The Bamiyan project : multi-resolution image-based modeling

The goal of the Bamiyan project of the Institute of Geodesy and Photogrammetry, ETH Zurich, started in July 2001, is the terrain modeling of the area, the 3D reconstruction of the two lost Buddha statues in Bamiyan as well the actual empty niches and the documentation of the UNESCO cultural heritage site with a topographic and tourist information system. Different types of images, with different spatial and temporal resolutions, have been used. The integration of all the photogrammetrically recovered information requires powerful software able to handle large amounts of textured 3D data. Moreover at the end of the 1990’s the extremist Taleban regime started an internal war against all the non-Islamic symbol. This led in March 2001 to the complete destruction of the two big standing Buddha statues of Bamiyan (Fig. 2), as well as other small statues in Foladi and Kakrak. In 2003, the World Heritage Committee has decided to include the cultural landscape and archaeological remains of the Bamiyan valley in the UNESCO World Heritage List (http://whc.unesco.org/). The area contains numerous Buddhist monastic ensembles and sanctuaries, as well as fortified edifices from the Islamic period. The site symbolizes the hope of the international community that extreme acts of intolerance, such as the deliberate destruction of the Buddhas, are never repeated again. The whole area is nowadays in a fragile state of conservation as it has suffered from abandonment, military actions and explosions. The major dangers are the risk of imminent collapse of the Buddha niches with the remaining fragments of the statues, further deterioration of still existing mural paintings in the caves, looting and illicit excavation. The main goals of the Bamiyan project are: the terrain modeling of the entire Bamiyan area from satellite images for the generation of virtual flights over the UNESCO cultural heritage site; the modeling of the rock cliff where the Buddha were carved out; the 3D computer reconstruction of the two lost Buddha statues and the mapping of all the frescos of the niches; the 3D modeling of the two empty niches where the Buddha statues once stood; the documentation of the cultural heritage area with a topographic, tourist and cultural information system. The project is an excellent example of image-based modeling, using many types of images, with different spatial and temporal resolution. It shows the capabilities and achievements of the photogrammetric modeling techniques and combines large site landscape modeling with highly detailed modeling of objects (i.e. the statues) by terrestrial images. Automated image-based modeling algorithms have been specifically developed for the modeling of the Great Buddha statue, but, at the end, manual measurements revealed to be the best procedure to recover reliable and accurate 3D models. Figure 2. The explosion of March 2001 that destroyed the Buddha statues (Image Source: CNN). The two empty niches, where the Buddha once stood, as seen in August 2003 during our field campaign. 2 TERRAIN MODELING FROM SATELLITE IMAGERY For the 3D modeling and visualization of the area of interest, an accurate DTM is required. Aerial images were not available to us and the idea to acquire them was unrealistic, due to the absence of any surveying company operating in that area. Thus, space-based image acquisition and processing resulted as the only alternative to the aerial photos or any other surveying method. Nowadays space images are competing successfully with traditional aerial photos, for the purpose of DTM generation or terrain study in such problematic countries as the current Afghanistan. Also, the availability of high-resolution world-wide scenes taken from satellite platforms is constantly increasing. Those scenes are available in different radiometric modes (panchromatic, multispectral) and also in stereo mode. For the project, a B/W stereo pair acquired with the HRG sensor carried on SPOT-5 and a PAN Geo level IKONOS image mosaic over the Bamiyan area were available. The SPOT5 images were acquired in across-track direction at 2.5 m ground resolution while the IKONOS image has a ground resolution of 1 m. The sensor modeling, DTM/DSM and ortho-image generation were performed with our software SAT-PP, recently developed for the processing of high-resolution satellite imagery (Zhang & Gruen 2004, Poli et al. 2004, Gruen et al. 2005). The IKONOS mosaic orientation was based on a 2D affine transformation. On the other hand, the SPOT scenes orientation was based on a rational function model. Using the camera model, the calibration data and the ephemeris contained in the metadata file, the software estimates the RPC (Rational Polynomial Coefficients) for each image and applies a block adjustment in order to remove systematic errors in the sensor external and internal orientation. The scenes' orientation was performed with the help of some GCPs measured with GPS. The DTM was afterwards generated from the oriented SPOT stereo pair using the SAT–PP module for DTM/DSM generation. A 20 m raster DTM for the whole area and 5 m raster DTM for the area covered by the IKONOS image were interpolated from the original matching results (Fig. 3), using also some manually measured breaklines near the Buddha cliff. The matching algorithm combines the matching results of feature points, grid points and edges. It is a modified version of MPGC (Multi Photo Geometrically Constrained) matching algorithm (Gruen 1985, Zhang & Gruen 2004) and can achieve sub-pixel accuracy for all the matched features. For the photo-realistic visualization of the whole Bamiyan area, a 2.5 m resolution B/W ortho-image from SPOT images and a 1 m resolution RGB ortho-image from the IKONOS image were generated. The textured 3D model (rendered with Erdas-Virtual GIS) is shown in Figure 4 where two closer views on the 3D IKONOS textured model of the Bamiyan cliff and the old Bamiyan city (the pyramid-type hill to the left) are presented. Figure 3. The recovered 20 m DTM of the Bamiyan area displayed in color coding mode (left), overlaid by the 5 m DTM (right) (colour plate, see p._). 3 3D MODELING OF THE ROCK CLIFF For the reconstruction and modeling of the Bamiyan cliff (Fig. 5), a series of terrestrial images acquired with an analogue Rollei 6006 camera was used while ca. 30 control points (measured with a total station) distributed all along the rock cliff were used as reference. The images were digitized at 20 μm resolution and then oriented with a photogrammetric bundle-adjustment. Then manual measurements were performed on stereo-pairs in order to get all the small details that an automated procedure would smooth out. The recovered point cloud was triangulated, edited and finally textured, as shown in Figure 6. Because of the network configuration and the complex shape of the rock facade, the recovered geometric model is not really complete, in particular in the upper part. In some areas it was not possible to find corresponding features, because of occlusions, different lighting conditions and shadows. This is not such a big problem, because the cliff model is not meant to be used alone, but in a next step it will be integrated into the DTM for visualization purposes. Figure 4. Close view of the Bamiyan terrain model textured with an IKONOS ortho-image. Empty niche of the Great Buddha Empty niche of the Small Buddha Shahr-i-Ghulghulah, the old Bamiyan city Rock cliff with Buddha niches