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Documenting mausoleums of the Chinese Tang Dynasty  Prof Dr Wolfgang Boehler Institute for Spatial Information and Surveying Technology, University of Applied Sciences, Mainz, Germany boehler@geoinform.fh-mainz.de  Dipl - Ing Guido Heinz Roemisch-Germanisches Zentralmuseum, Mainz, Germany heinz@geoinform.fh-mainz.de The Tang project The 18 mausoleums of the Chinese Tang Dynasty (618-907) are scattered over an area of 5.000 km˛ in the Shaanxi province in the PR China. The size of a single mausoleum, including the surrounding wall, may reach up to 15 km˛. Nowadays only mounds and single findings remain from the former buildings, walls and towers, whereas numerous larger-than-life stone sculptures are still in good condition. In 1993, cooperation between the Shaanxi Archaeological Institute, PR China, and the RGZM was established for the documentation of the mausoleums. i3mainz is a partner in this project and carries out all tasks concerning surveying and geoinformatics.  Fig. 1: Perspective view showing a part of a Tang Emperors’s mausoleum in Shaanxi, China. IKONOS Data have been draped over a DEM derived from a local tacheometric survey. Vector data from archaeologic prospecting (findings, tumuli, location of an ancient wall) are superimposed Image data During the course of working on the project, we had the oppurtunity to acquire and to evaluate many different images from the same area. Among those are:
A common reference coordinate system, preferably the state coordinate system, must be used for all surveying and mapping activities. This allows the relocation of any site and a correct union with former and future documentation. A common system is also required when different data sources, such as maps, remote sensing, photogrammetry, and local field surveys, have to be combined. In the Tang project, a custom designed coordinate system is used for all ground surveys, as no information about the state coordinate system was available. The spatial positioning was determined using GPS. GPS receivers have also been successfully used in our project to supply coordinates for remote sites and control points. When brought together within a common reference system, data can be visualised and analysed in various combinations for use in research, publications, and museum presentations.
Stereo / DEM A SPOT PAN stereo model was used to generate a digital elevation model of the target area. GPS was used for the determination of control points. The image data with a resolution of about 12 m allow to calculate an elevation model with about 50 m spacing and an accuracy of about 10 m. This model is suitable for the visualisation of extended mausoleums. The accuracy is not sufficient for the generation of maps in scales of 1 : 5,000 for the area of one mausoleum. Hence, it cannot replace the tacheometric measurements in the field to record topography. Multispectral Data / Image Merging Multispectral data can be used for landuse classification and natural coloured images. Landuse maps are suitable for overlays in views showing the current situation in combination with historical findings. The standard product for classifications is LANDSAT data with seven bands from visible to thermal infrared. Colour images are very useful for a natural visualisation of the site as they are more attractive as compared to black and white images. They can be produced if a red, green, and blue band (RGB) is supplied by the sensor. The combination with panchromatic data with a higher resolution improves the colour image. Several methods, including RGB-IHS-RGB transformation, principle components transformation and sub-pixel merging have been used and tested in this project for the combination of SPOT PAN with LANDSAT TM RGB as well as the panchromatic and RGB bands of IKONOS. Combination of Image Data with Ground Surveys With the aid of image processing software, different types of data can be combined to show relations between information from different sources, e.g. archaeological objects, topography, and present landuse. Point, line, and area information from maps or local surveys can be visualised as vector symbols with CAD tools and combined with orthoimages or perspective views from satellite images (cf. Fig. 1). As mentioned above, it is essential to reference all information involved in a common coordinate system. Using Satellite Image Data for planning and orientation in fieldwork Raw digital or film images, as supplied by the distributors, can already be very helpful as a source of information about the project site and the surrounding area. The location of findings in relation to modern infrastructure like cities and roads can roughly be measured and used to plan further field work. Using simple plots of the image together with its approximate georeference as supplied by the distributor in combination with a simple handheld GPS receiver is very useful for orientation in the field and the determination of point positions in remote areas where maps are poor or not available. For this purpose, high accuracy is not essential, so the uncertainty in the geographic position of the image and in the GPS positioning is acceptable in most cases.
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