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Potential of sunlight simulation to support conservation of Bayon archeological site Nobuya Watanabe Research Postgraduate, MAG-SFC Keio University, Japan Email: nov@sfc.keio.ac.jp Hiroyuki Yoshida Post-Doctoral Research Associate SFC, Keio University, Japan Yuichiro Usuda Remote Sensing Technology Centre of Japan, Japan Abstract An important determinant for surface temperature of an object on the ground is the quantity of heat brought by sunlight. Heat reaches surfaces, and deterioration may begin from the torrid superficies. This is particularly crucial for an archeological site when its conservation is considered an urgent issue. In this study, solar energy influx to Bayon, a major part of the Angkor Monuments in Cambodia, is examined through simulations. The objective of this attempt is to clarify the way in which sunlight covers the site over a day as well as over seasons. The approach taken in this study is complementary to conventional field surveys in two ways. Firstly, while thermographic instruments and other tools enable accurate but only temporary measurements of changing temperatures and other variables in the field, simulations of geometry of the site in relation to the sun enables analyses of effects of solar energy on the site for a prolonged period. Secondly, while field surveys consists of sampling from the target inevitably, simulations enable spatially synoptic observations of the site having complex compositions. Simulation of sunlight, simulation of changing shadows around the three-dimensionally modeled site in other words, is a comprehensive approach which sets data derived from field surveys in temporal and spatial contexts. The sunlight simulation carried out in this study presents geometrical patterns through which heat taking the form of sunlight is given to the site of Bayon. On the basis of such geometrical patterns, the possibility to shed light on concrete mechanisms in which not just heat but also precipitation, humidity, permeability of building material and structure of the site itself result in damages will be envisaged. Introduction Simulation of a phenomenon by using a computer has become common throughout the wide spectra of science and engineering. This trend is reaching the domain of archaeological studies. Among various approaches of computer simulations, production of computer graphics (CG) has a significant possibility for many archaeological researches. Graphics produced through simulations can be interfaces for qualitative observations / analyses and compilations of quantitative facts taking forms of data sets. In many archaeological studies, one of the most important characteristics of CG to be exploited is its ability to visualise scenes that are difficult, or impossible, to see under normal circumstances. In this study, attempts were made to visualise shadows around the large and complex ruin in Cambodia, the Bayon, over a day and a year to formulate a conservation strategy to be deployed in future. Systematic observation of shadows of, and insolation over, an archaeological site like the Bayon in situ is not an easy task to carry out. While it might be possible to observe a site on one day or for one week in a scientifically sound manner, human and economic resources required to do the same for one year could be too enormous to proceed. The difficulty becomes an impossibility for many archaeological sites in areas where sufficient logistics can be hardly maintained. Moreover, the impossibility cannot be confined in the temporal aspect of fieldwork when a target is spatially large and complex. Field observation consists mainly of sampling of variables at points on and around, or at angles to, the target. The number of samples is limited under any circumstance. If a ruin of interest is not simple enough, the limit on sampling forces the observer to omit certain important features from analyses. In the context explained above, visualisation of shadows of the ruin of Bayon in Cambodia is significant in three ways. Firstly, such an approach enables an observation for a prolonged period. Secondly, computer simulation is not restricted by insufficient logistics. Thirdly, and most substantially, it is almost the only way to comprehend the spatially complex target as a whole. Conventional methods to measure insolation over a target like the Bayon are usages of instruments such as sunshine recorder, temperature sensor and thermography. Sunshine recorders and temperature sensors measure the variables only at points, and their locations and numbers are highly likely to bias consequent analyses and inferences. This problem has to be considered critical when structural complexity of the Bayon is taken into account. A thermography can record a series of frames at an angle to the target, but it is hard to set up a number of the costly instruments in the site for a long-term observation. The simulation of sunlight by using a 3D model of the ruin and its geographical location data set in this study has advantages over such conventional methods. It has to be, however, noted that the production of CGs through the simulation in this study is not a substitution for conventional field observations. Data collection in fields, simulation and ground truthing are all supplementary to each other in many scientific disciplines. IT intensive archaeology such as this study is not an exception. Bayon in Cambodia The Bayon in Cambodia is a Buddhist temple set up in the ninth century. Several modifications and expansions have been applied to the ruin in the past, and it is a three-dimensionally complex structure today. The dimension of the site is approximately 150 m by 150 m, and has about fifty towers forming a stone-built maze. Structural collapses and surface deterioration are conspicuous on this site, and efforts have been made by both Cambodian and foreign teams for conservation and restoration. Uneven distribution of heat over the ruin is considered one of the causes for structural collapses (e.g. JSA, 1998, p334). More concretely, heat brought by the sunlight might be resulting in thermal expansion of particular stone blocks and joints, hence, imbalance of structures.  Figure 1. Bayon on a sunny day 3D Model of Bayon Construction of a 3D model of the ruin of Bayon was carried out by using a 3D modeling software. The basis for the modeling process was the plan made by the French institute, Ecole Francaise D'extreme-Orient. This plan was produced through field surveys in the 1920s and 1960s, and contained outlines and height information. The plan was first scanned, then, lines on it were digitised on screen. The 3D modeling software used for this study extruded towers, terraces and other structures to the heights as indicated by this plan. The number of primitive 3D shapes produced by the above procedure was more than 1200. Every effort was made to make the 3D model as precise as possible. The following two points, however, have to be realised. Firstly, the plan on which the 3D model was based was about 50 years old. It is not necessarily an accurate illustration of the status quo of the ruin of Bayon. Secondly, details of the ruin such as fine relief engravings were omitted. In the production process of the 3D model, features less than 1m had to be neglected. The "spatial resolution" that the model had in the end seemed approximately same to, or slightly less than, that of standard aerial photographs taken for photogrammetric purposes. These two points, however, do not have to be regarded as defects. The age of the plan suggests that the 3D model produced from it can be taken as a reconstructed model showing the state in 1960s rather than the state in 2000s representing the effects of insolation for many decades. The lack of details economizes the available computing power so that production of many scenes having different viewing angles become possible.  Figure 2. 3D model of Bayon
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