C1PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey 
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at an oblique angle from, for example, the triforium in a 
cathedral. When the photography is to be used for 
rectification or photogrammetry the camera needs to be as 
square on as possible to the subject. Also each individual 
photograph must have a negative scale commensurate with 
the anticipated scale of the final product. As a general rule 
the final product should be no more than a six times 
enlargement from the negative scale - so a 1:10 scale 
orthophotograph would require photography at a minimum 
of 1:60 scale. If using a 50mm lens this means the camera 
would need to be a maximum of 3 m from the subject. These 
constraints can be met in a number of different ways. 
Figure 1 Rollei 6006 semi-metric camera set up on an 
extension bar with a Macbeth chart in the image. 
The most common method is to use an extension bar and a 
tripod (see Fig. 1). This allows the camera to look straight 
down at the surface of the floor without imaging the tripod 
legs. There is, however, a compromise to be made between 
the length of the bar and the number of photographs required. 
A long bar will allow a photograph without any interference 
from the tripod legs but as the bar will tend to oscillate, it 
will be necessary to wait a long time after moving the camera 
set up (until the camera is still) before taking the next 
photograph. The possibility of vibration will also limit 
exposure times. A shorter bar will not oscillate but it will be 
necessary to take more photographs because tripod legs will 
appear in a portion of each photograph. Each strip of 
photography will have to be substantially overlapped by the 
next in order to achieve complete un-obscured coverage. 
Another method of achieving the correct camera to subject 
distance is to take oblique photographs from a tall tripod. 
Whilst digital rectification software and to a lesser extent 
digital photogrammetric systems can accommodate oblique 
photography it is not an ideal solution. It should be 
remembered that because the photograph is oblique the scale 
will vary across the image. Therefore it will be necessary to 
either take a photograph where part of it is at a larger scale 
than required or to discard part of the image because it is at 
too small a scale. Using a tall tripod has its own practical 
difficulties. In order to look through the viewfinder the 
photographer will require a step ladder and, if the surface is 
rough, a colleague to steady it. If the floor is particularly fine 
or delicate it will be necessary to protect it from the feet of 
the ladder in some way. 
If the only suitable camera available has a longer lens it will 
be necessary to set the camera further away from the subject 
to achieve economical coverage. A camera with a 100 mm 
lens could be set up 6 m from the floor in order to produce a 
negative scale of 1:60. A vantage point 6 m above a floor 
could be found using a scaffold tower or a hydraulic lift. This 
sort of equipment brings its own problems, however, and 
would only be appropriate in very specific circumstances. An 
archaeological site where a floor has been revealed and has to 
be recorded rapidly is perhaps one example. Otherwise the 
problems of using a hydraulic lift inside a building or moving 
a scaffold tower without damaging the floor are probably 
insurmountable. 
The English Heritage Metric Survey Team generally use a 
standard photographic tripod with a short extension bar. 
Using a 50 mm lens with the camera about 1.5 m above the 
subject gives a negative scale of 1:30, allowing for final 
products at up to 1:5 scale. Although good quality 
photography is obtained relatively easily this approach does 
result in more photographs and therefore more control than 
strictly necessary. For most work a Rolleimetric 6006 semi 
metric camera is used with Kodak NCI60 colour negative 
film. Colour negative is used as opposed to colour 
transparency because of its wider exposure latitude. Kodak 
NCI60 is a professional film for use in controlled lighting. 
Each batch is guaranteed consistent so only film from a 
single batch is used on any one project. The NC (Natural 
Colours) prefix means there is no particular colour bias and 
the very fine grain gives better resolution, although it 
requires a longer exposure time. The camera has a reseau 
plate fitted and has been calibrated so that it can be used in a 
digital photogrammetric workstation (DPW). The use of 
calibration information also improves results in Rolleimetric 
MSR, the digital rectification software used by the team. To 
obtain correct and consistent exposure a professional spot 
meter is used. 
4. CONTROL 
A photographic survey product, be it a single scaled image, a 
rectified photography montage or an orthophotograph, 
requires some sort of control. This can range from a simple 
scale bar through to targets with 3-D co-ordinates. 
A single rectified photograph can be produced using only a 
scale bar. To achieve accurate scaling in both axes, however, 
requires two scale bars or known distances. The distances can 
be measured between points of detail on the floor or targets 
can be attached. If more than two photographs are to be 
montaged together, a grid of targets will be required with at