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Measured Survey

© Dr Roland B Harris - Rectified photography of the Locutory

The importance of measured survey

Works to the fabric of Norwich Cathedral never cease. These mostly comprise conservation to the historic stonework, but in some instances – such as the new Refectory and Hostry – represent modification. All such works demand accurate scale drawings, as a basis for planning the work and for recording changes made to the historic fabric for posterity. Such drawings also underpin the archaeological study of the Cathedral, which, in turn, furthers the understanding of the development of the building and informs the conservation. To meet this need the Cathedral commissions a wide range of accurate measured surveys.

Plans – tacheometric survey

In the late 1990s the Chapter of Norwich Cathedral commissioned a comprehensive survey of the Cathedral (funded by the Friends of Norwich Cathedral and English Heritage), comprising plans of the Cathedral church and cloisters at the numerous levels from basement boiler rooms to the roofs. These plans record every paving slab and complexity of the mouldings of arches and doorways, and when completed provided Norwich Cathedral with the most accurate and comprehensive plan survey of an English cathedral. They have become a vital tool for the Cathedral Architect. With the surveys held in a CAD (Computer Aided Design) system, the drawings are able to look (and print) like conventional plans, whilst simultaneously holding 3-D data. The survey data for these plans was acquired by use of an Electronic Distance Meter (EDM), which is essentially a very modern version of a theodolite that, when equipped with a laser beam (a Reflectorless EDM), allows accurate non-contact measurements (handy for lone working, or reaching inaccessible points).

Elevations – photogrammetry

The external faces of the walls of the Cathedral are most exposed and, thus, have the greatest need for conservation. The main measured survey requirement for these is for accurate drawings of the elevations showing every stone. Such drawings are needed in advance of the work – and before scaffolding is erected – and the best survey method is stereo-photogrammetry. For this a special calibrated large-format camera is used in conjunction with a theodolite or EDM to acquire photography of the elevations, with stone-by-stone line drawings produced by a sophisticated and computerized version of tracing (again in a CAD system, and also in 3-D). Such surveys need to be up-to-date at the time of conservation works, so tend to be done only when needed: so far the Cathedral has photogrammetric elevations for the north Transept, the tower, and the south side of the Nave. When conservation works starts, archaeological data is added to the drawings from the scaffolding: this includes details of tooling on the stones, their geology and masons’ marks.

Elevations – digitally-rectified photography

While photogrammetry is ideally suited to recording the large ashlar (cut-stone) elevations of much of the Cathedral, it is less suited for measured survey of flint-rubble elevations (there are so many small stones that the cost tracing, or plotting, them becomes exorbitant), or areas where there are subtle features such as wall-paintings. For such areas Chapter has commissioned digitally-rectified photography. That is high-resolution photographs taken with a conventional camera, again in conjunction with a theodolite or EDM, with the images then corrected in a specialized computer program to produce accurately scaled photographs or, more commonly, a mosaic of many photographs. The technique has been used to record the cloister walls (where there are wall paintings), and the flint-rubble walls of the medieval Refectory and Hostry.

Three-dimensional features

Many parts of Norwich Cathedral cannot be reduced to largely flat plans or elevations: examples include the curved chapels at the east end, the stone vaults, or the spire. The 3-D environment of the CAD system means that survey data is easy to handle for such areas, and the measurement is done by a variety of approaches. For example, for a survey of the Gothic vaults of the cloisters, the laser-equipped Reflectorless EDM was used. The inaccessibility of the spire, however, meant that laser scanning was more appropriate: all the data could be captured be a few hours ground-based site work.

Who does the surveys?

A variety of specialists have been involved in the measured survey of the Cathedral. Stereo-photogrammetry and laser scanning is undertaken by external companies, but Norwich Cathedral is unique in its in-house expertise – most of the measured survey work at the Cathedral has been undertaken by Phil Thomas (when the Archaeological Surveyor: he is now Cathedral Estates Manager) and Dr Roland Harris (Cathedral Archaeologist).

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