Geotechnical News • March 2012
29
GEOTECHNICAL INSTRUMENTATION NEWS
Remote monitoring of deformation using
Robotic Total Stations (RTS)
Rob Nyren, Ryan Drefus, Sean Johnson
Robotic total stations (RTS) are
remotely operated theodolites that can
deliver continuous (24/7) near-to-real-
time survey measurements on reflec-
tive prismatic targets. They are also
referred to as automated total stations
(ATS) and automated motorized total
stations (AMTS). In the past 3-5 years
RTS systems have become an essential
component of performance monitoring
programs for urban infrastructure proj-
ects across North America. The essence
of the RTS system operation has been
explained by others in this publication,
including David Cook (GIN December
2006) and Allen Marr (GIN September
2008).The authors refer the readers to
these issues for additional information.
Applications
RTS systems are most frequently used
as a tool for monitoring deformation
of buildings and structures due to large
civil works. However the authors have
used these system to monitor many
other applications including load tests
(pile loading, lateral loading of bridge
foundations, static and dynamic load
testing of bridges), MSE wall per-
formance (wall face monitoring and
internal strain), ground deformation
monitoring around deep excavations
for power (please clarify), compaction
grouting beneath various structures,
automated crack monitoring on base-
ment walls. The application of RTS
systems is seemingly limitless.
Accuracy
The best instruments available coupled
with proper installations and best
operating practice deliver accuracies of
+/-0.5mm (0.02in). For this accuracy
it is reasonable to expect about 90% of
the readings within +/-1mm, and to see
statically “real” readings up to +/-2mm
every now and then. Consideration of
“relative movements” of targets can
yield much better accuracies (nearer
+/0.3 mm (0.01 in).
Main advantages
RTS systems deliver the highest quality
survey data from a fixed survey layout
with little manual field effort once
installed; multiple readings done at the
instrument instantaneously improves
overall precision, (why do you need to
refer to precision?) accuracy, and helps
to identify erroneous readings. Systems
can easily accept the addition of new
targets to accommodate unforeseen
monitoring needs with low cost. Newer
systems can capture photographic
images in conjunction with monitoring
to provide additional information and
insight.
Main limitations and other
performance considerations
Measurements from RTS systems are
optical with accuracy and precision (as
above) limited by many conditions,
such as weather changes, atmospheric
conditions, suspended particulate in
air due to construction, traffic, and
vibrations. Poor installations of RTS
instruments expose them to vandalism
and other severe weather issues. Main-
tenance of difficult-to-access locations
(e.g. an RTS high on a building facade)
can be both dangerous and expensive;
careful planning and system design can
reduce maintenance. The RTS system
by design concentrates all the monitor-
ing effort to the RTS; any failure of the
RTS (including power, remote access,
computer software) results in a total
failure of the monitoring program until
the problem is mitigated. Monitoring
points installed at extreme angles from
the reference points used for re-section-
ing the RTS can contribute to errors.
Large zones of construction influ-
ence often make finding an adequate
quantity of reference point locations
problematic.
Challenges
Many RTS monitoring systems used
for civil projects in the U.S. are com-
prised of multiple instruments in urban
settings. It has been the experience
of the authors that multiple units can
be ‘networked’ to overcome some of
the common limitations listed above
– notably a lack of good reference
sights. In a networked solution each
RTS shares common targets with other
RTSs. These common targets establish
redundant geometries between the RTS
positions and known reference loca-
tions, and the position of each RTS
can be solved using a least squares
adjustment solution. This process
minimizes random and systematic error
associated with raw measurements,
gives better solutions on RTSs with
poor referential control, and allows the
overall movement calculations to be
more statistically qualified. With these
improvements also come new limita-
tions: the loss of measurements from
any one RTS that provides observa-
tional continuity along the network can
cripple the ability for commercially
available software to process raw mea-
surements into monitoring data. Based
on this experience, it is recommended
that one (or more) spare RTSs be
maintained on each project to respond
quickly to potential issues when using
networked systems.
Commercial sources
Robotic total statin instrument
manufacturers include Leica, Sokkia,
Trimble. Implementing these systems is
best done by professionals experienced
with RTS systems (e.g. design, installa-
tion, operation, and maintenance); these
professionals are most often not tradi-
tional land surveyors but instrumenta-
tion specialists/engineers with broad
geotechnical and structural monitoring
expertise.
Rob Nyren, Ryan Drefus, Sean Johnson
Geocomp Corporation, 125 Nagog
Park, Acton, MA 01720
139 Fulton St., Suite 917 New York, NY
10038
E:
