This paper describes the photogrammetric 3D modelling of complex buildings using low-cost automatic image matching (AIM), consumer-grade digital cameras and low-altitude imagery. To verify the potential of this method, it was applied to the documentation of a specific case: the towers and roofs of the Cathedral of Santiago de Compostela in Spain, a UNESCO World Heritage Site. The design and development of a mechanism for coping with the elevation, based on telescopic masts, was essential. Models, orthophotos and plans have been obtained to determine and rigorously measure the geometry involved. Thereby it was possible to accurately record the materials and decorative elements based on the restitution of the granite stones. Furthermore, close range photogrammetry made the analysis and quantification of the inclination of the south tower possible.
Photogrammetry is a commonly used technique in three-dimensional image-based modelling for cultural heritage documentation. Three-dimensional image-based techniques that combine close-range photogrammetry and automatic image matching are presently attracting a great deal of interest. This paper evaluates the feasibility of the application of photogrammetric three-dimensional modelling of an archaeological site with close-range automatic image-matching software and a consumer-grade digital camera. In order to correctly model upward-facing surfaces using this technique, the photographs were taken from the air with the camera in low-oblique position. A mast pole was used as camera platform to obtain low-altitude imagery. To verify the usefulness of this method it was applied to the hill fort 'Castro de Formigueiros', located in Lugo, Spain. In this case, low-cost automatic image-correlation photogrammetry has proved to be a powerful, cost-effective and versatile technique for the documentation, analysis and cultural dissemination of archaeological mapping. The mast used is inexpensive, easily transported and handled on site, and allows oblique photographs of the ground and wall-tops to be obtained at distances similar to photographs of vertical wall faces taken from the ground. As a result, the resolution and accuracy of the point clouds of all the surfaces of the final model are similar. Given the geometric peculiarities of hill forts (the occupied surface area, height, volume and distribution of the objects) and the relief (they are generally located in areas difficult to access and often on high ground), the mast is particularly suitable for photographing low-altitude imagery at archaeological sites, as the final accuracy of the three-dimensional model demonstrates. The tools that have been applied to the final digital model of the hill fort show the information analysis potential that can be obtained for archaeological work.
The pseudoscopic effect in satellite imagery causes perception problems for rugged terrain. The topographic relief is perceived in reverse in images with southeast illumination because of the position of land shadows and the mechanisms of human vision and depth perception. This article presents a correction method for false topographic perception phenomena. Superposition of the orthoimage and the correctly shaded digital elevation model (DEM) provides the correct three-dimensional visualization of the relief. This study demonstrates the applicability of this processing technique for the correction of such effects to provide cartography with a more useful interpretation. The resolution of the DEM employed should be in accordance with the spatial resolution of each image. The opacity level proposed for the overlapping DEM is 50%, 30% and 45% for each image type. The selection of the most appropriate local incidence angle is determined by the level of terrain roughness in the work area.
Orthoimages have become a useful tool for photointerpretation and they are currently replacing conventional vector maps in many applications. Very high-resolution satellite images such as QuickBird, provide an important source of data for the creation of such cartography. However, the inverted perception of relief in optical sensor data significantly diminishes the visual quality of these orthoimages. This problem is caused by illumination from the southeast during data collection. In the present study, the pseudoscopic effect is resolved by means of a DEM and simple image processing tools available in commercial software. The superimposition of the orthoimage and the properly shaded DEM will provide the correct three-dimensional feature visualization of relief. In the absence of stereo pairs, the limitations of the two-dimensional surface of the satellite image are overcome for the proper perception of three-dimensionality. The map obtained is improved in quality to better understand the earth's surface and provide important information about relationships between land shape and slopes.
The perceptual problems of viewing topography on geo-images are caused by illumination from the southeast during data collection. This problem affects the majority of satellite images. The aim of this work was to obtain a stereoscopic effect of shaded relief in such images. Techniques available in commercial digital processing programs were used in the absence of a digital elevation model. The images used were taken by the Landsat TM and SPOT P satellites; the software used was the EASI -PACE and ACE programs (Canadian PCI Geomatics Group). The pseudoscopic effect was solved by using the first principal component obtained in principle components analysis of the three channels resulting from the weighted merging of the Landsat and SPOT data. The map obtained provides the observer a view with shaded relief.
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