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Potential of sunlight simulation to support conservation of Bayon archeological site Comparison with Images Recorded by Thermography A series of images recorded by a thermography were compared to a series of computer generated scenes having the corresponding field of view. The thermal images were recorded as a part of a field work in 1999 (Keio University, 1999). They indicated that roofs of galleries and terraces had kept having high temperatures. On the computer generated images, the same parts of the ruin were illuminated by the sunlight for prolonged periods. A point to be noticed is that some parts kept relatively high temperatures on the thermal images even when the corresponding simulated images indicated that those parts were already covered by shadows. This inconsistency is suggestive for future study to gather more information on heat capacity held by each component of the ruin.  Comprehension of Seasonal Changes of Isolation Distribution of insolation over the site for a year was estimated by production of hourly overhead images for four particular days, namely, two equinox days, the Summer Solstice and the Winter Solstice. At the location of the Bayon, the date for those four days were March 21st, September 19th, July 4th and December 19th. The previous experiment implied that the heat held by a particular part of the ruin was roughly proportional to the length of insolation for that part. This was applied to the whole of site to enable a synoptic observation in this experiment. The viewpoint for this experiment was set to 400m above the ground to include every part of the site. Orthorectification was applied to the rendering phase of the image generation to make areas of polygons as accurate as possible. Duration of the daytime in those four days was within the range between 5:00 am and 6:00 pm. One orthorectified overhead image was produced for every hour for the daytime of each of the four days, which resulted in 4 sets of 14 images. The landscape simulation software used in this study output them as 8 bit unsigned images. They were exported to an image processing software for remotely sensed data, and the 8 bit unsigned values from 14 images were summed up in a 16 bit unsigned channel. The 16 bit unsigned pixel depth was re-scaled to 8 bit unsigned for display purposes. In the end, four images indicating the total of insolation on the vernal equinox, the summer solstice, the autumnal equinox and the winter solstice came into existence as shown below.  Figure 5. Insolatin on Vernal Equinox, Summer Solstice, Autumnal Equinox, Winter Solstice (From left to right) N.B. Areas of shadows change over seasons. To characterise insolation in summer and winter, a false colour composite was made by assigning the summer solstice image to red, the vernal equinox to green and the winter solstice to blue. Furthermore, a classification algorithm was applied to the four 16 bit unsigned images. The particular procedure used was the K-means (minimum distance) clustering. The false colour composite and classification image are shown below.  Figure 6. False Colour Composite (see text) and Figure 7. Classification image The images derived from this experiment denote that the Sun's movement along the east-west line shifts between north and south according to the season. This results in seasonal change of locations of shadow. More concretely, the north side of a wall laying between east and west, for example, is heated for a long period in summer, while the south side of wall in winter. On other types of structures, the balance between insolation and shadows may exhibit a more complex pattern. Scatter Plot Analysis A scatter plot was produced by assigning the Summer Solstice image to the X axis and the Winter Solstice image to the Y axis. The two axes were highly correlated to each other. The plot were dominated by two sets of rectangulars. The set of rectangulars close to the bottom left corner represent shadows on the ground, and the other larger set of rectangulars represent shadows within the ruin itself. An examination on domains outside of the latter (as annotated on the figure) revealed the following tendencies: Domain 2) Areas having high pixel values on the Winter Solstice image and various pixel values on the Summer Solstice image. Their distributions were only on southern edges of surfaces at more than a certain height; Domain 3) Pixels in this domain occupy the west to the northwest edge of the top of the central tower; and Domain 4) Pixels in this domain occupy the west to south edge of the top of the central tower.  Figure 7. Scatter Plot The observations above can be further linked to analyses specific to particular components of the ruin on perspective images. Conclusion This study illustrated the potential that computer visualisation / simulation had for archaeological researches. The simulated images of the Bayon showing insolation and shadows of the Bayon was verified and made a number of useful implications. In fact, they showed what field observations can never show. It is, however, necessary to recognise the necessity of conventional field observations and data gathering practices. Without them, simulation / visualisation cannot be carried out and verified. Field works and simulations / visualisations are supplementary to each other. One inspires the other, and synergy between them will push researches forward. The future research scope from this attempt could include not just insolation but other variables and phenomena such as precipitation, humidity and land subsidence. References
We would like to acknowledge the JSA Project for supporting us in the data acquisition phase.
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