Historic building information modelling (HBIM)

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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Contents lists available at SciVerse ScienceDirect ISPRS Journal of Photogrammetry and Remote Sensing journal homepage: www.elsevier.com/locate/isprsjprs Historic Building Information Modelling – Adding intelligence to laser and image based surveys of European classical architecture Maurice Murphy a,⇑, Eugene McGovern a, Sara Pavia b a b Dublin Institute of Technology, Bolton Street Campus, D1, Ireland Trinity College, D2, Ireland a r t i c l e i n f o Article history: Available online 9 January 2013 Keywords: CAD Cultural heritage Modelling Architecture Building Software a b s t r a c t Historic Building Information Modelling (HBIM) is a novel prototype library of parametric objects, based on historic architectural data and a system of cross platform programmes for mapping parametric objects onto point cloud and image survey data. The HBIM process begins with remote collection of survey data using a terrestrial laser scanner combined with digital photo modelling. The next stage involves the design and construction of a parametric library of objects, which are based on the manuscripts ranging from Vitruvius to 18th century architectural pattern books. In building parametric objects, the problem of file format and exchange of data has been overcome within the BIM ArchiCAD software platform by using geometric descriptive language (GDL). The plotting of parametric objects onto the laser scan surveys as building components to create or form the entire building is the final stage in the reverse engineering process. The final HBIM product is the creation of full 3D models including detail behind the object’s surface concerning its methods of construction and material make-up. The resultant HBIM can automatically create cut sections, details and schedules in addition to the orthographic projections and 3D models (wire frame or textured) for both the analysis and conservation of historic objects, structures and environments. Ó 2012 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS) Published by Elsevier B.V. All rights reserved. 1. Introduction 1.1. Definition In this paper Historic Building Information Modelling (HBIM) (Murphy et al., 2009) is described beginning with a short review of literature concerning parametric modelling and Building Information Modelling (BIM). The methodology for constructing a library of interactive parametric objects based on historic architectural data is presented illustrating the sourcing and analysis of historic architectural data and how the parametric architectural elements are coded using geometric descriptive language GDL. The building of the library is followed by an example of mapping the interactive parametric objects onto the laser scan and image survey data, resulting in the automation of survey engineering drawings and schedules, demonstrating the complete HBIM process. In conclusion, the evaluation process, which is, now under-way is outlined; initial results indicate the potential for HBIM for the conservation of historic structures and environments. Historic Building Information Modelling (HBIM) is a novel solution whereby interactive parametric objects representing architectural elements are constructed from historic data, these elements (including detail behind the scan surface) are accurately mapped onto a point cloud or image based survey. The architectural elements are scripted using a geometric descriptive language (GDL). The design and detail for the parametric objects are based on architectural manuscripts ranging from Vitruvius to Palladio to the architectural pattern books of the 18th century. The architecture of the renaissance introduced and documented advanced scientific rules for the production of architectural elements, which support the design of parametric models. The use of historic data introduces the opportunity to develop detail behind the object’s surface concerning its methods of construction and material makeup In the final stage of the HBIM process, the prototype libraries of parametric objects (see sample of library in Fig. 11) are mapped onto the point cloud and image survey data using a system of cross software platform management. Full engineering drawings orthographic, sectional and 3D models can then be automatically produced from the Historic Building Information Model. ⇑ Corresponding author. E-mail addresses: [email protected] (M. Murphy), [email protected] (E. McGovern), [email protected] (S. Pavia). 0924-2716/$ - see front matter Ó 2012 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS) Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.isprsjprs.2012.11.006 Author's personal copy 90 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Fig. 1. Pattern book details. 1.2. An additional and new approach Historic Building Information Modelling has been described in previous work (Murphy et al., 2009) which concentrated on the identification of data collection using laser scanning and the processing of the scans in order to isolate and test the most suitable survey products for further modelling in HBIM. These were identified as segmented point cloud sections and ortho-images, used as frameworks for plotting parametric. The concept for designing library objects in GDL was introduced in addition to the design of a plotting procedure. The motivation for this work has evolved from attempts to automate conservation documentation in the form of engineering drawing and schedules from laser scan and image based surveys of historic structures. In the field of practice, particularly our work with historic structures in Ireland, it emerged that conservation experts found it difficult to use point cloud survey data as a basis for developing conservation documentation. With the result that much of the valuable research in the areas of remote sensing for architectural heritage was limited as a visualisation tool whereas its potential to automate documentation for the whole conservation cycle for historic structures is not yet realised. In this paper the following new and additional aspects are developed for modelling architectural heritage:  A historic framework for building a parametric library of architectural elements is proposed, through assessing the evolution of architectural manuscripts in order to map and identify significant rules that represent a wide range of classical buildings, and can be applied to computer modelling. Secondly the interpretation and understanding of these rules is essential and can be more easily adapted from the architectural pattern books which emerged after the renaissance and beginning of the enlightenment period of the 17th and 18th centuries, these patterns are interpreted for both geometric shapes and non uniform shapes.  