The YellowScan Surveyor: 5cm Accuracy Demonstrated

Transcription

1 The YellowScan Surveyor: 5cm Accuracy Demonstrated Pierre Chaponnière1 and Tristan Allouis2 1 Application Engineer, YellowScan 2 CTO, YellowScan Introduction YellowScan Surveyor, the very latest lightweight UAV borne lidar system developed by YellowScan was tested in real time conditions in the field. The aim of this paper is to provide detailed information on the level of accuracy that can be expected from the system and the methodology adopted to assess it. The YellowScan Surveyor lidar system was flown on a multicopter UAV platform over an area surveyed by a professional certified surveyor. Detailed comparisons between the two acquisition methods are described in this paper. YellowScan s turnkey solutions YellowScan, a French company has developed lightweight professional grade lidar systems made for UAVs since Prototyping phases evolved into a viable commercial solution, the Mapper a fully integrated self-powered 2.1kg lidar system which started to be commercialized in The 4 key principles behind each YellowScan system are: Turnkey solution Ultra-light weight Independent from the aerial platform Simple to use Additional 2 years of R&D development and intensive component testing led to the conception of the YellowScan Surveyor. This latest product concentrates the best high-end components with the highest accuracy/weight ratio available on the market. YellowScan Surveyor The YellowScan Surveyor includes: The Applanix APX-15 UAV, a single board GNSS-inertial solution chosen for its unprecedented post-processed position and orientation accuracy in its category The Velodyne VLP16 (also known as Puck), a dual return laser scanner An onboard computer for continuous data acquisition and processing Battery (up to 1.5 hours autonomy) Worldwide technical and operational support YellowScan Surveyor key specifications: Laser scanner frequency: 300 khz Weight: 1.6 kg, battery included Power consumption: 15W Autonomy: 1.5 hours typical Size (mm): 100x 150 x 140 1/6

2 Study Site and Dataset Study site description The site selected for demonstrating the accuracy level obtained with the YellowScan Surveyor is YellowScan s calibration site located in Assas, south of France, directly north of Montpellier. It consists of a 150x60m model aircraft training site which exhibits gravelly airstrip and a 40m long row of 9 concrete tables (1x2.3m) together with relatively sparse low and high vegetation. The overall terrain is mainly flat with occasional drainage ditches (20 to 50cm deep) surrounding the air field. GCP - Survey methods to establish control points In order to establish the most accurate validation points for comparison purpose to the lidar dataset the following method was conducted: 5 ground points were materialized at the edges of the surveyed ground using wooden beacons and surveyed using a double frequency GPS/GLONASS Septentrio APS NR2 receiver receiving RTK corrections via GSM connection from a national private network of automated GNSS base stations (TERIA). 5 readings were taken per control points achieving a RMS3D of 5mm. From this 5 points baseline grid, a professional certified surveyor installed a Leica total station TCRP and shot multiple points over the surveyed field including table corners and table top flat surfaces, airstrip gravelly surfaces and natural grassy ground (see Figure 1). Polygonal error to setup on the baseline grid was +/-10mm and points were subsequently picked up at +/- 2.5mm accuracy. Overall 70 points were picked up and used for the lidar point cloud validation. This included: 36 materialized points suitable for checking XYZ accuracy. Those points correspond to table corner points (4 corners for each of the 9 tables) 34 ground points for checking Z accuracy, in details these can be classified as: 9 flat table surface points (picked up in the middle of the table) 14 ground points picked up along the gravelly airstrips of the site Figure 1. Ground surveying of the validation points 2/6

3 11 ground points located on natural grassy surfaces in the vicinity of the tables. Method Flight plan parameters System set-up The YellowScan Surveyor system was installed on the OnyxStar Fox-C8 multirotor UAV (Figure 2) which has a flight time of about 25min and a takeoff weight of 9kg. Lever arm measured and entered into the Applanix system via Ethernet cable communication. The Surveyor was switched on 5min prior to the flight to optimize satellite acquisition and locking procedure. Prior to the flight the YellowScan team set up a Septentrio GNSS base station APS-NR2 next to the surveyed field on a fixed control point (see GCP description above). This was left reading 30min prior to the flight and stopped at the end of the mission. The logged data was saved and used subsequently during the PPK process of the trajectory. The flight plan adopted for this survey included 8 flight lines, 3 flown centered and along the table row, 3 flown perpendicular to the table row and the last two flown as a cross-like figure with one axis along the table row and the other perpendicular to it. The flight line spacing was set at about 20 to 25m, fly height at 20m and speed at 3m/s (see Figure 3). Overall flight time recorded for the mission was 5min and the total area covered summed up to about 0.5ha. The flight also displays two bathtub geometries flown at 8m/s at the start and at the end of the flight which are recommended as the initialization procedure for aligning the heading measurement of the Applanix GNSS-Inertial unit. Figure 2. YellowScan Surveyor onboard the OnyxStar Fox-C8 3/6

