416 
Blake 
A number of the original castings have cracked or snapped due to the above stresses. The failure and subsequent repair or 
replacement of components reveals much of the movement take up prior to 1973. The snapping of the radials on the S side of 
the span, the compression of the chords of the main ribs and the displacement and twist across the deck are all recorded to a 
consistent 3D positional precision of +/-25 mm for the model and +/- 10 mm in the photogrammetric wireframe. 
Recording the repair histories of the components in a 3D framework to enable the survey data to be used as a GIS for informing 
future projects 
3. SURVEY TECHNIQUES 
3.1 Control 
A prerequisite of producing a complete survey of the bridge was the use of a common control. Thirteen stations were set out on an 
adjusted traverse. These were then used to ensure that the data produced by the different survey techniques was compatible. Control 
for photogrammetry required the stations to be occupied for the recording of 600 control points on the bridge. Observation was 
carried out by two-point intersection to detail points on the structure rather than marked targets. 
3.2 Metric Survey Methods 
The Iron Bridge presents a number of problems to the surveyor; the need for a wide range of scales from large [1:50] to small [full 
size], line of sight obstructions and access affected all the applied techniques. Lighting and vegetation were difficulties for photo- 
based techniques. When access was possible by scaffold instrument and hand survey techniques could be used, however gaps in the 
data set remained. Filling these gaps provided an opportunity to evaluate the performance of laser scanning, when applied to the 
rigorous levels of precision required to the survey of an historic structure of this kind. 
3.3 Photogrammetry 
Stereo-photography for photogrammetry was acquired from camera positions on the riverbank and also under the bridge at the 
footings, the photography was lit by available daylight. Vegetation obstructing sight lines from the riverbank and obscuring the 
retaining facades required the supplementary use of historic (1972) stereo-photography. The soffit of the span was obscure due to 
limited access, poor lighting and near camera obstructions. This resulted in a lack of cover of the soffit at the centre of the span. The 
camera used in both cases was a Wild P31 at a range of approx. 40 m and the Ziess UMK 300 mm for ranges exceeding 40 m.