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Documenting mausoleums of the Chinese Tang Dynasty Mapping topography around the site Any archaeological fact can only be explained and understood if the surrounding geographical facts are taken into account. In the case of the Tang mausoleums, geomancy played an important role for the selection of the sites and their arrangement. Nevertheless, the mapping of topography around archaeological sites is often neglected in documentation. An important reason for this is the comparatively large amount of work necessary. The use of remote sensing data can be a good compromise between expenditure and benefit, especially when other data, such as aerial images, are not available. In the current project, the use of standard data like SPOT and LANDSAT TM are sufficient for most mapping and visualisation tasks. Compared to standard mapping procedures, a lot of time and work in the field can be saved.  Fig. 2: Huiling Mausoleum (ca. 30 m in diameter) in different images Upper row from left to right: IKONOS PAN, IKONOS MS, KVR-1000 Lower row from left to right: SPOT PAN, IRS-1C LISS III, LANDSAT Mapping the Site / Replacing Ground Surveys Obviously, detection of objects depends on pixel size. The reduction in pixel size has pushed the limit towards smaller objects. Single findings of small size and topographic detail plans still will have to be recorded locally in conjunction with the archaeological survey. In maps with smaller scales, a part of the field work can be replaced by remote sensing techniques. For these tasks the ground resolution of the data is a most important factor. Figure 2 shows Huiling Mausoleum with a mound of about 30 m in diameter and 15 m in height in different images. It is visible only in the high resolution images due to topography and the effects of light and shadow. Smaller objects, like stone sculptures with a base area of about 1.5 m x 3 m can only be noticed in the IKONOS PAN images, if their location is known. In the Tang project, the localisation of the numerous tomb mounds of officials and relatives consumes a lot of time. Many of these are not documented in Chinese literature and have to be searched for in the field. Since their diameters area at least around 5 m, satellite images can be used to locate these features by visual inspection or automated detection using image processing techniques. Visualisation of archaeological objects The most important point in using remote sensing image data for visualisation is the image information itself. A recent image of the region often brings much more information as compared to standard topographic maps. The combination with ground surveys and landuse information leads to a highly meaningful document. The combination with digital elevation models adds the topographic component and understanding. Almost photorealistic perspective views and animations can give an excellent impression of the region, the sites and findings, and the relations between them (cf. Fig. 1). The required image resolution is dependent on the scale of the representation (cf. Table 2).
 Fig. 3: Query result of an archaeological GIS. Information Systems for archaeological documentation In the Tang project, a desktop GIS (ArcView) was used to examine the potential of such a system (Heinz, 1997). In the documentation of such extended sites enormous amounts of data have to be managed. This can sensibly be done using database and information systems. As the geographical and topographical location of all findings are an important attribute and topographic dependencies are of importance for the relation among the findings, the use of GIS tools can improve data storage and retrieval (Fig. 3). GIS allows the storage of different data types like vector drawings, tabular data, image data, text information, maps, elevation models, photos, etc. By managing the documentation in a GIS, different data queries can be made possible:
Satellite image data are suitable for the documentation of extensive sites. Images with a standard resolution of about 10 m can be used for visualisations in map scales up to about 1 : 25.000. High resolution data (1 m) is suitable in scales up to 1 : 5.000. These images can match the quality of medium scale aerial photogrammetry. If multispectral data (usually with poorer spatial resolution) are available, they are useful in addition to panchromatic data for visualisations and animations in natural colour. The combination of image information and locally recorded single findings in maps helps to understand the topography of the site and can reduce the amount of time necessary for documentation on the site. The visibility of archaeological findings is obviously limited to objects of about pixel size. Ruins of buildings or long walls can be found in high resolution data whereas single sculptures are not visible. Since high resolution images from different systems are now available on the commercial market, large archaeological sites and their environs can be mapped, visualised and documented in GIS. This opportunity will not replace field work on the ground, but it can speed up the documentation process allowing the field personnel to concentrate on the investigation of the historical remains. References
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