D module) allowed drawing 3D polygons directly using an existing model as a base by means of spatial snapping tools, an indirect method for the creation of 3D thematic maps was followed. In fact, drawing a 3D polygon
directly would have produced features matching with the base model only on external boundary (in correspondence of the snapping points) without recording any aspect of the inner complexity of the geometry. Furthermore, the proposed method allowed to overcome some limitations related to the application of GIS tools (typically conceived to operate from a top-down perspective) to three-dimensional geometries (see Section 3.3 about the use of traditional GIS tools to describe the geometry of a facade).
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Fig. 5. This image shows the assignment of the elevation map (a, 2) as texture for the 3D model (a, 3) of the facade using the same parameters used for the projection of the original textures – camera UVW projection – (a, 1) and its implementation inside a 3D geographic information systems environment (b).
A pipeline aimed at the creation of 3D features generated by inheritance of attributes from bi-dimensional maps was then proposed. To achieve this goal a set of strategies were defined stepping through the following actions: • setting up the data; • creating 2D thematic maps documenting the actual state of conservation of the historical building;
• transferring attributes from 2D to 3D layers; • generating 3D layers related to bi-dimensional maps.
The entire process was performed with ESRI ArcGIS software, a widespread GIS system provided with versatile modules for 2.5D (ArcMAP) and 3D (ArcSCENE) management, visualization and analysis of data. In particular, ArcMAP was adopted to create bidimensional maps, perform analysis and transfer values from 2D
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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Fig. 6. This image shows one the potential outcomes of this study, the production of cross-sections (a) and their linkage inside to the model inside a 3D geographic information systems environment (b).
to 3D, while ArcSCENE was used to perform 3D data visualization and analysis. A minor recourse to a 3D modeling software was required in the first phase of this study to post-process point-cloud data and generate an accurate textured model of the building (Fig. 2). 3.1. Setting up the data Creating thematic maps is mainly a simplification process. Since each monument can be seen as a complex stratification of
heterogeneous information (geometrical, historical, artistic, structural, etc.), the main aim of creating a thematic map is to extract from this composite context only the data sought by each preservation professional to highlight a specific topic. This means that complex geometrical structures, split into bi-dimensional elements, turn into meaningful bases for the critical digging out of the specialists. The scope of this phase was thus to divide the original 3D model into approximately plane parts (facades), create an horizontal clone for each of them to use as a base to draw thematic map and define a plane to be used as a reference system to perform
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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Fig. 7. This image shows the features composing the 2D to 3D value chain: multipatch (a–b, 1); footprint layer (a–b, 2); centroid layer (a–b, 3).
Fig. 8. This image summarize the workflow used to pass values from a 2D feature (1) to a 3D multipatch (4) through centroid elements (2) and footprint (3) with correspondent object identification (ID). The process is based on the correspondence between the elements (ID) of the clones of the same facade. The horizontal clone (3) is used to develop 2D maps (1) and receive the related attributes (2). This information are then transferred to the vertical clone (4) using the mentioned correspondence.
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Fig. 9. This picture shows three-dimensional features recording some aspects related to the state of conservation of the facade. In (a), each 3D feature describes a specific issue of decay and is linked to an attribute table reporting relevant information (e.g. the length/width of crack patterns, treatments needed to mitigate each issue). In (b), 3D features describe the lack of verticality of the facade. Graphs reporting data trends of an issue (e.g. lack of verticality progression), relationships, distributions and proportions can be easily produced using standard graph tools.
bi-dimensional analysis and easily switch between space dimensions (2D and 3D). As previously mentioned, the proposed work is focused on the case study of the North House of Caecilius Iucundus. The 3D model of the building was originally split into architectural subelements (facades) in order to add color information projecting
ortho-images by means of planar mapping techniques (texture mapping). Exploiting this partition thus appeared to be the most effective choice for the purpose of this research. A reference plane was assigned to each facade as an ideal approximation of its surface and the faces of the mesh were exploded into single objects to make them readable by GIS system as parts of the same feature.
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Fig. 10. This image shows three-dimensional features describing the various levels of overall risk associated to the specific facade. A table describing the rating category and the matrix used to assign the level of risk is linked as a raster to each specific colored area.
A horizontal clone of the facade was then created using the associated plane as a reference combination (plane/facade) for spatial transformations (roto-translation). Both horizontal and vertical meshes were exported as virtual reality modeling language (VRML) file to have faces recognized as subsets belonging to the same multipatch element. Moreover in order to get feasible results in ArcMAP, the horizontal reference plane (and accordingly the associated facade), was centered at coordinates (0,0,0), while the vertical facade was imported and placed in its correct location in ArcSCENE. The described steps were all developed using commercially available software (Autodesk 3D Studio Max) for 3D modeling (Fig. 4a). 3.2. 2D Thematic maps creation Thematic maps were generated in ArcMAP using different GIS tools depending on the specific topic to be highlighted. Vector data were used to create and edit decay maps and masonry stratification, raster maps to generate and algebraically add risks maps, digital elevation modeling (DEM) tools to assess facades tilting. In specific, deterioration issues were visually identified on an orthoimage (already used to texture the model) georeferenced to the horizontal facade (Fig. 3a) and drawn using editing vector features (Fig. 3b). A ‘type’ field was then assigned in the attribute table connected to each feature to identify single issues, based on the glossary of the International Scientific Committee for Stone [38]. Different levels of risk, calculated as product of the probability for a certain damage to occur multiplied by the severity of the damage, were identified for each feature and converted into raster dataset using the risk level of the original feature as an input field. Since an in-depth investigation of the risk prediction model was out of the scope of this research, simple qualitative considerations were adopted to define the above-mentioned matrix (e.g. missing
mortar was considered a less risky factor compared to a group of cracks that are more likely to severely endanger the monument).
