Maps

To analyze a plan, it must be of good quality. You can’t just pull a plan from a brochure or an old book! If you compare several plans of the same cathedral, you’ll often notice differences in measurements. In the case of Amiens, I found a discrepancy of more than three meters in the length of the transept!

Another issue compounds that of the measurements: the precision of the buildings’ construction. As you’ll see later, some cathedrals suffer from irregularities and asymmetries. The work is splendid, the craftsmanship inspired and perfectly executed, but the construction methods are sometimes empirical. The eye doesn’t pick up on these details; only the plans reveal them. It’s important to understand that for the builder, the stability of the monument, the overall harmony, and the quality of the details took precedence. We must realize that a master builder could not simply mark out a layout on the ground and then calmly begin building. The cathedral’s layout on the ground gradually took shape as construction progressed. This took years.
Construction began at the apse, and the layout at the crossing of the transept. To determine each wall’s position and each proportion, one had to rely on the existing structure. I’m referring to masonry elements and columns that had already been erected. It was necessary to determine their axes and exact centers—a task that is only simple on a computer or a drawing board. This difficulty is likely the source of the inaccuracies found throughout the floor plan surveys.

Fortunately, Gothic techniques allowed for great freedom of execution. While masonry blocks are “standardized” in the quarry, each stone used for an arch or vault could be adjusted on site. Consequently, slight irregularities between bays are of only relative importance. In the worst-case scenario, it was sufficient to modify the profile of the arch (see glossary) or the templates to correct the misalignment. Note that this discrepancy was usually invisible to the naked eye and had no structural impact.

In the Middle Ages, intent and practicality took precedence over strict numerical precision, which, it must be admitted, made no sense at that time.
This raises the question: how can we justify work based on approximate plans for buildings that are sometimes irregular in shape? Methodologically, this position would be untenable, but recently the situation has changed. Laser scanning techniques—notably thanks to the work of Professor Andrew Tallon—have made it possible to digitize several French cathedrals. A new analysis based on these data has confirmed the validity of my work in principle. The decision to prioritize logic and intention over formal precision was the right one.

Leica Geosystems ScanStation C10 - Credit: Andrew Tallon - CC-BY 3.0

Laser scanning model of Chartres Cathedral (Leica Geosystems)

Laser scanning of the Abbey Church of Saint-Denis - Credit: Andrew Tallon - CC-BY 3.0

It should be noted, however, that even a scan accurate to within a few millimeters does not eliminate the building’s irregularities. The problem remains the same. The compass reveals the concept, while the digital result is lost in a cloud of data that is largely meaningless. We would need to be able to correlate the information and interpolate it through statistical analysis. For now, these processing algorithms—and indeed the very concept behind them—do not exist. While having metric data is an invaluable aid, traditional compass-based analysis remains the standard approach.

I should note that the plans used on this site are intended solely for illustrative and explanatory purposes. I have redrawn some of them for greater clarity. I urge anyone who wishes to reproduce or verify the example projects to use only plans created by laser scanning.