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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