3.3. New outcomes using traditional GIS tools Geometries of the past are seldom perfectly stereometric because of their original construction flaws or preservation conditions, or because of some extrinsic factors like earthquakes or differential subsidence. Resulting permanent deformations (e.g. bulges and leanings) usually mark a damage, a condition that can lead to failure and collapse [39]. A crucial task for conservation specialists is pointing out portions of an asset most suffering from issues and cross-checking these information with other data in order to define strategies and prioritize treatments. To assess the regularity of a facade traditional landscape analysis [digital elevation model (DEM)] tools were chosen. As for other procedures described in this paper, the starting point for this analysis was an horizontal clone of the facade, here considered in the same way of a raised formation and a reference plane used to place the model in ArcMap and set the zero elevation. A raster map was thus created in ArcGIS storing the elevation (Z) values of the facade element (multipatch to raster tool) (Fig. 4). The result of this analysis was a false color map describing the actual deformations and leanings of the facade by assessing points shifting from an ideal XY plane (reference plane). The proposed method could allow specialists: • to easily visualize local deformations of the entire model both qualitatively and quantitatively replacing mapping textures with leanings maps (Fig. 5) or creating 3D thematic layer by means of the procedure described in the following paragraph (Fig. 9b); • to retrieve detailed information about the geometry of the facade using typical DEM instruments like cross-section graphs (profile graph) (Fig. 6).
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Based on the above-mentioned procedure, a risk raster map interpreting the hazards related to the deformations of the facade was created using the DEM representation as a base and algebraically added to other raster dataset to provide an overall map. 3.4. Transferring attributes from 2D to 3D layers A fundamental step of this research was to pass information from bi-dimensional to three-dimensional layers in a GIS environment. To address this issue a 2D to 3D attribute chain was developed setting up a ‘centroid method’. The basic principle of this pipeline was transferring a value from a traditional 2D layer (raster or vector) to the corresponding mesh element (triangle) belonging to the horizontal copy of the facade superimposed to that 2D layer (Fig. 8). Since ArcScene identifies corresponding triangles in cloned meshes with the same code (in the attribute table), was then possible to transfer a value belonging to a triangle from the first model (horizontal) to the corresponding triangle in the second (vertical). What follows is the description of the procedure in detail: footprint representing the bi-dimensional area occupied by the horizontal multipatch of the facade was generated resulting into a polygonal shapefile. Every sub-element of the footprint was identified with a unique code identical to the one describing the corresponding triangle of the original vertical multipatch. Applying a polygon based ‘feature to point’ tool a centroid (point placed at the center of gravity) for each triangle of the footprint was defined as a means to collect values from 2D layers: vector shapefiles (spatial join) or raster datasets (values to points extraction). Two one-to-one relations were then established: a spatial join between centroids and footprint triangles as intersecting features and a table join between the polygons of the footprint and the corresponding polygons of the multipatch on the basis of a common field value (ObjectID). A fundamental advantage of this method is given by the possibility of updating attributes in real-time, so that by changing a value for a feature sub-element (triangle or selection set of triangles) in the 2D layers (footprint) would result in a change of the corresponding 3D multipatch sub-elements (Figs. 7 and 8). Accordingly, since centroids are directly related to the original 3D model (because of the chain 3D triangle-2D triangle-centroid), different algorithms originating different triangular configurations would lead to different centroids distribution. A clone of the layer pack composing the 2D–3D attribute chain (vertical multipatch, horizontal footprint layer, centroid layer) was created for each bi-dimensional thematic map (decay, risk map, lack of verticality). Transferred values were finally used as a base field to draw categories as colored symbology for the different 3D thematic layers (Figs. 9 and 10). 4. Results and conclusions Based on the assumption that the advantages coming from the application of GIS technologies to cultural heritage preservation management could be crucially improved by a three-dimensional approach, the aim of this study was to set up a pipeline to provide preservation specialists with a methodology for 3D implementation of thematic maps to be developed through the use of standardized software packages and without any support from IT specialists. So far, most of the research in this field of application has been characterized by the use of approaches where 3D models were mainly used as geometrical reference for the production of traditional bi-dimensional data or as containers of hyperlinks to planar drawings. According to the obtained results, a direct connection between 2D maps and 3D layers can be pursued into a GIS environment avoiding ad hoc software applications. The result is a novel
