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Title
New perspectives to save cultural heritage
Author
Altan, M. Orhan

CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
least two targets appearing in the overlap between the frames.
It is possible to co-ordinate a grid of targets using a tape
measure by a system of braced quadrilaterals. The distances
between each target are measured as well as the distances to
the targets diagonally opposite. Once back in the office the
grid can be constructed on the drawing board using ruler and
compass or in a Computer Aided Drafting (CAD)
programme. This is, however, time consuming work which
must be undertaken with meticulous attention to detail if
accurate results are to be achieved.
Figure 2 A Leica TCRM1103 is used to co-ordinate
control targets
A more efficient and accurate method is to use a theodolite
and Electromagnetic Distance Meter (EDM). There are
various approaches that can be employed. Intersection is
probably the most accurate but requires angular observations
to each target from two different survey stations using both
faces of the instrument. To speed up the process it is useful to
have an assistant to identify and point out the targets, as they
can be difficult to see from a distance. To avoid confusion
such as observing the wrong target or the same one twice, it
is sensible to number all the targets individually before
applying them to the floor. To reduce the number of
observations required with only a minor reduction in
accuracy it is possible to co-ordinate the targets using angle
and distance observations. If using a standard EDM, a mini
prism with a small spike at the back is required. It is
necessary to first observe the angles and then have an
assistant introduce the mini-prism for the distance
measurement. For the best results it is still advisable to
observe the angles using both faces of the theodolite. The
advent of the Reflectorless EDM (REDM) has made the
mini-prism redundant, as distance measurements can now be
taken direct from the target. The REDM is probably not quite
as accurate as conventional EDM with a mini-prism,
especially at more oblique angles. The improvement in the
speed of measurement, however, means that the distance can
be measured each time the angles are observed, i.e. using
both faces, thus allowing an average distance to be
calculated.
A Leica TCRM 1103 total station theodolite in REDM mode
is used by the English Heritage Metric Survey Team to co
ordinate targets for photographic surveys of floors (see Fig.
2). The theodolite is motorised which means it can drive back
through the targets to allow the angles to be observed on the
other face and a second set of distances to be measured. The
observations are computed using Landscape software and
latterly Geosite from Survey Supplies Ltd. The targets used
are 10 mm in diameter and printed on self-adhesive vinyl.
They can be applied to and removed from most historic floors
without causing damage.
5. PROCESSING
The rapid development of computer technology over the last
10 years has made the use of digital imagery common place,
and allowed access to digital rectification and digital
photogrammetry software by most practitioners. Having said
that, the production of rectified photography by traditional
means is still a useful method.
5.1 Rectification
Rectified photography relies on the fact that a photograph
taken square on to a completely flat plane is analogous to an
orthographic projection of that plane. For this reason it is
most suitable when a floor is completely flat (see Fig. 3).
Slight undulations will produce slight scale errors but they
may be acceptable if within the scale tolerances required for
the survey. Larger undulations mean that an orthophotograph
will be required.
5.1.1 Analogue
The traditional analogue method of producing a rectified
photograph involves taking photographs as square on as
possible to the floor. They are then printed to scale in the
darkroom using an overlay plot of the control points. Each
individual print must then be spliced together so that it fits to
the adjacent prints and the control overlay. The edges of the
prints will be cut to follow lines of detail such as the edges of
tiles. In order to improve the appearance of the join when the
prints are stuck together, it is usual to feather the print by
removing some of the substrate paper using an abrasive. This
allows the remaining emulsion to be stuck down flush. The
resulting montage of prints is a one-off, so to make copies it
must be re-photographed with a large format copy camera.
5.1.2 Digital
The power of the standard desk top PC has reached a stage
where digital rectification is available to all. Professional,
fully featured rectification software such as Rolliemetric
MSR does cost in the region of 2500 Euros but less
sophisticated packages such as Monobild are available for as
little as 350 Euros. Digital rectification obviously requires
digital images and these are usually acquired by scanning
conventional negatives. Digital cameras and digital backs for
conventional cameras are reaching a stage of development
where it is now perfectly possible to use ‘bom digital’ images
for digital rectification. The size of the digital chip is still
smaller than a medium format film frame so more
photographs, more control and more time spent on
processing will be required. Professional rectification
systems usually allow for the correction of lens distortion
assuming calibration information is available. Others will
contain a calibration routine. Here, digital rectification has a
distinct advantage over analogue methods that allow no
possibility of correcting for lens distortion.
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