The development of parametric and shape rules to re-produce the classical elements detailed in the pattern books using GDL is presented, these are illustrated in Figs. 2 and 3, and described in a sample code. The shape commands and new library of primitives allow for all configurations of the classical orders in relation to uniform geometry. Non-uniform and organic shapes are developed in GDL through a series of procedures attempting to maximise parametric content of the objects (see Figs. 4 and 5). These shapes are stored as individual parametric objects or combined to make larger objects in a library and when used in the HBIM platform can be varied and deformed to match their requirements. The library range and size is best illustrated by their use in models, this range is illustrated in Figs. 6 and 9–11.  The problems of plotting onto point cloud and image survey data is addressed and solutions are proposed and tested, the fact that Building Information Modelling generates its 3D models through plotting in 2D onto different planes, requires that survey data be segmented and processed in 2D in the BIM environment. This has been solved through a series of procedures, see Fig. 7. Secondly objects are re-generated and deformed through changing parameters and this is based on numeric data. To facilitate this, a photo scaling application, which is web-based, has been developed and is used for plotting and measuring distances and angular values using two-dimen- Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 91 Fig. 2. Parametric and shape rules. Fig. 3. Doric column. sional segmented data. This automates the production of numeric parametric data for revising and plotting the library objects onto the laser survey data.  Finally a design for end user scenario testing is proposed to assess the suitability of HBIM as a tool for the automation of engineering drawings for the conservation process. Author's personal copy 92 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Fig. 4. Ionic capital. Fig. 5. Corinthian column. The HBIM approach is new as most applications of Building Information Modelling are applied to designing new buildings and new innovations that are concentrated around plug-ins for energy, structural, economic analysis and scheduling of components as an addition to new architectural design. With the exception of (Fai et al., 2011), very little work has been done in relation to modelling historic buildings and also generating BIM models from laser scan survey data. Their work, concentrated on the problems associated with combining laser scanning and BIM through plotting generic library objects onto the laser scan-based survey in a BIM environment. This approach did not include the creation of parametric libraries or improved plotting of objects onto the scan surveys. The advantages of HBIM over other modelling approaches is that the end result provides automated documentation in the form Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Fig. 6. Example of architectural rules in façade plot. Fig. 7. Point cloud interrogation. 93 Author's personal copy 94 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Fig. 8. Plotting objects. of engineering drawings for precise conservation of architectural heritage. This is in contrast to highly sophisticated visualisation products developed from procedural and other parametric modelling approaches whereby the main product is a visualisation tool. HBIM differs from these approaches, as the product is the creation of full 3D models including detail behind the object’s surface concerning its methods of construction and material makeup. In addition 3D documentation is produced which includes orthographic projections, sections, details and schedules (energy, cost decay etc.), adding intelligence to point cloud data. Using historic data to re-create the past or to restore or conserve historic artefacts and buildings is common in the wider area of conservation (ICOMOS, 2011) and is a wide area of research. Within both research areas of procedural and parametric modelling of architectural heritage the use of architectural knowledge to inform the creation of models is now becoming a common part of a design approach (Chevrier et al., 2010, 2009; De Luca et al., 2011; Muller et al., 2006; Wonka et al., 2003). While these works inform the HBIM approach they differ in their analysis of historic architectural data. HBIM focuses on the emergence of architectural pattern books to define architectural rules and detail. In addition a narrative is presented, which defines the evolution and form of European classical architecture for computer-based modelling. The aim of producing conservation documentation as opposed to sophisticated visualisation models requires different levels of accuracy especially in the specification of construction detail behind the scan surface. Wider and deeper historic sources in addition to different software tools are therefore required. 2. Architectural 3D modelling – previous work 2.1. Architectural modelling using shape grammars In documenting the classical orders, renaissance architects formulated a language whereby the rules, which govern the distribu- tion and combination of parts, resulted in a grammar of ornament and composition. The elements (mouldings, profiles, symbols, etc.) become the architectural vocabulary; the whole composition relates to a linguistic structure, this linguistic analogy offers architecture a basis for analysis and understanding (Clarke and Crossley, 2000). More recently, linguistics is used for representation and semantics in the field of computing for procedural modelling of buildings and virtual environments. Shape grammars (Stiny and Gips, 1972), introduced in the 1970s is now commonly used for conceptualising and analysing architectural design for computer modelling. Buildings are based on different architectural styles and can be divided and represented by sets of basic shapes, these shapes are governed by replacement rules where a shape can be changed or replaced by transformations. Shape grammar therefore can recognise architectural styles and urban planning configurations. The introduction of procedural modelling, which is based on shape grammars, can automatically reconstruct and generate these styles and configurations in a virtual environment. In the case of the generation of 3D city models, maps and land water boundaries are used to generate roadways and streets and the geometry of buildings and their position (Parish and Müller, 2001). Procedural