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True digital orthophoto for architectural and archaeological applications 3. True Orthophoto Generation Amhar and Ecker proposed an original solution for the generation of true orthophoto in 1996. The procedure, devoted to the production of orthophotos in urban areas, uses a DSM managed by means of a relational database. All images are classified in terrain and building surfaces and the orthopoto is generated in separate phases: first the terrain then the roofs. The results of these treatments are then merged in a single digital orthophoto. Hidden areas are eliminated through superimposition of the orthophoto generated from other images. The solution proposed in this paper tries to simplify this approach. The input data for the generation of the true orthophoto of a rough object are: a dense DEM, generated by a laser scanner device, and a series of oriented images containing the radiometric description of all the points to be orthoprojected. The aims of the procedure are: to maintain complete automation so as to guarantee the same performances of traditional orthoprojection software and to avoid the previously highlighted problems (see par. 2). Let us consider the object in figure 3. In perspective images, higher points hide a lower point, therefore the procedure must run from the highest to the lowest point.  Figure 3. True orthoprojection The procedure starts from point R. The best recording of the grey value of this point can be found in the image which has the projection centre nearest to the point itself (image I1). In order to avoid the duplication of the images (see fig. 2), this pixel should be inhibited: for this reason a "flag image" is created where each pixel records the height used for the orthoprojection of the correspondent pixel on the original image. Point R has also been recorded in I2 and, for the same reason, the pixel representing point R on I2 should also be inhibited. The procedure orthoprojects point S with the same criteria (point S will only be recorded in I1). When the procedure orthoprojects point P, it finds the pixel on I1 that was used before for point R. The flag image inhibits the second use of this pixel, because the height recorded on it is higher than the height of point P. Then the procedure looks for the grey (or colour) value in I2. The pixel is not inhibited and the orthoprojetion of point P is possible. After this, the procedure orthoprojects point Q. The first attempt is to use the corresponding pixel on I1, but this pixel has been used for point S and the "flag image" then inhibits the radiometric value reading. The second attempt is to use the corresponding pixel on I2, but this pixel has been inhibited because it contains the grey (or colour) value of point R. In this case no more images are available and the orthoprojection of point Q cannot be defined. This simple example describes all the possible cases of the true orthoprojection. 4. The Accortho Software The procedure described in the previous paragraph has been implemented in a specific software called ACCORTHO (ACCurate ORTHOprojection). The input data are a regular dense grid generated from the irregular DEM acquired using a laser scanner device and a set of oriented images. The software runs in two separate steps. In the first step, the software:
 Figure 4. Flow-chart of Accortho 5. Test of the Procedure The software was tested on a set of data acquired by TUV and Politecnico di Torino for the "KARLSPLATZ C.I.P.A. test". The DEM was acquired (summer 1999) using an LMS-Z210 laser scanner, manufactured by RIEGL-Laser Measurement System, located in Horn - Austria. The LMS-Z210 laser-scanner is a fully portable sensor, specifically designed for the acquisition of 3D images. The LMS-Z210 device operates with any standard PC or notebook, has a measuring range up to 350 m and a field of view of up to 80° x 340°. The scanning time is approx. 30 to 240 s, with a nominal metric accuracy of ± 2.5 cm. Four different scenes have been acquired to build the 3D model of the principal façade. The acquisition time varied from a few seconds to a couple of minutes; less then one hour was necessary for the acquisitions, even when taking the displacement of the instruments into account. The post-processing phase includes the 3D reconstruction using tie points, materialised onto the monument through reflective stickers. The software is able to locate through an autocorrelation procedure the 3D position of the reflective stickers (characterised by a high reflectivity in the near-infrared wavelength), and imposes the correspondence of the co-ordinates (in a given reference system) of them in the different images (tie points). The 3D reconstruction generates 3D files (in a 3DD file format) that can be easily viewed using free-of-charge Internet downloadable software such as COSMOS. The 3D-model reconstruction can be easily performed thanks to the capability of the LMS-Z210 laser-scanner and the good potentialities of the post-processing software. No problems arose during the surveys nor in the image-processing phase. The capability of the acquiring system to pass through glass surfaces (with high transmittance coefficients and refraction index) could cause some problems for architectural surveying, particularly due to object distance misplacement (all the features behind glass surfaces) and shift, caused by the refraction coefficient. Approx. 100.000 points that form the resulting original DEM have been processed. A regular DEM (5 cm point spaced in the X and Y directions) has been generated with a new procedure based on a statistical approach that allows the detection of the outliers and gross errors. All the images acquired by means of a ROLLEIFLEX 6006 semimetric camera have been digitised using a calibrated UMAX DTP scanner at 1400 dpi resolution. Figure 5 shows the orthophoto generated using ACCORTHO. Observing the image in figure 5, it can be seen that the lamp in front of the station hides a significant portion of the façade. This phenomenon is very common in terrestrial images: parked cars, people, lamps and so on are frequent obstacles. This noising effect can be avoided if, between all the used images, at least one shows the area that is hidden in the other images. The crosses of the semimetric images cause the same problem. In order to avoid this effect, and to generate a "clean" orthophoto, the pixel of each image that contains noising information have been blocked directly into the "flag images", recording an height that is greater than the maximum height of the used DEM. Following the implemented procedure, these pixels will not be used during the orthophoto generation process.  Figure 5. Rough orthophoto of KARSPLATZ Figure 6 shows the final results of the entire process. Using a standard PC (PENTIUM III - 880 MHz - RAM 512 Mb), the computation time (including the generation of the regular DEM and the generation of the final orthophoto) is of about 5 minutes. Figure 7 shows the superimposition of a traditional plotting of the façade. This result is almost perfect: the mean discrepancies are less than one pixel (which size on the object is 5 mm x 5 mm).  Figure 6. Final true orthophoto  Figure 7. Raster-vector superimposition 6. Conclusions ACCORTHO is an easy software that is able to produce true orthophotos of architectural objects thanks to a completely automated procedure. Using a correct dense regular DEM, it is possible to generate orthophotos with an accuracy that is sufficient for most architectural survey applications. The proposed procedure can be applied to any kind of discontinuous objects (urban areas, monumental and archaeological sites, museum documentation, etc.) and allows a great diffusion of orthophoto benefits in unexplored fields. The simplicity of the proposed solution is due to the correct mixture of different techniques: photogrammetry, laser scanner, digital image processing. Acknowledgement This study has been financed by Italian Ministry of University and Scientific Research (MURST) as part of the project "Digital surface modelling by laser scanning and GPS for 3D city models and digital orthophoto". Terrestrial photos and data are from "KARSPLATZ" CIPA test material produced by TUV and Politecnico di Torino. References
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