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system combining a complete three-dimensional replica of the monument and GIS tools, where every 3D graphic layer describing a specific topic (level of decay, geometry singularities, stresses analysis visual results coming from finite-elements software package, multispectral imaging, etc.) is linked to an alphanumeric information, and can be used as fully queryable feature to conduct advanced management operations. Among those is worth to mention the possibility of performing GIS analyses like: • assessing the overall preservation conditions of heritage structures (e.g. combining crack patterns and stress analysis visual results for failure mechanism identification); • highlighting critical areas of the asset affected by several issues by means of enhanced 3D vector operations (geometric intersections of selected features). The present findings, although having important implications for preservation managers and professionals of the Italian Superintendency of Pompeii, might be virtually extended to any kind of heritage building or monument. The results of this work suggest that in the future 3D GIS could become the standard tool for crossing-board management of the entire cultural heritage preservation process, especially for the development of automated or semi-automated predicting models for damage scenarios. Acknowledgements This research activity was funded by C.M. Lerici Foundation, by the Birgit och Sven Håkan Ohlssons Foundation and by the Swedish Research Council Grant (340-2012-5751) Archaeological information in the digital society (ARKDIS). References [1] ICOMOS, International Charter for the Conservation and Restoration of Monuments and Sites, Decision and Resolutions, Venice 31.V, ICOMOS, I, Paris, 1964 (Retrieved from 1966. . . http://www.icomos.org/charters/venice e.pdf). [2] G. Carbonara, An Italian contribution to architectural restoration, Front. Archit. Res. 1 (1) (2012) 2–9. ˜ [3] M.A. Núnez Andrés, F. Pozuelo, Evolution of the architectural and heritage representation, Landscape Urban Plan. 91 (2) (2009) 105–112. [4] S. Janvier-Badosa, C. Stefani, X. Brunetaud, K. Beck, L. De Luca, M. Al-Mukhtar, Documentation and analysis of 3D mappings for monument diagnosis, in: L. Toniolo, M. Boriani, G. Guidi (Eds.), Built Heritage: Monitoring Conservation Management SE - 29, Springer International Publishing, Switzerland, 2015, pp. 347–357. [5] L. Barazzetti, L. Binda, M. Scaioni, P. Taranto, Photogrammetric survey of complex geometries with low-cost software: application to the ‘G1’ temple in Myson, Vietnam, J. Cult. Herit. 12 (3) (2011) 253–262. [6] M. Aldenderfer, Introduction, in: M. Aldenderfer, H. Maschner (Eds.), Anthropology, Space, and Geographic Information Systems, Oxford University Press, New York, 1996, pp. 3–18. [7] R. Opitz, J. Nowlin, Photogrammetric modeling + GIS. Better methods for working with mesh data, ArcUser 57 (2012) 46–49. [8] N. Dell’ Unto, D. Ferdani, A.-M. Leander Touati, M. Dellepiane, M. Callieri, S. Lindgren, Digital reconstruction and visualization in archaeology. Case study drawn from the work of the Swedish Pompeii Project. In: Addison, A., Guidi, G., De Luca, L. and Pescarin, S. (eds.), in: Digital Heritage International Congress, 2013, pp. 621–628. [9] J. De Reu, G. Plets, G. Verhoeven, P. De Smedt, M. Bats, B. Cherretté, W. De Maeyer, J. Deconynck, D. Herremans, P. Laloo, M. Van Meirvenne, W. De Clercq, Towards a three-dimensional cost effective registration of the archaeological heritage, J. Archaeol. Sci. 40 (2) (2013) 1108–1121. [10] N. Dell’Unto, The use of 3D models for intra-site investigation in archaeology, in: S. Campana, F. Remondino (Eds.), 3D Surveying and Modeling in Archaeology and Cultural Heritage. Theory and Best Practices, BAR international series, Oxford, 2014, pp. 151–158. [11] K.M.S. Allen, S.W. Green, E.B.R. Zubrow (Eds.), Interpreting Space: GIS and Archaeology, Taylor and Francis, London, 1990. [12] G. Lock, Z. Stancic (Eds.), The Impact of GIS on Archaeology: An European Perspective, Taylor and Francis, New York, 1995. [13] D. Wheatley, M. Gillings, Spatial Technology and Archaeology: The Archaeological Applications of GIS, Taylor and Francis, London, 2002.
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Original article
3D GIS for cultural heritage restoration: A ‘white box’ workflow Danilo Marco Campanaro ∗ , Giacomo Landeschi 1 , Nicoló Dell’Unto 2 , Anne-Marie Leander Touati 3 Department of Archaeology and Ancient History, Lund University, Helgonavägen 3 Box 192, 221 00 Lund, Sweden
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Article history: Received 13 March 2015 Accepted 15 September 2015 Available online xxx Keywords: Pompeii 3D GIS Digital archaeology Cultural heritage conservation
a b s t r a c t Structures of architectural heritage are constantly exposed to natural and human-made threats that can compromise their cultural and artistic values. This is the case of the Roman city of Pompeii, whose preserving conditions nowadays are so critical to let a group of Unesco-appointed experts consider the possible inscription of the property on the list of “World Heritage in Danger”. In this respect, improving the effectiveness of preservation strategies becomes a crucial task. A great contribution in this direction is given by the combination of digital technologies such as laser scanning, photogrammetry and computer vision-based techniques and 3D geographic information systems (3D GIS), whose integrated use could exponentially increase the effectiveness of conservation strategies of ancient buildings. This paper presents the results of a research developed as part of the Swedish Pompeii Project, a fieldwork initiated from the Swedish Institute in Rome in 2000. Main objectives of this research were (i) to develop a set of integrated digital methods to be extensively adopted by conservation specialists in the practice of preservation management; (ii) to deal with several aspects connected to the preservation of an ancient structure in a ‘fully-3D’ environment; (iii) to take advantage of GIS analytic tools for investigating architectural structures in three-dimensions. © 2015 Elsevier Masson SAS. All rights reserved.