modelling can automatically generate virtual models of single buildings based on shape grammars. In the case of applying grammars to architectural styles new rules were devised to improve the automation of the virtual models, for example split rules, divide up architectural structures and elements into components, for example facades can be divided vertically into floors and horizontally into windows and their accompanying panels (Aliaga et al., 2007; Muller et al., 2006; Wonka et al., 2003). In the case of architectural heritage and archaeology the ‘‘Plastico di Roma antica’’, a large plaster-of-Paris model of imperial Rome (16  17 m) created in the last century was scanned and modelled as a mesh model. The model was then incorporated with other historic and specialist information and with rule-based generation the reconstruction of the ancient Rome was modelled, entitled ‘‘Rome Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 95 Fig. 9. HBIM including automated documentation. Reborn 1.0’’ (Guidi et al., 2007, 2008) and extended to the whole city in the following project ‘‘Rome Reborn 2.0’’ (Frischer et al., 2008). Shape grammar modelling contrasts with using an architectural language to build parametric objects, whereby these objects are then plotted onto laser/image surveys to build a model, the former is automatic and the later depends on human interaction combined with automation. 2.2. Parametric modelling onto point clouds The process of mapping vectors onto a 3D point cloud can be improved by automatically placing primitive 2D or 3D shapes onto the point cloud by locating/defining shapes on the point cloud as primitives. For example a primitive shape of a cylinder can be mapped onto the point cloud to represent a column, which is then textured from the associated image data (Abmayr et al., 2005). An improvement in mapping can be achieved by recognising that buildings are a set of elements, organised by spatial relationships determined by an architectural style or language. The architectural elements can be represented in libraries as parametric objects and mapped onto point cloud or image-based surveys (De Luca et al., 2007). In similar work (Deveau et al., 2005), primitive objects are mapped onto the scan and image data; these are detected through semi-automatic extraction of the objects where the object localisation is initialised by user interaction. This is then followed by fully automatic segmentation of both the image and 3D data where each object needs to be reconstructed from planar surfaces, general geo- metric primitives and generalised cylinders. When these models do not fit with the surface, triangulation is performed similar to laser data editing software (Deveau et al., 2005). A classical column is made up of a cylindrical shaft to one third of its height and a tapering shaft for the remainder of the shaft; this subtlety may not usually be detected by automatic or semi-automatic recognition of primitives. Parametric libraries of architectural elements or objects can be built with precision for mapping onto different survey data sets if they are based on architectural language and vocabularies. In their work (Chevrier et al., 2009), state, ‘‘only simple geometrical shapes are automatically adjusted to cloud points’’ and only visible parts of the objects can be modelled and rebuilt. Hidden parts are often predictable and can be created as parametric objects based on historic architectural data. In more recent work Chevrier et al., 2010, develop parametric components for 3D modelling of architectural elements, in a Maya Environment combined with a Graphical User Interface (GUI), they automatically construct 3D models of the objects based on point cloud and image survey data. In this study they concentrate on window openings, which they automatically generate as parametric models of walls and their openings, further parameters can be added based on historic and other survey data sources. Historic architectural and geometrical knowledge is essential in order to create architectural parametric objects or elements. De Luca et al. (2011, 2006, 2007) have developed a system for modelling and representing architectural heritage through a software platform, called NUBES. They propose a methodology for Author's personal copy 96 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 Fig. 10. From scan to HBIM to automated documentation. Fig. 11. Samples of object library. the semantic description of architectural elements based on historic architectural knowledge, which is used to construct a shape library and is organised spatially as completed structures within the NUBES framework. They establish their analysis of the classical language of architecture on historic architectural manuscripts, in their work they refer to three renaissance manuscripts; Alberti, Serlio and Palladio. They identify a number of concepts for analysing the geometrical character and make-up of classical buildings and for the reinstatement of the buildings’ shape within a virtual environment. As a first step in their modelling pipeline they iden- tify the dominant surface as the internal and external fabric or envelope of a structure, which is separated in the modelling pipeline from other parts of the artifact or structure. They describe architectural mouldings as the key building atoms of classical architecture, which make up and add complexity to the geometry of larger elements such as walls, columns and beams, windows doors. Their library of shapes is based on sets of geometric primitives, which form the architectural mouldings, which are described as basic elements (atoms). The fact that objects in classical architecture can be linked by intermediate shapes, which allow for tran- Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 sition between the atoms is essential in describing the behaviour of architectural elements within their spatial framework. In addition they identify the use of repetition of architectural elements (colonnades, façade symmetry and proportion) that make up the whole. They state that orthographic plans, elevations and surveys can also inform the creation of objects (De Luca et al., 2006, 2007) and compliment the knowledge taken from architectural manuscripts. In their most recent publications they outline a WEB based system for describing, analysing, documenting and sharing digital representations of heritage buildings using their improved methodologies (De Luca et al., 2011). 