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THE SEQUENTIAL TRACKING OF TARGETS IN A REMOTE EXPERIMENTAL ENVIRONMENT. 
T.A. Clarke, S. Robson, D.N. Qu, X. Wang, M.A.R. Cooper, R.N. Taylor. 
Centre for Digital Image Measurement & Analysis, School of Engineering, 
City University, Northampton Square, LONDON. EC1V OHB. UK. 
Email: t.a.clarke @city.uk.ac 
Phone: +171-477-8000 Ext. 3817 
Fax: +171-477-8568 
KEY WORDS: retro-reflective, centrifuge, targets 
ABSTRACT 
The application of digital image processing techniques within experimental apparatus from other academic disciplines is a recurring 
theme within photogrammetric research. For instance, H.G. Maas has applied 3-D measurement techniques to the mapping of 
turbulent flow (H.G. Maas, 1994). Such applications of photogrammetric techniques often produce new problems requiring solutions 
or improvements of existing techniques. Research has been conducted into monitoring the locations of targets within an environment 
subjected to high forces. Work to analyse video records is described which includes analysis of JPEG encoded sequences using a 
sensitive target detector and tracker as well as the use of a priori information concerning the initial target spacing to effect an 
approximate system calibration. An analysis of retro-reflective target properties has led to a redesign of the optics of the system for 
enhanced performance. 
1. INTRODUCTION 
In order to understand the detailed behaviour of geotechnical events and processes it is important to be able to observe how soils 
respond to load. Single element testing apparatus can be used to investigate the stress-strain behaviour of soil when subjected to 
particular stress paths. However, the response of geotechnical structures is the integrated effect of a large number of soil elements 
each following its own particular stress path. It is therefore of major importance to be able to measure displacements and hence 
strains during real geotechnical events. Instrumentation of prototype structures can yield valuable results, but much more can be 
learned from comprehensive test series on small scale geotechnical models. 
The behaviour of geotechnical structures is dominated by self weight effects. The driving force is often the self weight of the soil 
(e.g. embankment loading, tunnels) and the strength is related to in situ effective stresses which are in turn related to the weight of 
soil. In order to study the behaviour of geotechnical structures using physical models, the main requirement is to be able to create in 
the model stress profiles corresponding to those in the prototype. This can be achieved by accelerating small scale (1:7) models to 7 
times Earth's gravity using a geotechnical centrifuge. Thus a 10 m. layer of soil can be represented by a 10 cm. deep model of the 
same soil accelerated to 100 g because the reality and the model will then experience the same self weight stresses at homologous 
points. The development of centrifuge testing and its application to a wide range of geotechnical engineering applications are 
described by Taylor (1995). 
Centrifuge testing allows the study of geotechnical processes in scaled models with properly established scaling laws relating the 
model to the corresponding prototype. It is a technique particularly useful in the study of mechanisms of collapse and deformation; in 
this context parametric studies are often undertaken. Centrifuge tests have proved to be a major source of high quality repeatable data 
which are essential for verification of numerical analysis. Particularly valuable data are the movements in vertical sections of plane 
models which can be observed through a perspex window in the side-wall of a model container. These subsurface deformations can 
be compared directly with those from finite element predictions and can be used to test and improve constitutive models of soil 
behaviour. - 
In order to monitor such movements, the technique commonly adopted is to place markers or targets in the soil face which is in 
contact with the window. A closed circuit television system allows these targets to be viewed during centrifuge flight. Thus, by 
measuring the position of these targets in video images, displacements in the model can be determined. A model width is typically of 
the order of 500 mm. which in an experiment at 100 g represents a prototype distance of 50 m.. The most useful measurements of 
displacement will need to have an accuracy of 0.01 - 0.1 mm. i.e. 1 - 10 mm. prototype scale. This paper describes the development 
of a system which allows such measurements to be made. It is illustrated by reference to a project investigating subsurface ground 
movements due to tunnel construction. 
2. ANALYSIS OF THE CURRENT SYSTEM. 
The centrifuge optical system consists of a Toshiba IK-M36PK miniature colour camera mounted approximately 400 mm. from the 
front face of a thick perspex block. The signal from this camera, which was mounted in the 100g environment, is sent to the camera 
interface unit mounted close to the axis of rotation and consequently subjected to low g forces. From there the signal is transferred 
via coaxial cables through a slip ring assembly manufactured by Pandect. Two of the 130 Silver Graphite contacts on Silver Graphite 
rings were used. The video signal is then routed across several metres out of the environment of the centrifuge to a viewing station. 
IAPRS, Vol 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995