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this paper (Hartmann-Virnich 1996), is an excellent example of the refined ashlar two-shell masonry characteristic of 
the latter 12th century in southern France. The church is preserved in a semi-ruinous state, offering an insight into the 
internal structure of the walls, necessary to develop the relevant extrude vectors for an ARPENTEUR survey. The 
Romanesque builders of Aleyrac had obviously a good empirical knowledge of the mechanical properties of their 
building material, as they used deliberately different types of stone for different functional entities, such as 
cornerstones, arches and windows, the vaults alone being assembled with three different sorts of limestone (hard, soft, 
and tufa). This tends to prove that the criteria defining different types of ashlar material : ordinary blocks, cornerstones, 
voussoirs, are relevant, and can be used consistently for an ARPENTEUR survey. 
The stone-by-stone analysis of a medieval building implies a great number of interesting aspects, essential for the 
knowledge of the building process and chronology, the technical standards and skills of the builders, the mechanical 
properties of the edifice (deformations) as well as the economy and the history of its development. The dimensions, the 
finish of the stones (e.g. the precision of angles and sides, tool marks, mason marks etc.), the geological properties of 
the distinctive materials, the continuity or discontinuity of the courses, are irreplaceable clues (Hartmann-Virnich, 1998, 
1999). A virtual stone-by stone survey is of great interest, especially if allows to visualize relevant information for each 
constructional unit. 
The selected part of the building is the north transept. The study has been focused on the round arch opening on the 
north side of the nave. The sample is of particular interest as it can be easily defined as a coherent unit composed with 
similar elements, the voussoirs, which share the same properties : each voussoir course is assembled with three or four 
blocks, two of which have visible front sides. Ideally, they all share the same intrados and extrados, i.e. the curved inner 
and outer sides, and their joints converge towards one same central axe parallel to the intrados and extrados. On the 
other hand, their thickness and depth vary. Thus the structure offers several types of relevant parameters, variable and 
invariable, for a test of ARPENTEUR'S capacities. 
Our research project is originated from the dialogue between specialists of different disciplines who rarely collaborate 
to create new tools. The interdisciplinary exchange is at a beginning but will generate further exchange and, we hope, 
far-reaching results. Although ARPENTEUR is presently being developed to serve in the architectural field, its 
possibilities might go far beyond, the tool being adaptable to the photogrammetric survey of any type of structure 
composed of characterized units. 
3 THE PHOTOGRAMMETRIC SURVEY 
3.1 The photogrammetric campaign 
The photogrammetric campaign was achieved in August, 1999 in Aleyrac, Drome, France. The pictures were taken with 
a P32 Wild camera using Agfapan 25 roll film. No control point was taken, we used plumb lines and distances to create 
a local referential. Three stereo pairs have been used. The film was scanned by Kodak and the result file is 4860 x 3575 
pixel in GIF format. 
  
  
  
Figure 1. Example of picture used for the survey. 
3.2 Definition of blocks 
The survey first step is a building rigorous analysis to isolate the architectural entity and the ashlar blocks types to be 
measured. For each entity, arch, wall, gable wall, we have to determine which type of stone has been used and how to 
measure it. We can isolate two generic cases of measurement : 
» The geometry of the entity is easy to modelize (a wall offers the opportunity of determining a plane for extrude) 
> The geometry is not obvious and the extrude method needs to be informed about any indirect results from the 
measurement (intrados radius and axis of an arch). 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 189