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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 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 73