2.3. Parametric modelling – Building Information Modelling The basic parameters, which describe vector objects, are shape and volume and can be simply expressed as coordinate points and their orientation as an angular value within a 3D space. The specification for the materials and texture can accompany the numerical data. The 3D object as a parametric model can be edited to revise any or all of its parameters of construction, texture and orientation. Parametric CAD differs from generic 3D CAD, as parameters are assigned to an object prior to its use. For example, AutoCAD is an a C++ written object-oriented program, the objects which are used to create the lines, arcs, and dimensions that in turn create architectural elements are not parametric. These objects exist as graphic entities but they do not have intelligence (Ibrahim and Krawczyk, 2004; Ibrahim et al., 2003). Parametric modelling can be described as systems which solve object constraints by applying sequential commands to model variables such as geometry, shape, surface texture or feature (Shah and Mäntylä, 1995). Architectural elements are represented as real world entities by capturing their characteristics, function and performance under different conditions. The parametric objects can be adaptive to wider architectural scenarios reducing their level of detail or alternatively capture specific knowledge reducing their wider use (Garba and Hassanain, 2004). Other strategic evolutionary stages of 3D CAD are Boundary Representation (B-rep) and Constructive Solid Geometry (CSG) these were both developed in the 1970s and 1980s; Boundary Representation (B-rep) provides details of an object’s shape by describing the object’s faces, edges and vertices and their relationships. Constructive Solid Geometry (CSG) represents objects using primitive shapes and subsequently combines these in 3D spaces using Boolean operations to create additional objects. These are low-level operations and intangible in terms of a designer’s requirements, a further evolutionary stage introduced the concept of describing the features and characteristics of an object. Feature based CAD can refer to geometry, specification of materials etc., in addition to function which describes the objects role (e.g. wall, door, window, etc.) and performance which indicates how elements relate to each other; the window can cut an opening in a wall (Gross, 2001; Van Leeuwen, 1999). Feature based CAD enables the modification and variation of parameters by the user, the incorporation of object features (such as openings in elements) and interactions between elements within a spatial environment (Van Leeuwen et al., 1996). This is now described as Building Information Modelling (BIM) and differentiates itself as an object intelligent architectural CAD tool rather than a drafting tool. BIM can be described as the assembling of parametric objects within a virtual environment, these objects which represent building components are then used to create or form an entire building. The parametric building objects are not defined singularly but as systems using interaction with other objects and their own values (shape, texture, etc.) within a BIM. Objects are described according to parameters some of which are user defined and others, which relate to position in a 3D environment relative to other shape objects (Eastman, 2007). The 97 visualisation of objects is achieved through viewing 2D and 3D features, plans, sections, elevations and 3D views. BIM can automatically create cut sections, details and schedules in addition to orthographic projections and 3D models (wire frame or textured and animated). BIM uses building semantics to represent buildings and their components in a virtual environment (Boeykens et al., 2008). The evolutionary stages of architectural CAD have moved from 2D graphic computer representation to parametric modelling to nD modelling (Tse et al., 2005) and onto feature extraction and finally more recently to Building Information Modelling. The leading BIM software platforms are Autodesk Revitt (Autodesk, 2011), GraphiSoft ArchiCAD (Graphisoft, 2011) and Bentley Architecture (Bentley, 2011). ArchiCAD is an architectural design application, built around the BIM concept as a standalone application. In ArchiCAD the modelling of objects can be achieved through using standard parametric construction elements. These elements are embedded in the software (such as walls, columns, beams, slabs, roofs, etc.) or created as new objects using the embedded scripting language geometric descriptive language GDL. The use of GDL allows for the creation of any number of rich parametric BIM objects and for their storage in internal libraries or data bases for further reuse or modification (Tse et al., 2005). Revit is also a BIM modelling platform, where the user constructs a mass model with a combination of solid forms and void forms. The faces of the mass volume can be turned into building elements and floors and other architectural elements can be generated inside the mass model (Boeykens et al., 2008). Bentley differs from Archi-CAD and Revit in that it exists as a plug-in for other Bentley platforms. 3. Building a library of parametric objects 3.1. Historic framework for building a parametric library of architectural elements Data concerning historic construction techniques and architectural details can be found in architectural manuscripts, which have evolved from Vitruvius to the 17th and 18th century Architectural Pattern Books. It is essential to identify the correct sources for describing the rules and cannons of classical architecture. Initially in this section, the evolution of these manuscripts is summarised chronologically in order to map and identify significant rules, which represent a wide range of classical buildings and can be applied to computer modelling. Secondly the interpretation and understanding of these rules is essential and can be more easily adapted from the architectural pattern books which emerged after the renaissance and beginning in the enlightenment period of the 17th and 18th centuries. The analysis for modelling of architecture is confined to the classical period in the 17th and 18th centuries in Ireland and Europe. The classical architecture of this period is based on ordered components, geometric proportion and a limited range of material and texture and is an ideal subject for the building of parametric components for virtual models. 