Simple photogrammetric methods with arpenteur. 3-d plotting and orthoimage generation: The i-mage process

This paper focuses on an original approach to architectural survey whereby the measuring step, driven by an architectural model, isleft to the architect's or the archaeologist's initiative, with reduced participation by the photogrammetric expert. A new set of toolsto this end is available in the Web-based tool ARPENTEUR (Architectural PhotogrammEtry Network Tool for EdUcation andResearch).Surveys are accomplished with the so-called I-MAGE process (Image processing and Measure Assisted by Geometrical primitive)which enables the user, for instance, to obtain 3-D measurements while digitising a single image with "no loss" of accuracy. IMAGEis a new measuring method for digitising 3-D points, based on the coupling of image correlation and the expected shape ofobjects. Shapes are primitive surfaces or partial surfaces like planes, cylinders, cones and spheres. We have developed the I-MAGEprocess to assist the plotting phase: the homologous point is determined by the system using both the image orientation and thegeometrical primitive computed on a set of 3D points already measured.On the other hand, so far, accuracy has not always been the principal purpose of surveys in archaeology and architecture. Eachphotograph contains a tremendous amount of qualitative information which is not often used to its full potential. We suggest,therefore, the full use of all the information made available during the photogrammetric process, such as: camera orientation results,3-D primitive surface computation, and rectification or projection of the images onto the primitives. The result is a scaled imagecontaining all of the qualitative information. Accuracy is then a consequence of the hypothesis made when nearing the real surfacewith a primitive. We present as an example an orthoimage generation from a rubbed vault (estimated with a cylindrical primitive) atthe frigidarium in the Thermes de Cluny, at the Museum of the Middle-Ages, Paris.