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True digital orthophoto for architectural and archaeological applications Piero Boccardo, Sergio Dequal, Andrea Lingua, Fulvio Rinaudo Politecnico di Torino, Italy Dipartimento di Georisorse e Territorio E-Mail: boccardo@polito.it, dequal@polito.it lingua@polito.it, rinaudo@polito.it Abstract Digital orthophotos are cheap and efficient products used to represent the correct shape of any object. In the case where the surface that models the object is discontinuous, which is a frequent situation in architectural and archaeological applications, breaklines and hidden areas must be taken into account, and the orthoprojection procedure therefore needs to be more sophisticated. A complete description of the hidden areas is often obtainable using multiple images (present in the photogrammetric block), while a rigorous geometric description of the object requires the restitution of the breaklines or, as an alternative, a very small grid interval of the digital terrain model. In the case of architectural and archaeological objects, acquiring all the breaklines is almost equivalent, in terms of time and cost, to a complete photogrammetric plot. A "dense" DEM seems to be a more efficient solution: the new opportunity is offered by the laser scanning technique, which is now also available for terrestrial applications. A terrestrial laser scanner gives a very dense DEM (a few centimetres of grid interval) in seconds, with an excellent accuracy (better than ± 2 cm). The authors have conceived and developed an original software, named ACCORTHO (=ACCurate ORTHOphoto), which is here described in detail: it produces correct digital orthophotos of architectural objects, using multiple images and DTM acquired by terrestrial laser scanners. A practical example of the obtained product, based on the well known "Karlsplatz C.I.P.A. test", is shown. 1. Introduction Orthophoto is an efficient and economic way of representing photographic information in a 2D-reference system, which is useful when the user has to measure the surveyed object without the interpretation made by an unexperienced operator. If the projection has to be made on a plane and the shape of the object is smoothed, the procedure is very simple and the reached accuracy satisfies all kinds of applications. The two previously mentioned hypotheses do not usually occur in architectural applications. There are a great deal of smoothed objects that require different reference surfaces (e.g. a dome requires a sphere or an ellipsoid). More often the shape of man-made objects is not completely smooth and a simple shaped model, based on a regular grid (DEM), is not able to describe it properly. The digital solution of orthophoto production opens up an easy and inexpensive way of solving this kind of problem. The use of reference surfaces that are not plane is now a common tool in architectural surveying: over the last years a great number of authors have described the algorithms that are used and shown very interesting applications in the restoration and documentation of the cultural heritage. Orthoprojection of rough objects is still an unsolved problem especially in architectural applications: the main difficulty is that of the complexity of the description of the analytical shape of the object, where points with the same planimetric co-ordinates show different heights. Regular grids integrated by break-lines and DSMs (Digital Surface Models) are the most popular and investigated solutions used to build-up a mathematical shape description of such an object. In both cases complex algorithms and expensive computation times must be used before and during the orthophoto production. In this paper, the authors describe and test a solution, based on the use of DEM generated by modern terrestrial laser scanners, that preserves the practical benefits of digital orthophoto: the metrical accuracy of the product and the complete automation of the procedure. The work is a further development of the solution proposed by the authors for the production of true orthophoto in urban areas. 2. True Orthophoto of a Rough Object Let us consider an object described by means of a correct DEM (see figure 1). If one uses the traditional approach to digital orthophoto generation, all the points hidden by the perspective effects are not represented and the visible points are duplicated on the resulting orthophoto. For example, the radiometry of point Q is recorded twice: the first time in Q0, when point Q is orthoprojected, and the second time in P0 when one tries to orthoproject point P. Figure 2 shows a realistic demonstration of this systematic error which originates from the uncompleted radiometry description of the object by means of a single perspective image.
Considering the previously mentioned effect, it can be stated that, in order to generate a true orthophoto of an unsmooth object, a correct shape description and a complete radiometry recording of all points of the object must be used. This last requirement can be accomplished only by using different perspective images of the object itself and by avoiding the possibility of using the grey (or colour) value of a point Q for the orthoprojection of a point P lying on the same perspective ray. As far as 3D modelling techniques are concerned, deeper consideration is required on the present day evolution of the research activities. 2.1. 3D model generation The build up of a correct 3D-shape description of an unsmooth object can be obtained using different approaches. The first class of solutions tries to minimise the number of points and geometric information required to give a complete 3D description. The first approach is that of the use of a regular grid integrated by means of all the breaklines needed to describe the height discontinuities of the object. Simply considering the façade of a building, it is clear that the survey of all the breaklines would mean plotting almost the entire object: the survey of the break-lines is not automatic and therefore not economic for an orthophoto generation. The second solution is the definition of a DSM using geometric primitives to describe the boundaries of the object. This last approach has been studied in detail and set up to model buildings in urban areas: plane triangles and quadrangles are defined as geometric primitives and a relational database is used to manage this sophisticated instrument. When applying this solution to architectural objects more types of geometric primitives must be considered (e.g. cylinders, spheres, etc) to correctly describe the structural and decorative elements currently present in this particular kind of object. The generation of a DSM cannot be automated; the setting up and the management of such an instrument require complicated software and a large amount of computation time. Dense irregular grids represent the second class of tools that are useful to describe the shape of an object, where the distance between two points ranges from 50 cm to few centimetres. In these models the density of the points replaces the intelligence of the previous solutions. It is possible to conceive three different ways of generating a dense grid. The first is the manual survey of the points. Let us consider a small 10 m x 5 m façade, an acquisition of one point each 2 cm in the X and Y directions and a speed of acquisition of 2 s per point: an experienced operator, working 6 hours a day could record this dense grid in 3 days! The second possibility is the use of automatic DEM generation using matching algorithms. It is well known that this solution gives almost 70% of the required points or less for critical situations: the remaining points must be corrected or integrated manually by an operator. It is obvious that these two possibilities are not able to construct a dense grid in an economic way. The third way of generating a dense grid is the use of modern terrestrial laser scanners. These surveying devices are able to acquire thousands of points in a few seconds, with high accuracy. The laser scanner technology, based on optical-electronic devices, uses a high intensity pulse directed towards the object to survey, in order to have a distance from the device itself. A digital image is derived from thousands of different pulses where all the measurements on single points are pure 3D locations. Over recent years a great deal of tests have been set up on the use of laser scanners mounted onboard aeroplanes in order to derive high precision DEM, with the precision being independent of the sensor and target distances. As far as terrestrial applications are concerned, laser scanner devices guarantee different acquisition accuracies ranging from ±5 mm (e.g. CYRAX 2500 manufactured by CYRA Technologies Inc.) to ±25 mm (e.g. LMS-Z210 manufactured by RIEGL). These instruments are fully portable sensors, specifically designed for the acquisition of 3D images; a rotating mirror directs the internal laser range-finder transmitter beam over a precise angular pattern and the resulting range measurements comprise a very accurate 3D dimensional representation of the acquired scene. The grids generated by terrestrial laser scanners are irregular, the X and Y spacing depending on range and direction between the instruments and the measured point: this means that it is possible to manage the density of the points by simply changing the acquisition distance. The acquisition process is completely automated and the DEM generation of any object (acquisition and processing of the data) is an easy and quick procedure. For these reasons the dense DEM generated by a laser scanner device can be considered the optimal solution for a correct and complete 3D description of the shape of a complex object, both from the technical and economical points of view.
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