Geotechnical News • June 2012
27
GEOTECHNICAL INSTRUMENTATION NEWS
Remote monitoring of deformation using
Digital Photogrammetry
Raul Fuentes and Stuart Robson
Principle of operation
Digital photogrammetry is an optical
measurement technique that allows
the accurate computation of the size,
shape and position of a 3D object by
measuring discernible features in two-
dimensional images. The method sup-
ports single images, pairs of images
and networks of images taken around
an object. Images are captured either
in a single instance with several cam-
eras or as a sequential set over time
moving a single camera from location
to location.
3D coordination is based on triangula-
tion whereby every feature measured
in an image provides data analogous to
the horizontal and vertical angles pro-
vided by a theodolite. Key differences
are that multiple features of inter-
est are captured at the same instant
rather than sequentially and there is
generally no requirement to setup and
level a camera over a known point.
The location and orientation of each
image is modelled either, singly, as a
resection or in combination with the
complete constellation of images with
a network or bundle adjustment.
In its most accurate form, where a
single camera is used to take a net-
work of images that converge towards
the object, it is possible to utilise
off-the shelf camera technology and to
ascertain the optical properties of the
camera at the same time as imaging
the structure. This process is termed
self-calibration. Where a constellation
of cameras are used, cameras must
either be purpose designed for photo-
grammetry or pre-calibrated.
Main fields of application
Photogrammetry can be applied to
any structure (e.g. bridges, heritage
structures, deep excavations, build-
ings, dams, tunnels and wind turbines)
and is particularly effective for those
exhibiting complex or rapid motion.
Accuracy
Accuracies of the order of +/- 2.0mm
are achievable. Principal parameters
governing accuracy are: the features
to be measured; the geometry of the
imaging network, comprising the
number of images, their distance from
the object and degree of convergence;
the physical stability and calibration
of the camera(s); the effectiveness of
the features measured in the imagery
and; the geometry and accuracy of any
reference targets or scale bars used to
define the coordinate system.
The use of photogrammetric targets
allows image measurements to be
much more accurate and repeatable
than using natural features. For the
highest accuracies, circular retro-
reflective targets occupying between
5-15 pixels in each image are used.
Main advantages
The main advantages are: Equip-
ment is economical compared with
other remote monitoring techniques;
Photogrammetry is non-contact,
non-destructive and can be real-time;
Data capture and use is flexible, safe
and not time consuming; Simultane-
ous full-field capability gives it a
great advantage over single-point
sensors since a complete structure
can be captured through the instan-
taneous coordination of hundreds of
targets, features and surfaces allow-
ing “Monitoring for the unexpected”
and; Images add value: contributing to
construction records; as-built surveys;
characterisation of rock faces and;
area and volume calculations.
Main limitations
The main limitations are: Control
targets coordinated by conventional
survey are required if the results are
to be expressed in a particular coordi-
nate system. However, the stochastic
properties expressing the quality of
the 3D data and the coordinates must
be both transformed; Processing can
be time consuming as automation is
dependent on solving which feature
is which within the image network
and; In general, accurate photogram-
metry, particularly where real-time is
a requirement, needs the support of a
specialist.
Future challenges
A challenge for photogrammetry is
through its adoption within terrestrial
laser scanning instruments since this
offers the best of active and passive
imaging solutions. However due to
the low cost of off-the shelf cameras,
where targets and highly dynamic
structures are concerned it is likely
that digital photogrammetry will
continue to provide a leading edge
solution.
Mr R. Fuentes and Prof. S. Robson
Dpt. of Civil, Environ. & Geomatic
Eng., UCL, Gower St., London, UK,
WC1E 6BT. T: +44 207 679 1570,
E:
/ s.robson@
ucl.ac.uk
