For students of medieval proportional systems who lack such documentary evidence as James Ackerman had, in relative abundance, for his groundbreaking 1949 study of the Cathedral of Milan, the only choice has been to turn to the buildings. Yet Ackerman warned that such an exercise was destined to fail: “the analysis of remaining monuments,” he wrote, “provides insufficient evidence for [the] task.”

Ackerman's pessimism probably had much to do with the quality of survey data readily available for medieval monuments. One could not assume, without knowing the specific conditions of acquisition, that existing plans were accurate enough to sustain the scrutiny necessary to resolve differences among potential proportional schemes. Had the buildings in question been completely and carefully surveyed? Were the resultant plans and sections plotted using the drafting techniques necessary to minimize error? To what extent were measurements rectified—i.e., arbitrarily corrected—out of a desire to represent the building in a supposed perfected state, or from a lack of sufficient survey data to represent the building as it actually stood?

The scholar of proportions had two solutions when faced with the difficulty of establishing the representational reliability of drawings created by others. He might dismiss the problem by arguing that precision in plan and section was in any case not essential: since proportional systems were more often than not imperfectly executed in the buildings themselves for a host of reasons having to do with the reality of the construction site, a certain inaccuracy in an existing plan might be tolerated as a result. Though this might be satisfactory in a general sense—in order, for example, to locate a double square in the nave of a given building—more complex situations could not be resolved with any clarity. It was simply impossible to know precisely by how much a proposed proportional scheme was distant from the constructional reality of the building.

In this case the only choice was to undertake a new survey of the building in question using the most precise means available. Yet this could take weeks, if not months, and only in the best of situations would it be possible to acquire data in the upper reaches of the building: scaffolding is expensive and encumbering.

Today, over half a century later, Ackerman might have had a different response.