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they are designed for the relatively consistent exposure
conditions of aerial photography, but also because the
operators are generally photogramme trists rather than
scanning specialists. It has been found that it is possible to
use Kodak PhotoCD as an alternative to dedicated
photogrammetric scanning. (Thomas, Mills and Newton
1995). This system has been used by the English Heritage
Metric Survey Team to improve colour balance in
orthophotograhs (Clowes 2002).
6.2 True Colour
When studying the decoration of one floor it will usually be
acceptable to have each colour reproduced consistently as
long as the colours look approximately correct. When
comparing two or more examples or where a floor is likely to
be re-buried after excavation it will be desirable to reproduce
the true colour of the decoration as accurately as possible.
In order to give an exact representation of the colours of a
floor it is necessary to have a way of measuring those
colours. This could possibly be achieved by recording the
colours of the individual components of the floor using a
colour measurement device. However, the number of
different colours typically found in an historic floor would
make this impractical. Even a mosaic made up of, say three
different colours, will exhibit variations due to changes in the
natural materials.
A more feasible approach is to place a standard colour chart,
such as a Macbeth chart, in each photograph (see Fig.l). The
colours of the individual elements in the chart can then be
measured and corrected throughout the process. Each process
and piece of equipment used will have its own effect on the
colour so individual colour profiles are required for digital
cameras, scanners, monitors, printers and even the type of
paper used. Even then the colour rendition will only be
correct if the final product is viewed under the same lighting
conditions as the original. This is almost impossible to
achieve because the reaction of the eyes to the lighting of a
subject is also affected by the colour and lighting of the
surroundings. The general aim is to reproduce the colour as
accurately as possible but with the caveat that it will never be
absolutely correct.
7. CONCLUSIONS
Mosaic, tiled and other decorative floors are a valuable part
of our cultural heritage which should be conserved and
recorded for future generations. A photographic approach is
the most useful and is the only practical way of recording the
many textural variations exhibited by most historic floors.
Floors are usually quite flat and so can be successfully
recorded using the relatively cheap and accessible method of
rectified photography. Some floors, are however quite
undulating and so warrant the application of
orthophotography. The use of orthophotography although
more expensive provides added value in the associated
products such as contour maps. Colour balance is important
if an acceptable looking product is to be produced. Correct
representation of colour is important to some users but
actually very difficult to achieve. It is also difficult to know if
it has been achieved.
8. REFERENCES
Clowes, M., 1997. Mosaicing the Mosaic - Roman restoration goes digital. Surveying World, May/June 1997, pp 36-37
Clowes, M., 2002. Digital Photogrammetry at English Heritage: a Pictorial Review of Projects to Date. Photogrammetric Record,
17(99), pp 441-452.
Dallas, R. in Fawcett, J., 1998. Historic Floors: Their History and Conservation. Butterworth Heinemann, Oxford, pp 91.
Fawcett, J., 1998. Historic Floors: Their History and Conservation. Butterworth Heinemann, Oxford, pp 2.
Hunt, R.W.G., 1995. The Reproduction of Colour. Fountain Press, Kingston-upon-Thames, pp 71.
Thomas, P.R., Mills, J.P., Newton, I., 1995. An Investigation into the Use of Kodak Photo CD for Digital Photogrammetry.
Photogrammetric Record, 15(86), pp301-314.