1. Introduction and aims Strategies of documentation have always been object of special concern for preservation professionals, being universally considered a milestone in the restoration process, as stated by restoration charters [1]. The reason for this urge can be easily found in the very nature of the documentation itself as act of analytic in-deepening. Every work of restoration, in fact, is based on an earlier and detailed study whose strength is univocally connected to documentation (e.g. of geometries, levels of decay, crack patterns, materials characterization). The result of this cross-disciplinary process, in the framework of a job of restoration, overcomes the limits of a mere depiction of the actual state of a monument, acting more as prediagnostic/monitoring tool, eventually standing as the key element for an exhaustive investigation of the existing heritage [2].
∗ Corresponding author. Tel.: +393470000901; fax: +46 (0)46 222 42 14. E-mail addresses: danilo [email protected] (D.M. Campanaro), [email protected] (G. Landeschi), nicolo.dell [email protected] (N. Dell’Unto), anne-marie.leander [email protected] (A.-M. Leander Touati). 1 Fax: +46 (0)46 222 42 14. 2 Tel.: +46 (0)46 222 31 86; fax: +46 (0)46 222 42 14. 3 Tel.: +46 (0)46 222 79 34; fax: +46 (0)46 222 42 14.
Concurrently, due to the constant development undergone by digital technologies in the last decades, traditional bi-dimensional way of documenting appeared to be almost inadequate [3]: new data acquisition and processing systems (laser scanner, photogrammetry, GPS) have changed the concept itself of surveying; 3D digital graphics now allow to create and manage photorealistic models of the existing heritage while geographic information systems (GIS), linking information to graphic representation, offer enhanced tools to perform analysis and archive data. Aim of the present research, which has been developed in the framework of the Swedish Pompeii Project (http://www. pompejiprojektet.se/index.php), was: • to set up a novel three-dimensional system for the management of heritage structures by exploiting the combination of 3D visualization and geographic information systems (GIS) analysis; • to provide conservation specialists with an effective work pipeline for managing the architectural degradation through the use of standard software packages. In brief, the obtained results can dramatically contribute to the creation and management of fully usable 3D GIS thematic layers to record and analyse specific aspect related to the state of preservation of a historical building. As a case study, the north part of
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Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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the double atrium house of Caecilius Iucundus (North House of Caecilius Iucundus) in Pompeii has been investigated.
2. Previous work The diffusion of digital technologies has strongly affected the way researchers approach and manage the conservation of ancient buildings. Digital instruments are used in risk management at any level, and their application enhances the possibilities to detect, monitor and possibly restore ancient structures [3–5]. The improvement of site preservation management by means of digital technologies, as instruments to prevent and reduce damages, becomes an essential step to define time and cost effective preservation strategies. A great contribution in this respect has been provided by the introduction of GIS, a technology designed to store, manipulate, analyse and visualize database elements associated with geometric features [6]. The use of GIS for cultural heritage management purposes has grown considerably in the last decades, becoming a very widespread toolkit among preservation specialists. On the other hand, the theoretical debate has defined GIS as the most “natural” visualization platform to connect all different types of data [7–10] as well as one of the most influential technologies to manage and analyse information connected with cultural heritage in general [11–15]. Due to the very spatial nature of the discipline, this technology rapidly opened up new possibilities also in cultural heritage management appearing the most effective environment where to perform both technical and historical-critical assessment and where to analyse and control the entire conservation process in a more diagnostic sense. Furthermore, the evolution of visual technologies in the last two decades led to the introduction of 3D digitizing techniques both as an instrument to educate and engage the public about the past and as a means to study and analyse historical contexts [16,17]. This three-dimensional turn has strongly involved GIS technologies raising up several methodological issues and new potential outcomes in the field of heritage preservation [18,19]. So far, despite the growing body of literature related to this special issue, research on 3D techniques in preservation management has been mostly oriented towards data visualization rather than data analysis. In this respect, novel approaches for using 3D GIS system applied to restoration will be presented here, focusing on the case study of the North House of Caecilius Iucundus in Pompeii. Since a close examination of the general use and relevance of 3D GIS is out of the scope of this paper, being extensively discussed in literature [15,20–22], only those experiences strictly dealing with the use of 3D GIS for preservation purposes and thematic maps management (masonry analysis, architectural stratigraphic units description, decay, weathering forms and cracks patterns assessment, risk evaluation, etc.) will be reviewed. At an early stage, the use of GIS tools to address conservation issues has been mainly based on a bi-dimensional approach [23,24]. Canonical forms of graphic representation (plans, sections, facades) were used as a base to create vectorial maps linked to a database in a kind of vertical use of this technology. This solution, despite the undeniable advantage of making GIS-based analysis tools available to the architectural scale, has shown its limitations especially when dealing with complex geometries, a condition that in the field of cultural heritage tends to be the rule rather than the exception. Some experiments have been carried out introducing 3D models, generated by means of solid modelling techniques or by direct triangulation of point-cloud data coming from advanced 3D acquisition systems, such as laser scanners or photogrammetry. Thematic maps describing different aspects of the building were linked, as a part of the database, to the models. In this case, despite