4 Lidar Point cloud generation As per standard post processing procedure when operating the YellowScan Surveyor, the trajectory was imported into POSPac together with the raw data (RINEX file) gathered by the Septentrio base station. POSPac operates corrections to the trajectory and enables the user to specify whenever possible the exact coordinates the base station was setup at. In our case we specify its coordinates picked by the surveyor together with the offset of the tripod. The corrected trajectory together with boresight calibration angles of the Surveyor lidar unit were finally used to generate the corresponding point cloud (las format) using YellowScan s processing software. The Velodyne Puck laser scanner records 360deg field of view. For the purpose of this document, a 30deg corridor along each flight line was kept in the point cloud while the rest of the points were classified as overlap and thus discarded for the accuracy estimation. Automated shape detection method for XYZ accuracy estimation In order to exclude human / subjective intervention in materializing the table corners within the lidar point cloud used to check XYZ accuracy, an automated approach was favored. Using Terrasolid software (TerraScan, TerraModeler), lidar points corresponding to the tables were classified as a specific table class and used as an input class within the automated vectorize building function (Roof class table, maximum gap of 1m, planarity tolerance of 0.05m, increase tolerance of 0.05m minimum area of 1m², minimum detail of 0.5m² and max roof slope of 5deg). Corners of the produced shape were digitized, their XYZ coordinates extracted and compared to the validation points. Accuracy estimation method To assess the difference between the lidar point cloud and the validation points we used the Root Mean Square Error a well-known and vastly used Figure 3. Flight plan trajectory 4/6

5 accuracy estimator. RMSE (1) takes the difference between the observed and the estimated/forecast value for each user-defined point, squares it, finds the mean squared value and generates the square root of that mean. N 1 RMSE= N ( f i oi )2 i=1 (1) Where : N =number of forecast / observation pair f = forecast o=observation Two types of validations were produced from this survey: A Z-only validation which looks at the vertical distance between the control point and a triangulated surface generated directly from the point cloud (Terrasolid parameters during the output control point report being maximum triangle length to 20m, maximum slope to 20deg and Z tolerance to 0.2m). A XYZ validation which looked at comparing the 4 corners of each of the 9 tables using an automated shape detection method described before. Results Point cloud parameters The generated point cloud (Figure 4) for the 0.5ha surveyed area totalizes 21M points for a disk space of 580Mb. These parameters led to a point density of about 6000points/m² with all flight lines combined and translated into a measured density of 1600points/m² per flight line. Z validation The results obtained from the Z-validation process are summarized in Table 1. Considering all the points, the calculated RMSE is 2.1cm. The average error, also called absolute accuracy or bias, is -1.3cm. The standard deviation, also called precision or repeatability, is 1.6cm. The type of material seems to affect the precision level: as notionally expected, the smoother the surface the higher the precision. The elevation of flat cement slabs is identified and mapped with an RMSE of 1.9cm, an accuracy of -1.6cm and a precision of 1cm. Gravelly airstrip and natural grassy terrain measurements present an accuracy of Figure 4. Resulting point cloud 5/6

6 -1.3cm and -1.2cm respectively, and precision of 1.6cm and 2.2cm respectively. and introduce errors in the generation of the table shape during the Terrasolid step. XYZ validation Conclusion Table 2 summarizes the results (in m) of the comparison between the 36 table corner points picked by ground total station and the 36 table corner points automatically generated in Terrasolid software. The average error (absolute accuracy) for X, Y and Z is 2.5cm, -3.3cm and -2.6cm respectively. The standard deviation (precision or repeatability) for X, Y and Z is 3.3cm, 3.8cm and 0.9cm respectively. The RMSE for X, Y and Z is 4.1cm, 5cm and 2.8cm respectively. The materialized corner edges of the tables have a higher XY error than a Z error. This is mostly due to the fact that edges tend to be noisier than surfaces The YellowScan Surveyor used in parallel with an onsite GNSS base station reaches absolute accuracy of -1.3cm in Z with a precision of 1.6cm. As expected, the harder the surface is, the higher the precsion will be. In the study, cement slab surfaces are positioned with an accuracy of -1.6cm and a precision of 1cm. Assessing XYZ accuracy is relatively more demanding in terms of target material and automatic shape generation technique in order to exclude subjective interaction. However, the study shows an average error below 3.3cm and a precision below 3.8cm. Table 1. Z validation Table 2. XYZ validation 6/6