Aerial Mapping Spring '13 - Aerial Mapping Spring '13
Waking up from the calibration nightmare
by Konstantin Konakov, Director General, Opten ZAO
Delivering high quality, engineering-grade models of overhead lines (OHL) remains a challenge for even the most experienced airborne lidar surveyors. We are always trying to strike a balance between controlling costs and operating efficiently while still delivering accurate and actionable data products.
OPTEN, located in Moscow, Russia, specializes in transmission and distribution OHLs. We have flown over more than 100,000 km of power transmission lines in 15 countries over our 15-year history. We also develop engineering solutions for increasing the power transmission capacity of existing lines and modeling extreme weather cases to detect line clearance issues.
Calibration is critical for a survey’s success – but wrestling with the data and processing that often takes one to two days makes calibration a nightmare.
OPTEN’s experience with Optech ALTM systems goes back 15 years; we knew Optech could offer a solution that would lower operational costs, and reduce and simplify calibration while maintaining or exceeding our existing data accuracy. Recently, we selected the Optech Orion C300 ALTM based on its ability to capture thinner, underlying wires accurately. The Orion C300 outputs full-waveform lidar data at up to 300 kHz from 150-1000 m AGL, with an advertised elevation accuracy of <3-7 cm (1 sigma) and an excellent vertical target separation distance of only 0.7 m. For this particular survey we anchored the Orion C300 to a forward-tilting basket mount on our AS 350 helicopter.
We also chose the new Optech LMS, a high-performance production processing workflow.
By deploying Optech LMS, the one-to-two-day calibration effort became a thing of the past. We replaced it with a daily calibration process that was shorter, simpler and resulted in a higher-quality data deliverable. Processing the flight test data for calibration with Optech LMS in our particular case required only around 30 minutes a day, plus ten to 15 minutes of flight time for the crosscheck line. Reducing the workflow to minutes made a huge difference to the success of the project – the calibration nightmare was over!
The auto-calibration capabilities of Optech LMS enable us to deliver the lidar point cloud more quickly than ever. For every flight we first calculate corrections to the angles (roll, pitch, heading) on one patch of data, and then check the validity of the corrections on another patch of data from the same flight. In the end we output data from all the flights using confirmed corrections to the angles, a procedure that is becoming a part of our ISO Quality Assurance plan for Orion surveys.
After around 120 flight hours and 2000 km with our first survey project with Optech LMS, we found the parameter scan scale required calibration only once – at the beginning of the project. The corrections for roll, pitch, and yaw/heading were minimal. When we compared our data with ground truthing measurements we realized that we did not even have to adjust horizontal coordinates, as we could get the required accuracy with roll, pitch and yaw adjustments only.
Optech LMS processes lidar data with a multi-thread approach on multi-core computers, outputting point cloud data at an unprecedented rate. This fast processing is extremely important on the first day of the survey project, when we run iterative calibration quality checks to gauge the system’s readiness before signing off on it. We then process the Optech LMS output in our Moscow office with our in-house software. Our engineers produce PLS-CADD models of OHLs and detect all the structures that influence the safety and reliability of a network, such as telecom lines, metallic fences, SWER (singe wire earth return) lines, rivers, encroaching vegetation, and other features. Because different objects require different clearances, review of high-quality photo images is essential in order to properly classify all features in the models.
Investing in the Optech Orion C300 ALTM and Optech LMS production workflow was definitely a sound decision. Optech’s reputation for high-precision lidar systems ensures accurate results; the noise of the rangefinder is so low we are able to detect a <5 cm difference between flight lines in overlapping areas and correct them during auto-calibration. Furthermore, our survey flights confirmed that the vertical accuracy of the Orion C300 was very close to the accuracy of the resulting SBET file (<0.04 m, 1 sigma). Optech’s simplified workflow makes a significant difference in the calibration effort and makes post-flight checks for calibration drift – a recommended step – much more practical. The time saved in calibration means we can begin a survey almost right away, which lowers overall costs. With the calibration nightmare over, we now have sweet dreams of happier customers and smooth future projects.
About the author: Konstantin Konakov is Director General at OPTEN ZAO and has overall responsibility for lidar activities and power line modeling for the OPTEN Group of Companies.
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