Case Study: ALTM Orion C200
New Eye-Safe Airborne Corridor Mapper Provides Outstanding Data Quality, Accuracy and Precision
By Valerie Ussyshkin, Livia Theriault, Martin Pokorny, Optech Incorporated
Lidar data has been widely adopted as an efficient and effective tool in the planning and management of power line and utility corridor assets. To better serve this segment of the lidar community, Optech
developed a new class of Airborne Laser Terrain Mapper (ALTM™), the Orion C200. Designed specifically for maximum efficiency in corridor mapping applications, the Orion C200 delivers the highest data precision and accuracy possible from an airborne sensor.
The Orion C200 incorporates a 1.5 micron laser, enabling complete eye-safe operation at extremely low altitudes (aided NOHD = 7 m), a critical feature often required for power line surveys and other corridor-type projects. To ensure complete eye-safe operation, the emitted energy of the laser pulse has to be reduced. However, when laser energy is reduced, there is potential to affect overall data accuracy and precision, and specifically, the ability to detect small objects such as power transmission or distribution wires. Considering power transmission lines as one of the most typical examples of the surveyed corridors, the question about the accuracy of the lidar data comprising wires and ground data would be the most important for the further analytical evaluation of the surveyed transmission line. This case study shows that, owing to its uniquely efficient system design, the Orion C200 has overcome the challenge of balancing complete eye-safe operation with exceptional data quality, accuracy and precision.
A new level of lidar data accuracy and precision consistent with outstanding small object detection capability is demonstrated.
Methodology and Results
A fixed-wing aircraft equipped with the Orion C200 flew over a set of power lines and control ground at an altitude of 500 m above ground level (AGL) with the laser Pulse Repetition Frequency (PRF) set to 200 kHz. Additional datasets over ground control were collected at higher altitudes using various PRF to analyze the consistency of the lidar performance under different operational parameters.
The set of power lines in this study was arranged in the following configuration: transmission wires are positioned at four levels—three levels of conductor bundles, and a top-level wire. The physical properties of the top wire assembly—typically 3/8” (≈ 9 mm) diameter steel wire—make it the most difficult wire for an airborne lidar to detect. This is due to partial signal return, which is typically several times weaker than that of a full return from the ground beneath the set of wires. It therefore represents a challenge to the lidar receiver to detect without loss or saturation of signal levels from the top wire and ground returns, respectively.
The ability to do so effectively, while providing complete eye-safe operation, without sacrificing wire or ground data quality, accuracy and precision is an excellent measure of lidar system performance.
The lidar data used in this study was classified into two main categories: wire data and ground data. The wire data was then manually selected and classified into four sub-categories: top wire and three sets of conductor data. The focus of the study lies in analyzing the top wire data in contrast to the ground data. Summary statistics of the accuracy parameters of the ground data (i.e., RMSE and standard deviation) were calculated and compared with similar statistical parameters from top wire data. The RMSE for ground data was assessed against a control reference surface, a large open flat terrain, pre-surveyed with sub-cm accuracy using traditional methods.
The results of the ground data elevation accuracy characteristics are represented in the top portion of Table 1
. It demonstrates that the Orion C200 can achieve operational vertical standard deviations of less than 2 cm, and an RMS of less than 3 cm, independent of the flying altitude and data collection rate up to 1000 m AGL.
In order to quantify the quality of transmission and distribution wire detection, a recently developed approach was used to characterize the relative accuracy of lidar data comprising small linear objects. The top wire data were analyzed in two planes, vertical and horizontal, while XYZ coordinates were translated to a new origin to simplify further analysis by working on a relative scale. Figure 2
shows translated and fragmented top wire data in a vertical plane with respect to a best-fit analytical curve (catenary, for power lines), which was used to calculate basic statistical characteristics of the top wire data (i.e., standard deviation and RMSE with respect to the best fit).
Based on this analysis, and taking into account the 9-mm diameter of the top wire, one can conclude that the distribution of the 3D point cloud data comprising the power transmission wire is in the sub-centimeter range. Such outstanding data quality from the Orion C200 enables the highest level of precision and accuracy for quality catenary modeling in PLS-CADD, increasing the reliability of engineering calculations of the physical parameters of power lines.
shows the accuracy and precision characteristics of both ground and wire data collected at the same altitude. Of particular importance is the consistency in data quality, accuracy and precision, regardless of the lidar signal strength variations between wire and ground data.
The ALTM Orion C200 demonstrates exceptional performance characteristics for power line mapping and small target detection applications. The uniquely efficient system design of the Orion C200 also provides full compliance with stringent eye-safety standards without compromising outstanding data quality, accuracy and precision.
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