Stefano Brusaporci, Pamela Maiezza, Alessandra Tata
read A framework for architectural heritage HBIM semantization and development. Part 1
PART 2
3.1 Transparency and model reliability
With the spread of digital tools and applications, which have become increasingly accessible by everyone, the concept of transparency for modelling and visualization of architectural heritage plays a central role.
For transparency, essential references are The London Charter (2009) and the Principles of Seville (2012).
The London Charter defines “intellectual transparency” as “the provision of information, presented in any medium or format, to allow users to understand the nature and scope of “knowledge claim” made by a computer-based visualisation outcome” (p.12). According to Principles of Seville, “All computer-based visualisation must be essentially transparent, i.e. testable by other researchers or professionals, since the validity, and therefore the scope, of the conclusions produced by such visualisation will depend largely on the ability of others to confirm or refute the results obtained” (p. 8).
The principle of transparency has been developed in the field of archaeology, which immediately looked with interest at the potential offered by the digital reconstruction of artefacts no longer existing.
Transparency refers to the declaration of sources on which virtual reconstruction is based (survey data, documentary reference, etc.) and, therefore, of the reliability degree of the model itself.
It makes digital visualization testable by other professionals, who can thus confirm or deny the results obtained by other researchers.
The modeling of historical buildings for their documentation imposes specific reflections on the adherence of the model to the real object.
For transparency, an essential role is played by the paradata.
In accordance with the The London Charter, paradata are “Information about human processes of understanding and interpretation of data objects. Examples of paradata include descriptions stored within a structured dataset of how evidence was used to interpret an artefact, or a comment on methodological premises within a research publication. It is closely related, but somewhat different in emphasis, to “contextual metadata”, which tend to communicate interpretations of an artefact or collection, rather than the process through which one or more artefacts were processed or interpreted” (p.13).
The paradata constitute a kind of metadata useful for the philological reconstruction of the realization of the model. Bentkowska-Kafel, Denard, Baker (2012) highlight how “[…] the digital techniques – it is argued here – are only useful and valid if interpretative frameworks and processes are published” Hence the importance of paradata: “[…] the term borrowed from other disciplines that rely on recording information processes. Paradata document the process of interpretation so that the aims, contexts and reliability of visualization methods and their outcomes can be properly understood” (p.1).
As regards architectural heritage, a reflection on the theme of the trapsarence and model reliability is in Brusaporci (2017): “Considering digital heritage transparency, we could say that there are two main topics: Transparency of the model toward the existing physical reality; Transparency of virtual re- constructions, in particular when we don’t have an immediate physical reference. Focusing on digital heritage, we could indicate two kinds of paradata:
- Intrinsic Paradata: To declare choices about instruments and tools, applications, and their use, computing workflow, surveying pipeline and tools, 3D modeling and rendering, database modeling, etc. that is the architecture of the model and of the process. This paradata is similar to the traditional one in surveying process.
- Extrinsic Paradata: About the relation of the computer visualization with: Heritage’s nature and characteristics, Archival documents, Scholar’s experiences, skills, and decisions. The paradata has to describe the critical interpretation of sources by the scholar and, therefore, presets a degree of reliability.
The model is affected by: Kind of source; Source completeness; Source reliability; Level of interpretation of sources. Thus the model has a degree of Objectivity/Believability. The scholar(s) has to define critical degrees of reliability of the 3D model for the following items: Geometry; Location/Position; Date/Age; Colour/Texture; Material/Constructive system; Context (urban – rural – natural) /Landscape.
Obviously intrinsic and extrinsic paradata are related, and both
of them derives from scholar’s critical choices.” (pp.81-82). Since BIM is an information management system, it can easily provide paradigms relating to reliability.
In the case of BIM, the reliability of the model – essential metadata to define the informative framework for the construction of the model (Bianchini et al., 2017) – must
concern both the geometry and the information content connected to it (construction technology, historical phases, etc.).
Modelling in the BIM environment is based on the use of semantically recognized objects, which are enriched by information of various kinds (materials, technical features, costs, temporal phases, etc.)
The description of the architectural element also from the constructive point of view, as well as geometric, marks an important distinction from the use of BIM in new buildings.
In this last case, in fact, there is a progressive advancement of the project development which corresponds to a gradual increase in the definition of building components.
Step by step, the LoD are growing up to achieve a complete definition of the architectural and construction aspects of the building.
In HBIM, however, the object of study is the constructed building for which, even at the end of the analysis path, it is not said that a complete and uniform knowledge of the artefact is reached. Depending on the availability of information, in fact, parts of the building can be better defined and parts more unknown, with consequent non-homogeneous LoD.
In order to know the construction technology of a historic building, the references are archival documentation, diagnostic tests, building observation, comparison with buildings similar in age and characteristics, etc.
However, a complete knowledge of the components of a historic building, characterized by total reliability, can only be reached in the phase of construction site when, as in the case of the vaults consolidation, the element is uncovered.
The close connection between the representation of the building construction technology and the sources from which it is derived, leads to the problem of transparency and the consequent need to declare the level of reliability of the various modeled elements.
The reliability of the model in terms of geometric accuracy, on the other hand, is evaluated on the basis of the deviation between the model and the survey data (Fig.6).
Figure 6
In the case of unacceptable deviation values (in accordance with the desired LoD), the architectural elements are corrected by abandoning the ideal geometry and using the characteristics extracted from the survey.
For particularly irregular shapes, for which the Boolean operations are not sufficient, it may be appropriate to use NURBS (Non Uniform Rational B-Splines) which, then, taking advantage of interoperability between different formats, are imported in BIM environment and enriched by information.
Despite the possibility to parameterize these objects, it should be noted that the specificity of their shape, strictly tied to the characteristics of the historical object in question, hardly allows its reuse.
Consequently, the parameterization of these elements is limited to the attributes necessary for the case study.
4. CONCLUSIONS
The proposed study analyses advantages, limitations and problems in the use of BIM procedures for architectural heritage.
The peculiarities of architectural assets mean that the extension of the BIM methodology to them is not automatic, but it needs specific reflections aimed at declining this process to meet the historic buildings requirements.
Based on these considerations, the paper proposes a framework aimed at developing a BIM model of the architectural asset.
Taking into account the characteristics of the object of study and the goals pursued, the survey of the historical building is realized, from whose definition the accuracy of the model depends.
The proposed methodology provides for three different Levels of Development.
As the Lod grows, there is a progressive increase in information and geometric content: from a simplified model (LoD 200), a detailed BIM model is obtained from both an architectural and structural point of view (LoD 400), passing through an intermediate level between the two (LoD 300).
The difficulty in reconciling the complexity of the components of historical architecture (for example a vault with its articulated stratigraphy), with BIM objects also characterized by structural value, has been overcome by superimposing two different types of models: a model in-place that, according to LoD, is as close as possible to the real object from an architectural point of view (both for its single parts and for its geometric complexity), and a model with a structural value, which is superimposed on the first.
The geometric definition of the model is evaluated by measuring its deviation from the point cloud.
If this is not compatible with the desired LoD, the architectural element is re-modelled by abandoning the ideal geometries and, if necessary, using NURBS surfaces imported and parameterized.
A Level of Development of 400 requires a deep knowledge of the architectural object.
If on the one hand a high definition of the survey data is essential, on the other hand an important quantity of information of various kinds (constructive, historical, etc.) is necessary. Therefore, the transparency of the sources, which have substantiated the modelling and the creation of the database, and the consequent level of reliability of the model itself, become fundamental.
A similar model, including all the aspects necessary for documentation, can offer itself as a foundation for the restoration project, maintenance and management of the architectural heritage.
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