3.2. Evolution of manuscripts and rules for classical architecture The most important classical source for architecture is the treatise ‘‘De architectura’’ by Vitruvius, his treatise was possibly written before 27 BC, and during the first century AD. The text survived in various manuscripts during the middle ages. Marcus Vitruvius Pollio was a Roman architect working in the reign of the Emperor Augustus. Vitruvius observed design and geometry in ancient architecture of Rome and Greece and documented the classical orders, proportions, methods of construction and materials. Classical architecture was revived during the renaissance, introducing new and more scientific rules for the interpretation of Roman and Greek Author's personal copy 98 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 buildings and also for the production of drawings and surveys. Alberti, published his work ‘‘De re aedificatoria’’ (On the Art of Building) in 1452. This was as an attempt to interpret the work of Vitruvius and to improve on its philosophical and intellectual content. There were no illustrations included in the original and it was written in Latin. At the same time, Marini, published his interpretations of Vitruvius, unlike Alberti it contained illustrations, presenting the laws of classical proportion. In 1537, Sebastiano Serlio published ‘‘Regole generali d’architettura’’ (General Rules of Architecture). In 1562 Vignola published his ‘‘Regola Delli Cinque Ordini D’architettura’’ (The Five Orders of Architecture) which was mainly illustration and lacked text, resulting in a more practical aid for building (Chitham, 2005; Evers and Thoenes, 2003; Jokilehto, 1986). While Andrea Palladio’s 1570 work ‘‘Quattro Libri dell’Architettura’’ (The Four Books of Architecture) also documented a succinct account of the rules of classical architecture, his treatise, set out full design for buildings (in plan, elevation and section), and influenced greatly architecture in Europe and later its colonies, (Pain, 1788; Palladio, 2000). 3.3. Adapting design rules from architectural pattern books In the 17th, 18th and 19th centuries, architectural pattern books were written and devised by both architects, builders and theoreticians and were widely available in Europe and its colonies. The rules of the Renaissance Architects were more comprehensively documented in the Vernacular Pattern books which are specific to European and colonial regions. Architectural pattern books are a record of local design. These books contained the historic construction techniques used in the 18th century such as geometry and principles of the external and internal structure and fabric construction; positioning of openings; proportional relationship of the building’s elements; and classical detailing (Langley, 1756; Pain, 1788). 3.4. Building a library part – classical orders Classical proportioning consists of a series of modular relationships, which are based on the diameter of the base of the column, which represents a single module. Vignola’s manuscripts, which concentrated on the five orders, introduced additional and more precise methods for setting up classical proportions. He did this by dividing the order into a ratio of the pedestal, column and entablature, see column one in Table 1. Using these ratios for the larger elements reduced the complexity of calculations that arose if the whole building was related to the diameter of the base of a column. The sub-relationships between the column and details such as mouldings were usually devised using 60 divisions of minutes, which represented the diameter of the base of the column as a single module in Table 1, the measurements and rules for laying out the five orders are interpreted from a more recent manuscript (Ware, 1903), the proportions for the component parts are expressed as fractions of the diameter of the base of the column (as opposed to minutes). The choice to establish the design of the parametric columns on Vignola’s proportions relates to the fact that his illustrations focused on the five orders introducing a clearer method for setting up the proportions (Evers and Thoenes, 2003; Vignola, 1596; Ware, 1903). Fig. 1, detail 1 is an illustration from Pain’s 18th century which gives a description of the geometry of the Doric column capital and the modular arrangement of the base of the column, dividing it into 60 units (Pain, 1788). In detail 2, from the 19th century; fractions are introduced as minutes (60 divisions); this approach is much simpler for calculations (Ware, 1903). In the 20th century publication, (Chitham, 2005) metric divisions were introduced for the main elements, using four scales A, B, C and D. Scale A represents the main elements (pedestal, column and entablature etc.) ascending and descending from the underside of the column plinth, but further subdivided into tenths of the column diameter. Scale B shows the proportions of the principal divisions and subdivisions of the order. Scale C shows the proportions of the minor subdivisions, and scale D repeats these in running or cumulative figures. 3.5. Building architectural elements using geometric descriptive language Geometric descriptive language (GDL) is an open script based language embedded in Graphisoft ArchiCAD. ArchiCAD software divides parametric objects into built construction elements (walls, columns, beams, etc.) and GDL objects. GDL provides access to modelling of objects through a BASIC like language; these objects are specifically constructed for one or many uses and carry the required parametric information for the object’s function. All GDL objects are created within a three dimensional space, this space is measured by the x, y, and z-axes, the origin of which is called the global origin (0, 0, 0). The global origin and local coordinate system prepares the position, orientation and scale of objects, marking positions of objects or shapes, which can be moved on the x, y, and z-axis. The local coordinate system can be moved and provides a reference to the current point of an object with reference to the global origin. Shapes are scripted, based on primitives that represent the simplest solid objects; these are the building blocks of GDL and culminate to create the more complex parts, which are stored in libraries. The primitives are stored in the computer memory in binary format, and the 3D engine generates them within 3D space. The primitives are made up of all the vertices of the object’s components, all the edges linking the vertices and all the surface polygons within the edges. The primitives are formed together in groups known as bodies; these bodies make up the 3D model. Table 1 Elements according to vignola. Type of order Entablature Capital Shaft Base Height of cornice Height of frieze Height of architrave Height of abacus Height of echinus Height of necking Astragal Height of shaft Upper diameter of shaft Lower diameter of shaft Height of torrus Height of plinth Tuscan Doric Ionic Corinthian Composite 3/4D 1/2D 1/2D 1/6D 1/6D 1/6D 6D 3/4D 3/4D 1/2D 1/6D 1/6D 1/6D 7D 7/8D 6/8D 5/8D 1/3D 3/4D 1/2D 1/2D 7/6D 3/4D 1/2D 1/2D 7/6D 8D 8 1/3 8 1/3 5/6D 5/6D 1/2D 5/6D 5/6D 1/2D 5/6D 5/6D 1/2D 5/6D 5/6D 1/2D 5/6D 5/6D 1/2D Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 For shapes that become more complicated and for transformations, which are more abstract, additional values are required in their definition, which may not be found in simple primitives. GDL also includes Boolean operations, Meshing, NURBS and shape commands for creating organic and non-uniform 3D shapes (Watson, 2009). 