98 
Bryan, Andrews 
6. DISCUSSION 
Even though the size of the ceiling provided a challenge in its own right, the major problem encountered during this project was the 
colour balancing of the 23 colour images used during the production of the orthophotographic mosaic. Although geometrically 
correct scans were utilised the slight variations in exposure within each colour negative, caused apparently by the use of natural light, 
appeared to have upset the automated process usually utilised during the scanning of aerial imagery. This resulted in individual scans 
that were incorrectly colour balanced, and when mosaiced together, produced a very fluctuating colour image even after attempted 
correction using Adobe Photoshop. Alternative scanners were used in an attempt to minimise this problem, but accurate colour 
balancing still remained an issue eventually requiring re-scanning of the base imagery onto Kodak Photo-CDs in an attempt to 
produce colour images that could be evenly mosaiced together. It is now interesting to note the release by some software developers 
of image balancing wizards that could potentially take such a fluctuating image and automatically balance out the colour variations to 
produce a completely even image. Having such routines already installed within a digital photogrammetric environment would have 
greatly aided the presentational aspect of this project and no doubt other close-range projects. 
Since this project has been completed, a number of developments have occurred in theodolite/EDM technology which may have 
aided the progression of this survey. The introduction of total stations equipped with inbuilt REDM, working in a truly coaxial 
arrangement to precisions of at least ± 3mm, would have effectively halved the necessary survey control work. They could also have 
allowed more control points to be observed, using polar observations rather than intersection, and provide more redundancy within 
the processing stages. Within the latest phase of conservation work on the ceiling, REDM equipment has actually also been used to 
aid the survey and recording of the roof structure above, to which the ceiling is attached. 
0) 
(II) EAST mo 
(IV) 
KEY 
HI Ceiling Bolts Replaced Boards 
I Nail Ends > Protruding Nail Ends 
• Iron Screws • Galvanised Screws 
Nail Heads 
• Original • Added/Replaced 
t t Protruding Nail Heads 
DRAWING 
FIGURE No: 
SCALE 
1:30 
? 
1m 2m 
PETERBOROUGH CATHEDRAL - NAVE CEILING 
Conservation Programme Phase 1 (Jan - May 1998) 
stage PRELIMINARY 
TECHNICAL SURVEY 
DESCRIPTION 
Ceiling Structure, Upper Side 
POSITION 
Photogramme try courtesy 
English Heritage 
Survey Team 
of 
The Perry Lithgow Partnership 
5 Church Lane, Chipping Norton 
Oxon 0X7 5NS Tel 01606 643645 
Figure 9: Extract from preliminary technical survey showing upper ceiling structure overlaid onto underlying ceiling detail 
By remotely tracking around the edges of the many timber elements in this confined space, a three dimensional profile of each beam 
has been produced More detailed hand measurements have then been added to form a plan of each section of roof, which combined 
with the actual ceiling survey from below, forms a complete picture of the design and formation of the structure (Fig.9). 7 
7 CONCLUSION 
The lessons learnt during this survey, and the imagery already generated, are continuing to contribute to the progression of this 
project as subsequent phases of treatment are all planned, programmed, procured and executed based upon this base set of data. An 
important archive of high quality, stereo images also now exists and the use of the orthophotographic image, rather than a basic 
rectified photo-montage, has added to the documentation stage of the conservation work by greatly reducing the need to record every 
element of detail as a line. Work still obviously needs to be done on the scanning and processing aspects of the technology used, to