the use of a three-dimensional approach to the problem, the maps were still bi-dimensional and connected by an external link using models as ‘visualization container’ rather than acting as a 3D feature themselves [25–30]. To overcome these limitations, dedicated platforms have been designed by IT specialists so as to combine traditional bidimensional graphical information and 3D models for preservation. In the case of ad hoc conservation module [31], a plugin has been developed as part of a proprietary software (ad hoc 3D) operating on point-cloud models enabling users to draw map patterns of alteration and decay on ‘3D ortho-images’ (planar images that can give information about point coordinates) and link them to the model; a more relevant effort has been put in the frame of NUBES project [4,32,33] where a web-based platform was used to spatially associate data with a 3D model of any 3D structure. In this case, the mathematical replica of the ancient building was split according to architectural criteria into several sub-elements (spans and pillars for instance). Layer-like annotations, drawn on textures by means of a Javascript-driven Scalable Vector Graphics (SVG) editor were projected on the 3D model. Despite their potentials, these last two examples have shown some limitations, requiring a more robust IT contribution for core part development and being confined to buyer-seller (ad hoc 3D) or strictly academic (NUBES) based linkage. Furthermore, some authors have stressed the necessity of adding a three-dimensional component to traditional graphical documents, combining 3D information and GIS. In his seminal work on the digital 3D survey in archaeology [34], Fiorini et al. described new methods to obtain extremely accurate 3D graphical representation of different archaeological contexts of data, usually ignored by monoscopic photogrammetry and normal analytical documentation (plants, sections and elevations). In this case, despite the advanced findings raised up, some limitations were still present: thematic maps (stratigraphic surfaces) were developed as vector representation of the bounding edges without recording irregularities or local curvature; 3D maps were drawn by means of software outside the GIS environment; 3D GIS technologies were used for communication and visualization purposes only. To overcome these limitations, our work was aimed at defining a new methodology, to be used by conservation specialists with basic GIS knowledge, in order to manage several aspects connected with the preservation of an ancient structure directly in three-dimensions. Such a methodological workflow would provide specialists with a simulation system where 3D models are not mere ‘container’ of hyperlinks to 2D drawings but fully queryable GIS feature dataset. In particular, the proposed method would allow: • to perform different typologies of analysis for studying and assessing the structure degradation; • to build statistics capable to provide a more clear view of the temporal trends and the decay progress of the structures; • to generate new thematic maps (risk maps); • to make diagnoses (e.g. putting together crack patterns with masonry singularities to evaluate the weakness of a structure); • to define and visualize conservation actions and treatments. To define this pipeline a ‘white box’ principle was followed: the term, borrowed from engineering science, is used here to define a system where all the information regarding an heritage asset can be managed by culture heritage specialists using well-known GIS software. Such a definition is clearly inspired by the ‘black box’, a platform requiring a major input from IT professionals that can be only seen in terms of input and output without any knowledge of its internal working.
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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2.1. Pompeii – a 3D revival During autumn 2000, the Swedish Institute in Rome started a field documentation campaign in Insula V 1 in Pompeii with the goal to revisit a Pompeian city-bloc, unearthed mainly in the 1870s, for thorough documentation and study. The choice of a full cityblock as study object was made in order to allow an assessment of a whole social context of ancient city life, not just that pertaining to single large houses. Annual campaigns with the duration of six or twelve weeks each were carried out on site up to 2011. Initially, work was carried out according to traditional building archaeological principles, based on close study of walls, drawings and field photography. The area was divided into four parts, investigated in parallel by four teams. In order to obtain a homogeneous documentation a professional photographer with main task to document all walls, has worked aside the archaeologists since 2006. To facilitate communication and assure access to data in all areas, a web-based presentation of digital photographical montages, orthomosaics, accompanied by written descriptions and commentaries was designed in 2007–2008 and put in open access (http://www. pompejiprojektet.se/insula.php). This digital research platform, characterized by its unique transparency, permitted the first assessment of the history of urbanization and social relations in Insula V 1 to appear only two years later [35]. In the meantime, different typologies of documentation technique were tried in search of a less time and cost demanding workflow. Applied since 2011, laser scanning has proved without comparison the most efficient way to proceed [36]. The implementation of a way to import the orthomosaics from the already existing 2D research platform to the meshed pointcloud opened the possibility to obtain a textured model, which in its turn was imported into a GIS system. Thereby, completely new possibilities of analysis opened for investigation. The above described workflow involving laser scanning and 3D modelling was achieved as the direction of the Swedish Pompeii Project moved in 2010 to Lund University and encountered the technical experience and networks at hand at the Institute of Archaeology and Ancient History. Fieldwork, post-processing and the elaboration of new means of visualization were carried out in collaboration with the Humanistic Laboratory and the Digital Archaeology Lab (DARK) at Lund University and the National Research Council of Italy (Institute of information Technology and science “A. Faedo”) in a conjoint project of digital acquisition of Insula V 1 by means of integrated spatial technologies, such as: phase shift laser scanners and image based 3D modelling techniques [36]. Aim of this work was to acquire a high-resolution model of the entire city-block in order to: • provide a more accurate and all compassing documentation of the structures that characterize the insula;
Fig. 1. This image shows the entire dataset of the insula V 1 Pompeii aligned and meshed.
