Jubilee Line Extension
intercomparison
Project Summary
In February 2003, NPA with a team of consultants
were appointed by ESA to undertake one of the GMES projects.
The project, known as 'Terrafirma' involves the development
of an operational service for the pan-European monitoring
of terrain motion using the PSI technique from a satellite
platform. In Stage 1 of the project the focus was on monitoring
of terrain motion in urban environments. To assist in
the evaluation of the potential Terrafirma service offering,
NPA co-opted a Core User Group to consider user requirements
and applications. Arup, as consulting engineers, were
appointed by NPA to participate as part of this Core User
Group to provide an independent opinion as to the potential
application of PSI monitoring in the civil engineering
sector.
The Core User Group identified that to achieve
a sustainable penetration into the monitoring market,
convincing data were required that compared results from
the PSI monitoring with those produced by conventional
monitoring systems. To this end Arup with NPA undertook
an initial comparison of PSI monitoring results with terrestrial
monitoring data at a single site.
The Arup study considered a single building
(The Treasury Building) on the Jubilee Line Extension
Project in central London and semi-quantitatively compared
groundtruth data from precise levelling with PSI monitoring
results subsequently derived from the archive of radar
data. The PSI data was provided to Arup in a fully processed
form via NPA. The study considers PSI from viewpoint of
a civil engineer, treating the technique as black-box
technology. This is consistent with the aims of the Terrafirma
project, where the aim is to deliver a monitoring service
to a wide range of users who simply want motion data without
detail of the underlying technology.
Based on the initial study, the following
conclusions can be made:
-
The Terrafirma H-1
product, when presented in 'deformation map' format
appears to have the potential to identify areas of ground
hazard due to settlement in urban areas. This has potential
application at the site selection stage and in providing
a synoptic view for activities such as urban tunnelling.
-
At the moment deformation
maps are best used to assist in the identification of
potential areas of ground hazard rather than as a tool
to derive quantitative measurement data.
-
It is not clear at
this time why there is such a significant difference
between the PSI-calculated displacements and the research-quality
groundtruth data for the JLE project. Furthermore, work
should be undertaken to evaluate the accuracy and precision
of the vertical movement data in terms of 'deformation
maps' and 'time-series' . The accuracy and precision
of georeferencing of PS points also needs to be evaluated.
-
Ground motion due to
tunnelling is by definition a non-linear ground motion.
Guidance on the selection of 'linear' of 'non-linear'
displacement models is needed.
-
Results from the ESA
'PSI Codes Cross Comparison and Certification' (PSIC4)
project should be compared with the initial results
from this study.
-
The groundtruth on
the JLE is of outstanding quality and it is recommended
that further work is undertaken to understand the PSI
data at this site before embarking on validation studies
at similar sites.
