Fresno, California, United States
Davis, California, United States
Specifications for Using Small Unmanned Aerial Systems to Generate High Accuracy Mapping
Throughout the last decade, Small Unmanned Aerial Systems (sUAS) and light detection and ranging (LiDAR) systems have been increasingly utilized for digital mapping. The California Department of Transportation (Caltrans) updated their digital mapping standards to meet those of the 2014 ASPRS positional accuracy standards. This study was conducted to test how effectively sUASs equipped with LiDAR sensors meet new Caltrans standards, provides specifications for hardware, and identifies requirements for high accuracy digital mapping. Two test sites in Fresno and Davis, CA were established to test commercially available sUAS technologies. This project provides specifications for sUAS hardware (cameras, lenses, LIDAR sensors, and Global Positioning System (GPS)) and ground control requirements (e.g., flight planning and strip configuration, the spatial distribution of the ground control points) for high accuracy mapping. Eight sUAS mapping systems and six different camera systems were utilized. A total of 168 photogrammetric blocks were analyzed; the data obtained from 20 flights with RGB camera-equipped sUASs during 2018 to 2020 were used to evaluate the accuracy of LIDAR equipped sUASs.
- Fly sUAS systems equipped with LiDAR sensors such that there is a 60 percent overlap between successive photos to generate highly accurate terrain maps. To calibrate systems and further ensure mapping accuracy, fly at least two cross flights per project. It is recommended to aim for a side overlap (sidelap or lateral overlap) between 60 percent to 85 percent to accommodate terrain height and flying height variations.
- Utilize a frame camera with a global shutter and fixed focal length to generate highly accurate mappings. The camera should also have an electronic mid-exposure pulse (EMP) and Global Navigation Satellite System (GNSS) Post-Processed Kinematic (PPK) and Real-Time Kinematic (RTK) systems to further improve accuracy.
- Utilize a minimum of five control points and a total station (an optical surveying instrument that calculates angles and distances) for both horizontal and vertical leveling control. If the sUAS is equipped with a frame camera, the forward lap, the amount of image overlap in the flying way between successive photos, should be 80 ± 5 percent. The sidelap, the overlap between photos along adjacent parallel flight lines, should be 70 ± 5 percent. The average ground sampling distance (GSD) should be 2.5 cm or less.
- Collect GNSS data from a local base station for at least two hours, including 20 minutes before and after airborne data collection. Control points should be covered with at least three strips and placed outside of the photographic boundary (with a buffer or at least 25 meters or 82 feet). Process data with structure from motion (SfM) software.
- Avoid slow movements of sUASs with a LiDAR system to reduce Inertial Measurement Unit (IMU) drift. Minimize the drifting of integrated IMUs by flying the sUAS forward at full speed for 10 seconds, stopping it, then flying it again at full speed before stopping. To further improve the accuracy of LiDAR sensor systems, the surveying system should employ a dual frequency GNSS (a GNSS with both PPK and RTK).