Power lines, insulator marks and wire slack.
Accuracy assessment

Power lines inspection with UAVs

The survey and inspection of power lines with unmanned aerial vehicles have become quite widespread. Survey data are used for cadastre, engineering and surveying. Quite often the question arises: is it possible to obtain elevation marks of power lines, sags, wire dimensions and other metric characteristics using a UAV? And with what precision?

The main sensor of the UAV is a camera. The 3D image is built from overlapping photographs and information about the exact centers of photography. But as always, there are nuances. After processing a set of images of power transmission lines in photogrammetric software, you will not see supports and wires in 3D. The most that you can get is a dense cloud. Unfortunately, only a few of the many supports will be reconstructed properly. See example below:

The wires are not reconstructed in 3D by photogrammetric software because it is a very thin object for photogrammetry. The program simply does not pick up a long string several pixels thick.

In this case, computer vision comes to the rescue and there are a number of developments of various companies for the automated search for wires and their dimensions.

At Aerostream we developed our own technology for determining wire marks using standard tools.

As a result, we can provide marks of wires and ground with high accuracy, as well as determine wire dimensions, sags, angles of turns, etc.

Checking the accuracy of the power lines elevation marks measurements using UAVs

In order to check the accuracy of the wire marks we surveyed a small section of power line. First, we set up ground control points and obtained their coordinates using the EFT M3 GNSS in RTK mode from the Geospider network. Using the AR600E high-voltage cable height meter, we measured the height of the bottom wire at each mark.

Thus, the instrumental method was used to obtain the dimensions of the wires for each control point. Next, we conducted aerial survey of the site and determined the height of the wire above the identification using the photogrammetric method, using only the data from the drone survey. Last step was to compare the instrumental measurements with data obtained from the drone.

The aerial survey was performed with a Phantom 4 Pro v2 PPK survey grade quadcopter from uav-design.com

Coordinates of image centers were processed in Magnet Tools software. Photogrammetric processing was carried out in Photoscan Pro (Metashape). Image capturing was made from 100 meters altitude in two passes. Survey technology with a geodetic drone from uav-design allows you to consistently obtain high accuracy results. For our flight, we got average camera alignment error within 2 cm. The smaller the error in the relative position of the cameras, the more accurate the wire marks will be:

The root mean square error at ground control points was also several centimeters:

Using our own technology, the marks of power lines were marked in places above the signs. In total, 5 marks were taken by the photogrammetric method at different sections of 2 spans of power lines

 

The summary table below shows the difference in the height of the wires measured instrumentally and using photogrammetric method. The maximum deviations is 12 cm, with an average deviation of just a few cm

 

Conclusion: Photogrammetric survey of power lines with drones using the technology developed by Aerostream makes it possible to obtain wire marks with an accuracy of about 5-12 cm.