• use the 3D dataset of information at different level of details to increase the possibilities to study the relations between the different areas that characterize the Pompeian domestic architecture (Fig. 1). The virtual structures so far developed have been used as geometrical reference for the interpretation of several parts of the city-block [37], and have been post-processed and used in relation to the documentation previously realized in the frame of the project. The material developed so far has been employed to design a new data model for the archaeological documentation of the insula V 1, which combine a 3D GIS platform with a large use of textured 3D models of the ancient structures previously acquired by means of laser scanner and image based 3D modelling techniques [18]. For the reason that the maximum transparency should include also the 3D data we experimented the use of WebGL, to visualize the gathered three-dimensional data directly through web browsers, in order to connect this new experimental approach with the classic documentation disseminated during these years through the Internet. The development of such web access for the visualization of the 3D data would provide the opportunity to anyone interested in studying the insula V 1 to access directly the information elaborated by the project team. 3. Methodology In the frame of the Swedish Pompeii Project a geodatabase management system (GDBMS) was already set up with the purpose of providing archaeologists with an advanced instrument for data analysis and interpretation [18,19]. In the proposed case, considering the increasing interest among preservation specialists towards digital processes, a methodological pipeline was implemented in order to build a three-dimensional geographic database that could be used to manage all the thematic maps recording specific aspects
Fig. 2. This image summarizes the general workflow followed to build fully usable thematic maps in a 3D geographic information systems database. The original 3D model is split into facade elements. A horizontal copy of each facade is created and used as a base to draw 2D maps. A link between each 3D facade and its horizontal clone allow to pass attributes from 2D features to 3D multipatches in order to generate 3D thematic maps.
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Fig. 3. This image shows the ‘Masonry types’ thematic map (b) drawn using the georeferenced texture (a) as a base map. One of the pictures used to map the 3D model of the house of Caecilius Iucundus has been imported as a raster map in ArcMAP, georeferenced (a) using the horizontal copy of the facade as reference and used as a base to draw bi-dimensional thematic maps (b).
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Fig. 4. This image displays the horizontal clone and its reference plane (a) used as base for the elevation map (b) describing the lack of verticality [digital elevation modeling (DEM)]. Once created, the horizontal copy of the original mesh and the reference plane are translated to the origin of the local coordinate system and exported to ArcMAP to be used as a base to create a DEM.
of a heritage asset for preservation purposes. The main aim was to create 3D features that could be superimposed onto the existing textured model and used as fully queryable GIS features. Although the software used to carry out part of the proposed work (ArcScene, ESRI ArcGIS 3D module) allowed drawing 3D polygons directly using an existing model as a base by means of spatial snapping tools, an indirect method for the creation of 3D thematic maps was followed. In fact, drawing a 3D polygon
directly would have produced features matching with the base model only on external boundary (in correspondence of the snapping points) without recording any aspect of the inner complexity of the geometry. Furthermore, the proposed method allowed to overcome some limitations related to the application of GIS tools (typically conceived to operate from a top-down perspective) to three-dimensional geometries (see Section 3.3 about the use of traditional GIS tools to describe the geometry of a facade).
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Fig. 5. This image shows the assignment of the elevation map (a, 2) as texture for the 3D model (a, 3) of the facade using the same parameters used for the projection of the original textures – camera UVW projection – (a, 1) and its implementation inside a 3D geographic information systems environment (b).
A pipeline aimed at the creation of 3D features generated by inheritance of attributes from bi-dimensional maps was then proposed. To achieve this goal a set of strategies were defined stepping through the following actions: • setting up the data; • creating 2D thematic maps documenting the actual state of conservation of the historical building;
• transferring attributes from 2D to 3D layers; • generating 3D layers related to bi-dimensional maps.
The entire process was performed with ESRI ArcGIS software, a widespread GIS system provided with versatile modules for 2.5D (ArcMAP) and 3D (ArcSCENE) management, visualization and analysis of data. In particular, ArcMAP was adopted to create bidimensional maps, perform analysis and transfer values from 2D
Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006
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Fig. 6. This image shows one the potential outcomes of this study, the production of cross-sections (a) and their linkage inside to the model inside a 3D geographic information systems environment (b).
to 3D, while ArcSCENE was used to perform 3D data visualization and analysis. A minor recourse to a 3D modeling software was required in the first phase of this study to post-process point-cloud data and generate an accurate textured model of the building (Fig. 2). 3.1. Setting up the data Creating thematic maps is mainly a simplification process. Since each monument can be seen as a complex stratification of
heterogeneous information (geometrical, historical, artistic, structural, etc.), the main aim of creating a thematic map is to extract from this composite context only the data sought by each preservation professional to highlight a specific topic. This means that complex geometrical structures, split into bi-dimensional elements, turn into meaningful bases for the critical digging out of the specialists. The scope of this phase was thus to divide the original 3D model into approximately plane parts (facades), create an horizontal clone for each of them to use as a base to draw thematic map and define a plane to be used as a reference system to perform
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Fig. 7. This image shows the features composing the 2D to 3D value chain: multipatch (a–b, 1); footprint layer (a–b, 2); centroid layer (a–b, 3).
Fig. 8. This image summarize the workflow used to pass values from a 2D feature (1) to a 3D multipatch (4) through centroid elements (2) and footprint (3) with correspondent object identification (ID). The process is based on the correspondence between the elements (ID) of the clones of the same facade. The horizontal clone (3) is used to develop 2D maps (1) and receive the related attributes (2). This information are then transferred to the vertical clone (4) using the mentioned correspondence.
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Fig. 9. This picture shows three-dimensional features recording some aspects related to the state of conservation of the facade. In (a), each 3D feature describes a specific issue of decay and is linked to an attribute table reporting relevant information (e.g. the length/width of crack patterns, treatments needed to mitigate each issue). In (b), 3D features describe the lack of verticality of the facade. Graphs reporting data trends of an issue (e.g. lack of verticality progression), relationships, distributions and proportions can be easily produced using standard graph tools.
bi-dimensional analysis and easily switch between space dimensions (2D and 3D). As previously mentioned, the proposed work is focused on the case study of the North House of Caecilius Iucundus. The 3D model of the building was originally split into architectural subelements (facades) in order to add color information projecting
ortho-images by means of planar mapping techniques (texture mapping). Exploiting this partition thus appeared to be the most effective choice for the purpose of this research. A reference plane was assigned to each facade as an ideal approximation of its surface and the faces of the mesh were exploded into single objects to make them readable by GIS system as parts of the same feature.
