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COMMERCIAL FEATURE
Lidar for fibre optic cable
retro-fitting Southern Mapping Company has been providing solutions to the power supply
industry since 1997, when it first began surveying new and existing power line routes
for Eskom. In 2013, an Eskom partner approached the company with a problem
regarding a power line in the Cape.
E skom needed to communicate
between two substations and
wanted to place a fibre optic
cable on the 56 km power line to achieve
this. To engineer a solution, accurate
knowledge of the position or sag length
of the conductors and the earth wires, the
power flow through the line as well as the
conductor temperature was required.
To measure the conductor geometry
in a useful manner, the entire line needed
to be surveyed under the same ambient
and power load conditions. This was
achieved by flying a Lidar survey of the
line, as rapidly as possible, at a time of
day when the weather conditions were
relatively stable, while the power flow
through the line was held constant. As the
power line crossed some of the highest
mountains in the Cape, a variety of
weather conditions were expected and the
terrain rose steeply from the inland side
of the mountains (540 m over 33 km) and
then dropped even more rapidly (900 m
over 23 km) from the highest point of the
line to the substation in the valley below.
The flight was designed in such a
way that thousands of Lidar pulse returns
were collected from each phase of each
span on the conductors, as well as the
earth wires. This would be important for
the geometric reconstruction in the post
processing phase.
During this flight, Southern Mapping’s
ground control team, in addition to
conducting a GPS survey for the Lidar
control, deployed its portable weather
station on site to collect the weather
data required to calculate conductor
temperature. The raw data was processed into
a Lidar point cloud, containing several
hundred million discrete points comprising
positions of ground, vegetation, buildings,
and other above ground man-made
objects, including, of course, power
line conductors and towers. This point
cloud was then classified to separate the
important features into separate classes.
These classes differ according to the
requirements for the survey, but in the
ESI AFRICA ISSUE 1 2014
case of a survey for an existing power line,
conductor as opposed to the earth wires.
the important classes would be ground,
Southern Mapping uses a variety
power line and other above-ground points
of technologies to provide solutions
(mainly vegetation and buildings).
to the energy sector. Its technologies
There are a number of reasons for
include airborne Lidar with orthoimagery,
conducting such a survey of an existing
hyperspectral and thermal imagery, as
power line. One is to identify whether
well as optical and radar satellite imagery
the clearances exceed the statutory
at a variety of resolutions.
requirements, in which case the line may
These tools allow the company to
be overdesigned and additional power
offer the following services:
can be fed through the line at little extra
• Low to medium resolution imagery
cost; alternatively, to identify those spans
and accuracy elevation models for site
where the clearances are less than the
and corridor selection;
requirements, in which case there are
• High resolution imagery and design
safety issues that have to be addressed.
level elevation models for final site
Clearances are measured to ground as
selection and design;
well as to objects on the sides of the
• Maps for vegetation species, soil
line, typically trees. For this project, the
chemistry, surface mineralisation and
requirement was to accurately model
ground and water pollution maps
the sag length of the conductors and
for environmental baseline and site
earth wires, for the installation of the
selection planning and monitoring;
communication cable.
• Power line profiling for final route
To do this accurately, knowledge of
selection and templating;
the conductor/ earth wire temperature
• Profiling of existing lines for ampacity
at the time of survey is important.
and uprating studies to optimise
Because it is difficult to measure this
power flow as well as to identify safety
directly, a variety of ambient weather
violations due to under clearance;
measurements are made during the flight
• Regular hyperspectral surveys
and these are then used to calculate the
for vegetation management and
conductor temperature. This temperature,
environmental auditing;
as well as the power flow figures from the
• Satellite surveys for property and
utility, was used in line design software to
servitude encroachment monitoring;
calculate the length of each span. During
• Monitoring of infrastructure for early
this exercise, it was identified that a
detection of subsidence in vulnerable
number of the spans had no earth wires,
areas. ESI
nor were the towers designed for the
www.southernmapping.com additional load of an earth wire.
The project scope was
modified to consider routing
the fibre optic cable via the
conductors. The Lidar data
was used in the design
software along with the
additional weight loading
caused by the fibre optic
cable to model a solution.
This solution proved to be
successful and for the first
time in South Africa, a fibre
optic communication system
Top and long section showing Lidar strikes on the power line.
has been wrapped around a
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