International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004 
  
  
Figure 2: Front and rear surfaces 
materials from which the Retable was constructed. The wooden 
structure changes dimension anisotropically with fluctuations in 
relative humidity. Daily and annual cycles set up tensions 
within the Retable that are complicated by the cross-grain joints 
and attachments. These tensions are eventually released by 
permanent changes in the wooden structure, which in turn cause 
fractures in associated brittle components, such as stained glass 
and paint layers. [n addition to relatively large-scale movements 
(in the order of 1mm) that cause catastrophic failure and loss, 
small-scale movements (of less than lyum) can cause air-gaps in 
laminates that significantly alter the optical properties of the 
composite structures. Many components have therefore broken 
and, of those that remain mechanically intact, some have been 
visually distorted. 
The physical condition of the Retable prompted concern and 
tests in the 1970s and 1980s lead to the establishment of a major 
restoration project, of which photogrammetry is an integral part. 
This paper describes a new method of documenting the process 
of restoration which is based on a spatial database founded on a 
common 3D coordinate system which can be used both for 
referencing and monitoring purposes. 
2. PHOTOGRAMMETRIC MONITORING 
The (on-going) restoration project is addressing the immediate 
physical needs of the Retable — removing accumulated layers of 
dirt, a layer of paint that was applied in the eighteenth century, 
thinning a seven-hundred-year-old discoloured varnish and 
securing delaminated paint and glass. In addition, the project is 
addressing the environmental conditions in which the Retable 
will be displayed, when returned to the Abbey. For both these 
aspects of the project, a dynamic and accurate 3D assessment of 
the Retable was required. Monitoring change in dimensions 
with environmental change was necessary in order to establish 
safe limits for the control of relative humidity. It was also 
required in order to assess the response of the Retable to 
physical movement. Some of the conservation operations could 
be carried out with the Retable in its normal vertical position, 
  
of the Retable — prior to conservation 
but other operations were easier to execute with the Retable in a 
horizontal position — the conservators needed to know whether 
it would be safe to lay the Retable down for long periods of 
time. 
Monitoring was to be carried out on a periodic basis with an 
initial epoch to determine a starting size and shape followed by 
interim epochs taken at selected times during the restoration 
process. The precision of well-defined features was to be within 
0.2mm, with no monitored feature location to be worse than 
1.0mm. Such levels of tolerance for periodically measuring a 3 
metre-long object over several years could not be achieved 
using conventional techniques. A non-contact method of 
measurement was required because of the physical fragility of 
the surface of the Retable. 
      
a s SI Let m B a 
in its vertical supporting structure 
  
Figure 3: The Retable 
2.1 Data processing 
Photogrammetric monitoring is primarily dependant on ability 
to locate homologous imaged "features" to sub-pixel accuracy 
and the dimensional stability of the digital camera and subject 
during imaging. Monitoring requirements are well within the 
precision attainable from multi-photo digital photogrammetric 
   
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