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Fig. 10. This image shows three-dimensional features describing the various levels of overall risk associated to the specific facade. A table describing the rating category and the matrix used to assign the level of risk is linked as a raster to each specific colored area.
A horizontal clone of the facade was then created using the associated plane as a reference combination (plane/facade) for spatial transformations (roto-translation). Both horizontal and vertical meshes were exported as virtual reality modeling language (VRML) file to have faces recognized as subsets belonging to the same multipatch element. Moreover in order to get feasible results in ArcMAP, the horizontal reference plane (and accordingly the associated facade), was centered at coordinates (0,0,0), while the vertical facade was imported and placed in its correct location in ArcSCENE. The described steps were all developed using commercially available software (Autodesk 3D Studio Max) for 3D modeling (Fig. 4a). 3.2. 2D Thematic maps creation Thematic maps were generated in ArcMAP using different GIS tools depending on the specific topic to be highlighted. Vector data were used to create and edit decay maps and masonry stratification, raster maps to generate and algebraically add risks maps, digital elevation modeling (DEM) tools to assess facades tilting. In specific, deterioration issues were visually identified on an orthoimage (already used to texture the model) georeferenced to the horizontal facade (Fig. 3a) and drawn using editing vector features (Fig. 3b). A ‘type’ field was then assigned in the attribute table connected to each feature to identify single issues, based on the glossary of the International Scientific Committee for Stone [38]. Different levels of risk, calculated as product of the probability for a certain damage to occur multiplied by the severity of the damage, were identified for each feature and converted into raster dataset using the risk level of the original feature as an input field. Since an in-depth investigation of the risk prediction model was out of the scope of this research, simple qualitative considerations were adopted to define the above-mentioned matrix (e.g. missing
mortar was considered a less risky factor compared to a group of cracks that are more likely to severely endanger the monument).
3.3. New outcomes using traditional GIS tools Geometries of the past are seldom perfectly stereometric because of their original construction flaws or preservation conditions, or because of some extrinsic factors like earthquakes or differential subsidence. Resulting permanent deformations (e.g. bulges and leanings) usually mark a damage, a condition that can lead to failure and collapse [39]. A crucial task for conservation specialists is pointing out portions of an asset most suffering from issues and cross-checking these information with other data in order to define strategies and prioritize treatments. To assess the regularity of a facade traditional landscape analysis [digital elevation model (DEM)] tools were chosen. As for other procedures described in this paper, the starting point for this analysis was an horizontal clone of the facade, here considered in the same way of a raised formation and a reference plane used to place the model in ArcMap and set the zero elevation. A raster map was thus created in ArcGIS storing the elevation (Z) values of the facade element (multipatch to raster tool) (Fig. 4). The result of this analysis was a false color map describing the actual deformations and leanings of the facade by assessing points shifting from an ideal XY plane (reference plane). The proposed method could allow specialists: • to easily visualize local deformations of the entire model both qualitatively and quantitatively replacing mapping textures with leanings maps (Fig. 5) or creating 3D thematic layer by means of the procedure described in the following paragraph (Fig. 9b); • to retrieve detailed information about the geometry of the facade using typical DEM instruments like cross-section graphs (profile graph) (Fig. 6).
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Based on the above-mentioned procedure, a risk raster map interpreting the hazards related to the deformations of the facade was created using the DEM representation as a base and algebraically added to other raster dataset to provide an overall map. 3.4. Transferring attributes from 2D to 3D layers A fundamental step of this research was to pass information from bi-dimensional to three-dimensional layers in a GIS environment. To address this issue a 2D to 3D attribute chain was developed setting up a ‘centroid method’. The basic principle of this pipeline was transferring a value from a traditional 2D layer (raster or vector) to the corresponding mesh element (triangle) belonging to the horizontal copy of the facade superimposed to that 2D layer (Fig. 8). Since ArcScene identifies corresponding triangles in cloned meshes with the same code (in the attribute table), was then possible to transfer a value belonging to a triangle from the first model (horizontal) to the corresponding triangle in the second (vertical). What follows is the description of the procedure in detail: footprint representing the bi-dimensional area occupied by the horizontal multipatch of the facade was generated resulting into a polygonal shapefile. Every sub-element of the footprint was identified with a unique code identical to the one describing the corresponding triangle of the original vertical multipatch. Applying a polygon based ‘feature to point’ tool a centroid (point placed at the center of gravity) for each triangle of the footprint was defined as a means to collect values from 2D layers: vector shapefiles (spatial join) or raster datasets (values to points extraction). Two one-to-one relations were then established: a spatial join between centroids and footprint triangles as intersecting features and a table join between the polygons of the footprint and the corresponding polygons of the multipatch on the basis of a common field value (ObjectID). A fundamental advantage of this method is given by the possibility of updating attributes in real-time, so that by changing a value for a feature sub-element (triangle or selection set of triangles) in the 2D layers (footprint) would result in a change of the corresponding 3D multipatch sub-elements (Figs. 7 and 8). Accordingly, since centroids are directly related to the original 3D model (because of the chain 3D triangle-2D triangle-centroid), different algorithms originating different triangular configurations would lead to different centroids distribution. A clone of the layer pack composing the 2D–3D attribute chain (vertical multipatch, horizontal footprint layer, centroid layer) was created for each bi-dimensional thematic map (decay, risk map, lack of verticality). Transferred values were finally used as a base field to draw categories as colored symbology for the different 3D thematic layers (Figs. 9 and 10). 