LIDAR Surveys Electric Transmission Lines

Airborne Lidar is now a well proven technology for providing accurate elevation models for transmission lines, allowing utilities to measure the shape of the ground below the transmission line, the position of the towers and poles, the sag on the wires, and the up-growth of any vegetation incursions or other possibly illegal incursions into the right of way.

Lidar has shown its value in providing precise location of poles or pylons (usually within a foot or two), very accurate digital terrain models, the actual elevations of the conductors above the ground and vegetation (allowing the clearance to be measured), and the subsequent modeling of the change in sag if additional power is pushed through the conductors. Currently there is no other efficient way of accurately measuring the actual height of the lines above the ground.

Lasermap-GPR Consultants has been doing airborne Lidar surveys of transmission lines since 1997. A long established land surveying and mapping company (founded in 1980), they acquired one of Optech’s first Lidar systems in 1997 and subsequently in 2003 an Optech 2050. This is longer than any other commercial Lidar provider. The company has completed thousands of square miles of Lidar mapping projects, and during the last two years has developed proprietary software and methodology to detect small undulations in flat flood plains and eliminate much of the “noise” in the Lidar data which occurs where there is low vegetation. Their proprietary software is able to complete the interpretation of these data and in tests has proven more than 90% effective in determining true ground elevations in difficult vegetation covered areas.

For transmission line surveys, Lasermap provides accuracy of Lidar data in the region of 6 inches absolute and closer relative. This means Lidar point to Lidar point data is usually within 2-3 inches in accuracy, providing a very close accuracy rating between points along the line. However, compared to local geodetic benchmarks (ground control) the absolute accuracy of any point is around six inches.

At the same time as the Lidar data are collected, color digital images are collected to provide even more information about the conditions along the right of way. These are normally provided at a minimal cost unless they need to be rectified to form a seamless mosaic. Each individual picture is registered with its center point having a GPS photo center coordinate, so it is easy to locate the image anywhere in the system (i.e. it can have a UTM coordinate or even a latitude and longitude which effectively provides a specific location compared to anywhere in North America).

Utilities can effectively import the Lidar data and then calculate the amount of sag that will occur to the conductors if more power is pushed through the system. They will then indicate the amount of clearance to the ground or vegetation when this is done.

The Lidar data provides a lot of engineering possibilities, much of which can be done without actually leaving the office. New lines can be engineered with tower or pole placement without leaving the office. Some utilities have fully engineered transmission lines without actually going out on the ground until they are ready for construction. All of the tower placement can be done using the Lidar data in the office and programming a GPS unit so the construction crews know exactly where to locate each tower.

Costs associated with such projects are determined by how much of the system is fairly straight major power lines (say metal towers 132+ Kilovolts with relatively few bends, and how much is lower voltage pole lines where there are lots of twists and turns.) Very twisty lines tend to cost more because the aircraft or helicopter has to fly slowly or having to stop and change direction. Lasermap is able to compute the sag of wires on existing lines. However, this can add substantially to the cost of the project as each span has to be computed. Some clients want to keep costs down and perform these calculations themselves on an as needed basis. On the other hand, some utilities do not care to purchase the software to do all of these calculations themselves, and are willing to pay for a complete solution.

The Lidar data can be separated into 3 separate files: ground files, vegetation files and conductor files. They can be viewed independently or combined to do calculations. Typically, on larger structures more points are able to be collected, however on single pole structures the pole can be missed. In this instance, the conductors get picked up and by intersecting the catenaries of the conductors, the pole can be located, usually within about a foot. The image also helps in this respect - as you can usually see on the image what type of structure it is.

Following the acquisition, all of the Lidar data are processed in the field to ensure completeness and integrity. This step is a quality control process to ensure that coverage is sufficient and that all systems were functioning within specifications. If any problems with the data are determined, it allows the crew to isolate problem areas and re-fly the affected portions before demobilizing.

Most of the primary data processing is completed in the field. However, final processing is completed back in Lasermap/GPR’s headquarters outside of Montreal, Quebec. This includes a final classification of the Lidar data to complete the removal of vegetation and building effects. This involves a process whereby the data points are put through an intensive custom filtering process, and various classes of points are separated. Vegetation data are removed from the ground layer data, as well as buildings and other above ground structures. A final editing of the data ensures that no biases or data spikes are evident on the delivered data.

In the many years that Lasermap has been conducting transmission line Lidar surveys, they have found the key benefits are the ability to provide precise pole locations, very accurate digital terrain models, the actual elevations of the conductors above the ground and vegetation, and the ability to model the change in sag if additional power is pushed through the conductors. There is no other way of accurately measuring the actual height of the lines above the ground.

Lasermap/GPR Consultants is part of the Spatial Resources Network. To learn more about Lasermap or Lidar, how they can benefit your spatial information needs, or just for general information, contact us: On the web, www.spatialresources.com; by email, info@spatialresources.com; or call one of our advisers at 720-934-2482.

Until next time...Cheers!

Roland Mangold