3.6. Developing parametric and shape rules using GDL Although mouldings can be represented through the combination of cylinders and deformed ellipses and spheres, it is better to combine these with lathed prisms and revolved poly-lines in order to represent mouldings more accurately. The 2D profile of the objects are represented on the z and x-axis and then revolved to required value. An example of shape and parametric rules are illustrated in Fig. 2, these shapes represent classical mouldings and are designed to exploit and maximise the full range of parameters, deformation of shapes and abstract transformations. The sample script below is designed to create a series of parametric shapes, which represent the classical mouldings and their deformations. The diameter of the base of the column is represented by the cylinder radius (r1) allowing the moulding to deform in proportion to the column and other elements. The radius of the moulding profile (r2) can vary depending on the profile geometry also negative values can be inserted to change from convex to concave profiles. Numeric masking values are attached in the code to establish the construction of the curve in the profile, these can be radius, tangent or point and angle based syntax (Capo, 2006). !!! MOULDING GEOMETRY MATERIAL ‘‘Paint 02’’ HOTSPOT 0,0,0 variables r1= 1 !!!Cylind_rad r2 = .25 !!!curve profile a = 180 deg !!! profile angle !!! Transformations ROTy-90 REVOLVE 3, 360, 3, X coordinate Y coordinate Masking values 0 0 0 r1 a 1 1 2001 r2 !!! Masking values define visibility of edges and appropriate shape status for curves A Doric column is represented in Fig. 3, using coordinate transformations the primitives are stacked on the Z-axis or alternatively moved on the x and y-axis to form the column. The block represents the base of the column a cylinder and a cone, are added on the zaxis to represent the column shaft (the cone represents the tapering of the column one third up the shaft). The height and width of each element is represented by a variable expressed in terms of the objects height, width or diameter, for example cone_ht = the height of the cone and cone_rad1 = the first radius of the cone, which represents the upper shaft of the column, these primitives are combined with the mouldings in Fig. 2. The variables used to represent each value for the primitives are expressed in terms of the base diameter of the column, for example the cone_rad1 is equal to the half the diameter of the column base. A series of conical shapes can be used to represent further deformations in the column shaft. 99 The Doric Entablature (see Fig. 3a) consisting of decorated cornices, friezes and architraves are developed using similar mouldings and their profiles are achieved using 2D prism shapes, which are given depth or sent on linear paths. Mouldings whether used in columns or architraves are based on 2D profiles represented by the width and height, all of which can be expressed as variables in terms of the diameter of the base of the column (see Fig. 3c). Both primitives and mouldings are combined into a compound object (Fig. 3a), in this case a Doric column and entablature, additional transformations can re-scale the subsequent whole or parts of shapes or rotate the object around any of its axis. Non-geometric parameters such as texture, pen and fill are introduced to replace fixed values, making the object more flexible. These variables are accessible from the library parts settings dialog box within the software platform. When the object is placed, the variables and parameters can be changed to match the object it represents in real world terms; other common parameters are formation level and rotational transformations. The component or objects are placed in libraries or databases; the use of flow control, macros, subroutines and loops can re-introduce these objects in repetition or revised state. 3.7. Decoration and non-uniform shapes Organic shapes such as the Corinthian and Ionic capitol require more complex design based on NURBS, meshing and Boolean operations. The scroll in Fig. 4 was formed using NURBS and depth was added using slab commands, the scroll is based on the pattern book (see Fig. 4a) (Chitham, 2005; Pain, 1788), which lays out a complicated geometry for establishing the scroll radii. The Acanthus leaf (from pattern book) in Fig. 5a (Langley, 1756; Ware, 1903), was formed using NURBS to build a 2D profile and meshing was then used to add the irregular depth of the leaf. Group and solid operations were used to create the bend in the leaf around the column and the bend at the top of the leaf (see Fig. 5c); the object is bent in two in two directions. The range of parameters attributed to the Acanthus leaf is limited to its bounding box i.e. height, width and depth and the radius of the bend around the column. Finally the leaf is repeated to match the columns diameter through a looping procedure (see Fig. 5d). Finally in Fig. 6, the rules of classical architecture are illustrated in an ensemble of the classical orders combined with the structural elements of walls, roofs, windows, domes and entrance doorways. This plot was developed in a HBIM environment using the developed library and based on reconstruction from original historic drawings. The plotting process is detailed in the next section of this paper. 