4. Results and conclusions Based on the assumption that the advantages coming from the application of GIS technologies to cultural heritage preservation management could be crucially improved by a three-dimensional approach, the aim of this study was to set up a pipeline to provide preservation specialists with a methodology for 3D implementation of thematic maps to be developed through the use of standardized software packages and without any support from IT specialists. So far, most of the research in this field of application has been characterized by the use of approaches where 3D models were mainly used as geometrical reference for the production of traditional bi-dimensional data or as containers of hyperlinks to planar drawings. According to the obtained results, a direct connection between 2D maps and 3D layers can be pursued into a GIS environment avoiding ad hoc software applications. The result is a novel
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system combining a complete three-dimensional replica of the monument and GIS tools, where every 3D graphic layer describing a specific topic (level of decay, geometry singularities, stresses analysis visual results coming from finite-elements software package, multispectral imaging, etc.) is linked to an alphanumeric information, and can be used as fully queryable feature to conduct advanced management operations. Among those is worth to mention the possibility of performing GIS analyses like: • assessing the overall preservation conditions of heritage structures (e.g. combining crack patterns and stress analysis visual results for failure mechanism identification); • highlighting critical areas of the asset affected by several issues by means of enhanced 3D vector operations (geometric intersections of selected features). The present findings, although having important implications for preservation managers and professionals of the Italian Superintendency of Pompeii, might be virtually extended to any kind of heritage building or monument. The results of this work suggest that in the future 3D GIS could become the standard tool for crossing-board management of the entire cultural heritage preservation process, especially for the development of automated or semi-automated predicting models for damage scenarios. Acknowledgements This research activity was funded by C.M. Lerici Foundation, by the Birgit och Sven Håkan Ohlssons Foundation and by the Swedish Research Council Grant (340-2012-5751) Archaeological information in the digital society (ARKDIS). References [1] ICOMOS, International Charter for the Conservation and Restoration of Monuments and Sites, Decision and Resolutions, Venice 31.V, ICOMOS, I, Paris, 1964 (Retrieved from 1966. . . http://www.icomos.org/charters/venice e.pdf). [2] G. Carbonara, An Italian contribution to architectural restoration, Front. Archit. Res. 1 (1) (2012) 2–9. ˜ [3] M.A. Núnez Andrés, F. Pozuelo, Evolution of the architectural and heritage representation, Landscape Urban Plan. 91 (2) (2009) 105–112. [4] S. Janvier-Badosa, C. Stefani, X. Brunetaud, K. Beck, L. De Luca, M. Al-Mukhtar, Documentation and analysis of 3D mappings for monument diagnosis, in: L. Toniolo, M. Boriani, G. Guidi (Eds.), Built Heritage: Monitoring Conservation Management SE - 29, Springer International Publishing, Switzerland, 2015, pp. 347–357. [5] L. Barazzetti, L. Binda, M. Scaioni, P. Taranto, Photogrammetric survey of complex geometries with low-cost software: application to the ‘G1’ temple in Myson, Vietnam, J. Cult. Herit. 12 (3) (2011) 253–262. [6] M. Aldenderfer, Introduction, in: M. Aldenderfer, H. Maschner (Eds.), Anthropology, Space, and Geographic Information Systems, Oxford University Press, New York, 1996, pp. 3–18. [7] R. Opitz, J. Nowlin, Photogrammetric modeling + GIS. Better methods for working with mesh data, ArcUser 57 (2012) 46–49. [8] N. Dell’ Unto, D. Ferdani, A.-M. Leander Touati, M. Dellepiane, M. Callieri, S. Lindgren, Digital reconstruction and visualization in archaeology. Case study drawn from the work of the Swedish Pompeii Project. In: Addison, A., Guidi, G., De Luca, L. and Pescarin, S. (eds.), in: Digital Heritage International Congress, 2013, pp. 621–628. [9] J. De Reu, G. Plets, G. Verhoeven, P. De Smedt, M. Bats, B. Cherretté, W. De Maeyer, J. Deconynck, D. Herremans, P. Laloo, M. Van Meirvenne, W. De Clercq, Towards a three-dimensional cost effective registration of the archaeological heritage, J. Archaeol. Sci. 40 (2) (2013) 1108–1121. [10] N. Dell’Unto, The use of 3D models for intra-site investigation in archaeology, in: S. Campana, F. Remondino (Eds.), 3D Surveying and Modeling in Archaeology and Cultural Heritage. Theory and Best Practices, BAR international series, Oxford, 2014, pp. 151–158. [11] K.M.S. Allen, S.W. Green, E.B.R. Zubrow (Eds.), Interpreting Space: GIS and Archaeology, Taylor and Francis, London, 1990. [12] G. Lock, Z. Stancic (Eds.), The Impact of GIS on Archaeology: An European Perspective, Taylor and Francis, New York, 1995. [13] D. Wheatley, M. Gillings, Spatial Technology and Archaeology: The Archaeological Applications of GIS, Taylor and Francis, London, 2002.
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Please cite this article in press as: D.M. Campanaro, et al., 3D GIS for cultural heritage restoration: A ‘white box’ workflow, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.09.006