Photogrammetric stone-by-stone survey and archeological knowledge: an application on the Romanesque Priory Church Notre-Dame d'Aleyrac (Provence, France)

This paper focuses on a new approach of stone-by-stone surveying in which formalised architectural knowledge is used as a prerequisite to the photogrammetric measurement process. One of the major aspects of our approach regards the photogrammetric survey of historical architecture built from stone or brick,aiming at the three-dimensional and database treatment of the individual stones or bricks as elements composing structural units,such as arches,pillars,windows and vaults. The chosen object are the remains of the romanesque priory church of Aleyrac, in northern Provence (France), the semi-ruinous state of which gives a clear insight into the constructional details of its fine ashlar masonry. In addition to the morphological definition, the structural point of view is implemented in the model in order to consider some architectural elements as containing a set of ashlar blocks. In our approach of stone-by-stone surveying, which has been conceived for the study of historical ashlar masonry but can be applied to other types of investigation,archaeological or else,the measures are taken directly on each individual stone in its built context. A previous analysis of the building, conducted by an archaeologist, is necessary to define the characteristics of the construction, its relative chronology, and the properties of all the measured architectural entities. Retrieving architectural information (for example the intrados radius of an arch) is the approach we have developed in architectural surveying. We are currently working on this issue in the stone-by-stone surveying process on which this paper focuses. The photogrammetric approach is developed with ARPENTEUR,a web application for digital photogrammetry mainly dedicated to architecture (Architectural PhotogrammEtry Network Tool for EdUcation and Research) available at http://www.arpenteur.net. ARPENTEURhas been developed by two complementary research teams:the “Photogrammetry and Geomatics”group of ENSAIS-LERGEC's laboratory (Strasbourg, France) and the Gamsau-MAPCNRSlaboratory located in the school of Architecture of Marseilles (France). The survey process is made by a non-photogrammetrist operator using the I-MAGEmethod (Image processing and Measure Assisted by GEometrical primitive) which allows the operator to achieve the main part of the 3D survey considering one single picture.