Proceedings 18 Ih International Symposium CIPA 2001 
Potsdam (Germany), September 18 - 21, 2001 
USE OF INTEGRATED SURVEY TECHNIQUES: MEASURING THE IRON BRIDGE 
Bill Blake 
English Heritage: Metric Survey Team 
NMRC 2 Kemble Drive Swindon Wiltshire SN2 2 GZ United Kingdom 
Phone: 01793 414802 email bill.blake@english-heritage.org.uk 
KEY WORDS: Industrial Heritage Documentation, CAD, 3 D Modelling, REDM, Photogrammetry, TheoLt 
ABSTRACT 
The construction of a high parity 3D CAD model of the historic Ironbridge has demonstrated the value of integrating survey 
techniques to deliver spatial data of interest to historical analysts, conservators and presentation professionals. The performance of 
wireframe from photogrammetry as 3D CAD modelling source requires the use of infd techniques such as measured drawing, 
realtime CAD/REDM and laser scanning. The detailed survey needed to model the structure has added to the understanding of the 
manufacture and construction sequence. 
1. BACKGROUND 
The Iron Bridge at Coalbrookedale was scheduled as an Ancient Monument in 1934 and is the centrepiece of the World Heritage 
Site designated by UNESCO charter in 1986. The bridge is the world’s earliest major iron span and is the prototype for iron bridge 
construction. It is uniquely important as the first structure to use iron at an industrial scale. The manufacture of the bridge 
components is a unique example of 18 lh century quality controlled production of structural iron. 
The bridge is constructed of large cast iron parts (the largest weighing up to 5.5tonnes) cast, positioned and fitted in 1779 under the 
direction of Abraham Darby III master iron founder and Thomas Gregory his pattern maker. The form of the bridge is derived from a 
design by Thomas Famolls Pritchard, the architect directed by the bridge commissioners in 1776. There is no surviving copy of 
Pritchard’s drawing other than early scheme drawings for iron spans so it is an open question as to how much of the erected structure 
is from Pritchards design and how much is a result of foundry pattern work. 
The historic central span of cast iron is of 30.12m. It is made up of 5 frames supporting a roadway of 42 cast deck-plates. The span 
is an arch of a near perfect semicircle standing on stone abutments. The deck rises at an angle of aprox. 5deg to a shallow arc 
joining the 2 pitched sides of the deck. 
In 1999 the bridge was found to be in need of painting as surface corrosion had become extensive and there had been some loss of 
the bearing between the deck-bearers and the deck requiring consolidation. 
2. METRIC SURVEY REQUIREMENT 
The proposed works needed metric survey drawings for scaffold design, marking up the painting regime and recording repairs. 
Metric survey was also used for the following purposes: 
• The verification of historic photogrammetric survey from 1977, the previous survey was incomplete due to obscured areas in 
the photographs, the opportunity was taken to use the earlier work in the new survey; 
• Data from archaeological investigation was plotted onto 1:50 ink on plastic photogrammetric plots to provide a precise base for 
the record of the type and phase of the bridge components in 2D. 
• Acquiring a better understanding of the structure, there are many gaps in the knowledge of both the design and construction 
phases of the bridge, the 3D record enabled theories to be tested against true to scale information. 
• The presentation of the monument to visitors with the use of 3D survey data in virtual and on site interpretation using high- 
resolution CAD models. Visitors should be able to observe the structure from viewpoints not possible in any other medium. 
• Surveying in 3D enabled the twists caused by post erection deformation to be mapped, enabling stress analysis of the bridge and 
so failure mapping and prediction. 
The structure has undergone a number of movements since erection in 1779. The rotational thrust between the abutments and 
the pressure of the unconstrained stone work lead to a rebuild of the approach arches in 1821 and continued movement of the 
footings resulted in the below water ferro-concrete retaining work of 1972-3. The shape of the frames spanning the river has 
been distorted as a result of this and this needed to be recorded as a basis for monitoring and understanding future movement.