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Geotechnical News • March 2017
GEOTECHNICAL INSTRUMENTATION NEWS
Manual reflectorless total station monitoring (MRTS)
Colin Hope and Stephen Dawe
Introduction
MRTS is most commonly used to
monitor deflections of shoring and
structures where it is not possible to
install targets, most commonly due to
safety or access issues. It provides a
safer, easier and more cost effective
way to manage the risk associated
with working close to sensitive build-
ings, structures and infrastructure.
Under typical field conditions, if work
is carried out methodically with a high
standard of care, accuracies can range
from +/-4mm to +/- 2mm depending
on atmospherics, line of sight clear-
ance and the background surface that
the measurements points are on. The
background surface will impact the
reflectivity of the Electronic Distance
Measurement (EDM) technology used.
Measurements have less background
scatter when they are taken to flat sur-
faces while rough surfaces will cause
more scattering.
History
Some of the more recent advances
in distance measurement technolo-
gies occurred in the twentieth century
with the introduction of radar in the
1940s, and then in the 1960s with
advances in laser technology we saw
the emergence of Electronic Distance
Measurement (EDM) technologies.
Until recently, the only means of mea-
suring a distance electronically was
by combining EDM technology with
a retro prism. In the past 20 years, one
of the most significant advancements
in EDM technologies has been the
introduction of MRTS.
The following is a partial list of
some of the questions that need to
be answered when planning to use a
MRTS (this is in addition to any other
steps carried out for the monitoring):
• Is it repeatable?
• Is the correct instrument for the job
available?
• How far away or how close are the
points to be monitored?
• What is the background surface to
be monitored?
• What colour is the background
surface?
• Is the background surface smooth
or rough, are there holes in the
surface?
• Will the same geometry be useable
for the life of the project?
• What are the tolerances for the
project and what sort of accuracies
are needed?
• Is it doable?
• What sort of affects will the at-
mospheric conditions have on the
readings?
• Has this type of structure been
monitored before?
• Are there objects that lend them-
selves to becoming monitoring
points?
• Do you have traditional controls or
reflectorless controls?
In this article we discuss MRTS in
regards to when it should be used and
when it shouldn’t be used. Some of the
important considerations include:
• Instrument selection
• Background noise and scatter
• Selecting and initializing the moni-
toring points
• Accuracy and precision
• Measurement on two faces
• Perpendicularity
• A precise versus a MRTS experi-
ment
Instrument selection
It is important to understand that most
MRTS instruments are not suitable
for precision monitoring. A precision
MRTS instrument is generally one
with published specifications of 2 mm
+ 2 ppm or less in the Electronic Dis-
tance Measurement (EDM) and one
second or less in the angular measure-
ment. The instrument used in the writ-
ing of this article was a Leica TS30.
Ideally, instruments manufactured for
precision monitoring will offer the
best accuracy for the measurements,
as they are purpose built for the job
and come with on-board compensators
and specialized MRTS measurement
technology.
Background noise and scatter
By background noise we generally
mean the scattering of the laser as
it hits the measurement surface and
bounces back towards the instrument.
A stucco wall will have much poorer
reflectivity than a smooth concrete
surface. Also, the lighter the surface,
the better the return of the laser will
be. Some other forms of background
noise are taking readings over dis-
tances greater than 80 meters, which
tended to fall within an error bar of
+/-4mm while closer readings were
observed to be more accurate and fell
within the +/-2mm range. High humid-
ity, fog, precipitation of any kind,
bad lighting, vibration, dust, smoke
and strong winds will all impact the
accuracy of the measurements. Figure
1 shows a poor surface for MRTS
monitoring due to the rough texture of
the background surface.
Selecting and initializing the
monitoring points
To gain the most reliable repeatability,
it is vitally important to select points
that are readily identifiable through
