3D Digital Models for Scientific Purpose: Between Archaeological Heritage and Reverse Modelling

3D Digital Models for Scientific Purpose: Between Archaeological Heritage and Reverse Modelling

Luca Cipriani (Alma Mater Studiorum Università di Bologna, Italy), Filippo Fantini (Alma Mater Studiorum Università di Bologna, Italy) and Silvia Bertacchi (Alma Mater Studiorum Università di Bologna, Italy)
DOI: 10.4018/978-1-5225-0675-1.ch010
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Abstract

Reality-based digital models assist in the achievement of accurate analysis of historical buildings as well as archaeological sites and, more in general, of monuments featuring more or less complex forms. Their reliability is particularly useful when the state of conservation of masonries and vaults has been altered due to deterioration phenomena or as a consequence of incorrect interventions. In these cases, a highly detailed “digital copy” of the ancient constructions, if correctly observed via reverse modelling applications, can provide useful indications for an accurate and scientifically-based digital reconstruction. The Octagonal Hall of Small Baths at Hadrian's Villa, with its daring design of vaults and audacious building techniques, was chosen to test several interpretation techniques based on the customization of contemporary reverse modelling procedures integrated with standard protocols of design analysis and archaeological investigation.
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Introduction

In this chapter, a number of digital tools for archaeological investigation will be taken into consideration. Theoretical and practical aspects concerning how to use reality-based models will be discussed so to define the key topics of a procedure to interpret complex and partially collapsed buildings and their design principles, such as those concerning ancient cupolas. The case study that was chosen is the Octagonal Hall (Figure 1), which is part of the so-called Small Baths, an important monument partially in ruins located in the archaeological site of Hadrian’s Villa.1

Figure 1.

The unique shape of Small Baths’ Octagonal Hall

Source: Photo by Filippo Fantini

The analysis of a mesh model from the laser scanner survey provided a reconstruction of the vault: its exceptional shape is part of a sort of “catalogue” including the architectural experiments which were developed during Hadrian’s age and that can be seen in Tivoli (Rome) and Baia (Naples), particularly in thermal buildings.

Their uniqueness is the reason why they play a special role in the history of architecture and construction: “Daring experiments in the design and construction techniques of its buildings turned the site into a vast playground, with structures that had no equal in the ancient word” (Opper, 2008, p. 132). For this reason a typological comparison is hard to carry out; furthermore the main Small Baths’ dome structure presents more problems regarding its state of conservation: in fact, vast areas of the roofing were damaged and altered by both natural events and human interventions (restorations, reuse, spoliation, etc.). Restorations of such unique structures, that were carried out without a careful and complete survey of the masonry and concrete vaults, now harden current-day research, due to the presence of original and altered parts (Figure 2).

Figure 2.

Giovanni Battista Piranesi, The Octagonal Room in the Small Baths at the Hadrian’s Villa (Tivoli) (Study for Vedute di Roma, Dieta o sia luogo che da ingresso ... Villa Adriana), 1777. Chalk on paper, original dimensions 39.4 x 55.3cm.

Source: The Metropolitan Museum of Art, New York. Retrieved from http://metmuseum.org/collection/the-collection-online/search/339683

In order to provide the elements for a reliable virtual reconstruction of the baths’ central dome, a digital model of the Octagonal Hall was methodically analysed in search of the advanced design used by ancient architects to achieve such unique structures.

Hadrian’s Villa is the product of less than twenty years of building activity; in spite of it being built in such a short stretch of time, it is likely that originally the compound covered 120 hectares at least: 2 in terms of function and use, the imperial mansion appeared like a mixture of Versailles and the picturesque informality of a vast English country house (Opper, 2008, p. 240).

Due to the hectic condition of both the design and construction, a non-systematic lack of accuracy occurs from a close look at the ground plans of some of the major buildings (ichnographia).3 The short construction time affected the buildings’ accuracy at times; the Small Baths can be considered, in a way, an emblematic example that is well expressed by the Octagonal Hall, whose ground plan is “geometrically” compressed between the other volumes forming the main chambers of this outstanding thermal structure (Figure 3 and 4).

Key Terms in this Chapter

Reverse Modelling: The term describes a set of procedures aimed at converting a mesh model from active or passive sensors into a NURBS made of a continuous series of patches or a solid model. The process starts with a segmentation phase, namely the detection of primary and secondary surfaces, then followed by the detection of geometric features (curves) and relevant sections of the model. Curves are used for the construction of a CAD model that will be compared with the scanned data.

Subdivision Surfaces: Subdivision surfaces is a representation technique that includes a series of methods aimed at converting a coarse mesh model into a smooth piecewise surface. The smooth surface is the product of the conversion of a mesh called control cage into a theoretical limit surface obtained through of a recursive algorithm of subdividing (Catmull-Clark, Doo-Sabin, etc.).

Opus Caementicium: Also called Roman concrete, it is a building technique used by Romans for constructions starting from 2nd century BC. It consists of an aggregate of mortar and stones and even pozzolana for hydraulic purposes and thanks to its characteristics ensured the durability over time of still existing Roman monuments.

Small Baths: The building is one of the thermal baths existing in the Villa, called small referring to its dimensions and to distinguish it from the Grandi Terme (the Large Baths). The building is partially in ruin and consists in several rooms, some of which still preserve their vaults or domes.

Vitruvius: Marcus Vitruvius Pollio (80-70 BC, c. 15 BC) was a Roman author, architect, civil and military engineer during the 1st century BC. Around 25 BC he wrote the major masterpiece on architecture surviving from classical antiquity, the renowned treatise De Architectura libri decem¸ that is composed of ten books dealing with all aspects of construction. Some concepts explained in the work are still subject to interpretations, especially those referred to the famous Vitruvian Virtues (the triad of qualities: firmitas, utilitas, venustas) that a building must exhibit.

Ichnographia, Orthographia and Scaenographia: Concepts reported by Vitruvius as designing rules concerning dispositio, i.e. the basic graphic operations for organizing plans, sections and elevations of a building, also verifying its general proportions. Even though both are graphic algorithms for drawing plans and sections, they are not exactly referring to modern concepts of plans and sections: the first one deals with the spatial organization of the plan and its proportions; the second with the resulting dimensions on the vertical plane; the latter is often referred to perspective but can be related to elevations.

Hadrian’s Villa: Hadrian’s Villa (Villa Adriana) is an archaeological complex built during the 2nd century AD near Tivoli (Italy) by the Roman Emperor Hadrian. The complex contains over 30 important monuments, covering a large area (ca. 100 ha), and is still partially unexcavated. Since 1999, the complex has been part of the UNESCO World Heritage Sites because of its uniqueness representing the ancient Mediterranean culture.

Opus Mixtum: It literally means “mixed work” and is an ancient building technique used by Romans especially during the age of Emperor Hadrian (2nd century AD). It can consist of different Opus mixed in the same wall, for instance covering most of the elevation with tufa (opus reticulatum) while the corners are made of bricks (opus latericium).

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