4. Plotting parametric objects onto laser and image-based survey 4.1. Plotting vectors onto point cloud surveys The majority of current software platforms for creating engineering drawings from laser scan surveys are created by mapping vectors onto the point cloud or textured point cloud. This is a complex process as the data size of the point cloud is large and mapping in 3D space onto a point cloud is difficult because of point and edge detection and location. Segmentation of the point cloud is necessary in order to identify the correct surfaces for plotting of vectors. Vectors can then be plotted onto both the point cloud and the ortho-image; this process is largely manual, although additions to software platforms to automate the mapping process include profiling and the creation of paths. Author's personal copy 100 M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 4.2. Plotting parametric objects onto point cloud surveys Mapping parametric objects as opposed to vectors onto the point cloud can overcome the slow task of plotting and locating every vector onto the cloud surface. The use of parametric objects can also introduce the opportunity to develop detail behind the object’s surface concerning its methods of construction and material make-up. Within Historic Building Information Modelling, the library of parametric objects is designed as a plug-in for existing software platforms with the addition of a set of procedures and a framework for mapping these objects onto point clouds and image surveys. Before the parametric objects are plotted a range of software is required for processing the laser survey data prior to developing the data as a HBIM. The point cloud survey requires a series of pre and post processing stages, involving cleaning, sorting and combining of different sets of point cloud data, which is followed by surfacing and texturing. The initial processing stage, involves cleaning and removing erroneous data or artefacts followed by re-sampling and reducing the density of the data for overly dense point clouds. Registration is the combination of several point clouds taken from different observation points or the referencing of the scanned object in a global or project coordinate system. Polygonal surface meshing creates a surface on a point cloud; the created surface is made up of triangles connecting the data points into a consistent polygonal model. Following the creation of a triangular mesh the results are then textured from associated image information. The point cloud data can be considered as a skeletal framework, which is then mapped using parametric architectural elements to form the HBIM. Ortho-images and segmented point cloud sections and elevations are the initial data imported as image and geometric data for further processing within the HBIM platform. Ortho-images are photo realistic models containing width, breath and height of an object. The point cloud is segmented to supply floor plans, elevations and sectional cuts as a map for location of library objects. Further interrogation of the point cloud supplies numeric values for formation values (for z-axis location) and parametric values for the library objects themselves, these are recorded in data sheets. The dimensions and co-ordinates of openings and elements are calculated from the point cloud and orthoimage survey and transferred onto data sheets. A specialised WEB based ortho-photo scaling application has been developed as integral part HBIM to automatically supply the numeric and measurement data for adjusting the parameters and plotting the objects to establish the model, this is summarised in the following section. 4.3. Overview of photo scaling application for creating data sheets The photo scaling application is a web-based application used for plotting and measuring distances between two point and angular values using two-dimensional images (see Fig. 7a). The application is developed using Ruby on Rails and Javascript and is designed to be scalable allowing for further improvements in the future. The objective is to create an application that is easy to access and portable for use on any operating system and browser. Future work will include for working with three-dimensional data also extending the application to work on mobile devices. 4.4. Implementation and design Ruby on Rails is used on the server side for creating users, database storage and using special libraries called gems for uploading the images, for the image manipulation HTML, Javascript and JQuery are used. When a user uploads an ortho-image or segmented point cloud, it is displayed on screen with the same dimensions as the original image, this is important to maintain so as not to dis- tort the width or the height. Once the image is uploaded and displayed the user is asked to begin selecting two points. These initial are true coordinate values for these points based on survey data. Using javascript the position of the mouse is located in the window, and using JQuerys offset method the position of the image element is determined. To locate the point in the image the user is selecting, a HTML div which is over-layed on that particular point with the size of 1 pixel. Using Javascript in this manner, means that correct values will be determined regardless of the users screen resolution or for example if the browser is set a certain zoom level. The code sample below locates the current x and y position when the image is clicked at any point or location. $(‘img’).click(function(e) { var offset = $(this).offset(); alert(e.clientX – offset.left); alert(e.clientY – offset.top); }); The ortho-images and segmented point clouds are interrogated to automate linear and angular measurements which are contained in data sheets and used to establish the require parameters of library objects. These parameters are all numerical values and range from length, breath, height, formation level, diameters, radii and angular displacement position for the objects. 4.5. Adjusting parameters and plotting objects Before placing a construction element, or GDL object, in a HBIM, the default parameters can be edited, changing the parameters of shape, size or other properties to correspond with the survey data, the parameters for objects are calculated from data sheets. The objects are then positioned onto the segmented plan and orthographic image in elevation and adjusted in side elevation and section using segmented data for angular displacement (see Fig. 7b). All of these data sets represent the information for a particular plane on the x, y, and z-axis; this can therefore represent elevation, plan, or section of an object. The position of elements in the co-ordinate system relative to other objects is located by mapping directly onto segmented point cloud plan and section and into position in elevation onto the ortho-image. When a library part or parametric object is placed into the HBIM, it is placed as an icon in 2D in the floor plan position (separated by height or formation levels) and determined along the x, y-axis and in section and in elevation, on the z-axis (see Fig. 8a). The library objects are not plotted directly in 3D environments, with the result that the objects are not placed within the 3D point cloud. Upon completion of the mapping of objects in 2D the completed 3D model automates a full set of drawings and documentation and virtual nD models. In Fig. 8b, the sequence and process for mapping objects onto point clouds and image-based surveys of a façade is illustrated. 