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.
Figure 1. Orthoprojection with hidden areas
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Figure 2. Practical example
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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.
