CI P A 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
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5.3 Advantages and Disadvantages of Each Method
Each method has its own advantages and disadvantages. As a
general rule, however, the greater the accuracy required the
more time and money will be required to achieve it.
5.3.1 Rectified Photography
Rectified photography can be an economic and relatively
quick method of producing a record of sufficient accuracy for
most purposes. It requires less photography than
orthophotography and the equipment and software required
are cheaper. This means that it is possible for conservators
rather than specialist surveyors to undertake the work. For
larger projects, however, it is probably more economical to
employ an experienced contractor. The disadvantage is that
rectified photography will only work successfully on
relatively flat surfaces.
5.3.2 Orthophotography
The main advantage of orthophotography is that it can be
used to produce accurate records of undulating floors.
Inherent to the process is the acquisition of stereo
photography that can be viewed in 3-D using a conventional
stereo-scope or on the DPW. Another possibility is to
produce anaglyph prints that can be viewed with inexpensive
red/blue glasses. This allows conservators a greater insight
into the floor without the need for them to purchase
expensive software or visit a photogrammetric office.
As well as the orthophotograph it is also possible to produce
contours using the DEM - another way of presenting the fact
that a floor may not be flat. Draping the orthophotograph
over the DEM can be used to produce perspective views and
a number of perspective views can be combined to produce a
fly-through movie.
The greater number of photographs required plus the high
cost of the equipment and software, mean that producing an
orthophotograph is about twice as expensive as a rectified
photography montage of the same area. The high capital cost
of a DPW and the amount of operator training required
makes it uneconomical for non-specialists to undertake the
work. There are, however, a number of experienced
contractors willing to undertake such work.
6. COLOUR
Those floors deemed worthy of recording are usually
decorative and this decoration is normally achieved through
the use of various different colours. To facilitate
interpretation and analysis it is necessary to record colour
accurately, or at least consistently.
6.1 Colour Balance
Correct colour balance requires the reproduction of the
brightness, contrast, colourfulness and hues in an image to be
acceptable to the viewer (Hunt 1995). There should not be a
bias toward a particular hue. This becomes particularly
important when montaging together a number of
photographs, as biases towards different hues in different
photographs will become very apparent when juxtaposed and
will emphasise the join between two photographs (see Fig.
5). Consistent colour balance can be achieved by treating all
the photographs in exactly the same way. This means using
film from the same batch or a digital camera, consistent
exposure conditions, film development and scanning.
Figure 5 An example of poor colour balance - Brading
Roman Villa, Isle of Wight, UK
The use of artificial illumination, in particular daylight
balanced flash, can ensure consistent exposure as long as
there is no significant contribution from daylight. Floors
within buildings can be photographed at night to ensure this
or a tent could be erected over those revealed in an
archaeological dig.
Using exactly the same development and scanning process
can produce consistent images from conventional
photographs taken under consistent exposure conditions. The
images from a digital camera taken under the same
circumstances should also exhibit consistent colour balance.
As a digital camera warms up, however, the colour rendition
may vary.
Variations in colour balance can be particularly marked when
terrestrial photographs are scanned using a dedicated
photogrammetric scanner. This is probably partly because