5. Creating engineering drawings from laser scan surveys Where conservation or restoration work is to be carried out on an object or structure, conventional orthographic or 3D survey engineering drawings are required. To a large extent current research concerning automated surveying systems for cultural heritage objects has concentrated on the identification of suitable hardware and software systems for the collection and processing of data, as a result, the output is the accurate 3D model mainly suitable for visualisation of a historic structure or artefact. The laser scan survey of the Cathedral of Saint Pierre de Beauvais carried out by the University of Columbia Robotics Lab (Allen et al., 2003) is an example of an accurate 3D record Author's personal copy M. Murphy et al. / ISPRS Journal of Photogrammetry and Remote Sensing 76 (2013) 89–102 of a historic structure. As part of the scan results elevation, plan and section are based on the point cloud colour intensity. If the scan results are used to represent conservation or re-construction detail, there are then two significant limitations with this data. Firstly, upon close inspection of the point cloud, edges and planes, which define the construction elements that make up the structure, are not easily identified. Secondly, the survey data details the outer fabric or the surface of the structure and does not contain detail behind the surface. Part of the laser scan results for Beauvais are available on CyArc digital archive (CyArc, 2007) which is a cultural heritage archive providing access to data created by laser scanning, digital modelling, and other 3D technologies. The production of engineering drawings from laser and image survey data can be described as a reverse engineering process; whereby an object’s physical dimensions, geometry, and material properties are captured to produce orthographic plans, elevations, sections and 3D models (Cheng and Jin, 2006). The objects in this case are historic structures brought through the design process in the opposite direction, revealing information about the original design and construction. In the final stage of the HBIM process full engineering drawings orthographic, sectional and 3D models can then be automatically produced from the Historic Building Information Model, this is illustrated in Fig. 9. 6. Conclusion 6.1. Incorporating international standards for recording architectural heritage Early research concerning accuracy of laser scanning and digital photo modelling concentrated on smaller cultural objects, which require very high scan resolution. This is best illustrated by Stanford University and the University of Washington (Levoy et al., 2000) in the digitising of the sculpting of the Renaissance artist Michelangelo. The triangulation scanner at a resolution of 1/4 mm captured detail of the geometry of the artist’s chisel marks. The current commercial recording systems which combine laser scanning with digital photo-modelling have been proven to meet with accuracy requirements for recording and surveying and modelling historic structures and artefacts (Beraldin et al., 1997; Bernardini and Rushmeier, 2002; Jacobs, 2000). 6.2. Evaluation of HBIM as a conservation documentation and recording tool International and national standards for producing conservation documentation other than remote sensing, for historic structures are detailed in international guidelines such as the Historic American Building Surveys (HABS) (National Park Service, 2005) and English Heritage Metric Survey Practice (Bryan and Blake, 2000). An evaluation process for HBIM is now under-way, it is intended that as part of this process, the final product which is the automated models, documents and engineering drawings will meet the international and national standards for surveying and recording of historic structures as outlined in English Heritage documentation and the Historic American Building Surveys. The accuracy of the HBIM depends on requirements such as level of detail and end user application. While the end user requirements can be identified using scenario testing, geometric accuracy can be evaluated using conventional approaches. The required quality in the detail of elements and the definition of shape and texture of scan data will depend on the accuracy required for each survey. A sample of live survey data was utilised as ground truths to evaluate the accuracy of the mapped parametric data both as individual models 101 and integrated models (El-Hakim et al., 2007). The accuracy of mapped detail behind the surface of the structure, which is based on historic detail, can be compared for accuracy with existing survey data of the surveyed structure where detail is available from previously carried out surveys and openings of building elements. In addition to testing for geometric accuracy, the prototype is now being evaluated through a number of end user conservation scenarios identified by an expert group working within conservation. The user group will evaluate the new methodology using simulated conservation scenarios. From this, a series of historic building case studies have been identified; a sample of these studies is illustrated in Fig. 10. 6.3. Historic Building Information Model HBIM In conclusion, the evaluation process, which is now under-way indicate the potential for HBIM for use in the conservation of historic structures and environments. Initial results from conducting the conservation scenarios and end user groups have established its relevance for producing engineering survey drawings for architectural heritage conservation and its ease and speed in use in comparison to plotting vectors onto laser surveys for the creation of engineering drawings. The main improvements identified relate to the accuracy and content of the library of parametric objects, which represent the architectural elements and also in coding non-uniform geometric shapes. There is a need to introduce improvements into the plotting and mapping stage through further improving the automation process for creating data sheets from point cloud data in order to define the parameters of the architectural objects prior to their mapping onto the laser scan survey. The range of library objects need to be expanded and existing objects improved in their parameters, a sample of library parts is detailed in Fig. 11. A new methodology for the HBIM for historic structures and environments is proposed, this process involves the following stages: collection and processing of laser/image survey data; identifying historic detail from architectural pattern books; building of parametric historic components/objects; correlation and mapping of parametric objects onto scan data and the final production of engineering survey drawings and documentation. 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