QuestUAV DATAhawk PPK Accuracy Assessment and Validation

Transcription

1 QuestUAV DATAhawk PPK Accuracy Assessment and Validation Accuracy of 3.0cm(X), 3.3cm(Y), 3.9cm(Z) Nigel King 1, Kerstin Traut 2, Cameron Weeks 3, Chris Mills 4, Phil Mills 5, Ruairi Hardman 6, Stuart King 7, Francis Thompson 8, Callum Hewitson 9, Stefan Frizzel 10 1 Director QuestUAV, 2 Data Analyst QuestUAV, 3 Production Engineer QuestUAV, 4 Principal Survey Control Services, 5 Director Oakley Aerial Surveys, 6 General Manager QuestUAV, 7 Lead Pilot QuestUAV 8 Pilot QuestUAV, 9 Pilot QuestUAV, 10 System Administrator QuestUAV Glossary... 2 Introduction... 2 Objectives... 2 Claims... 3 Study Area... 3 Equipment and Workflow... 3 Methodology... 3 Reference Data... 4 Flight Reference Station... 4 Aerial Survey... 5 Post-Processing... 5 Statistics... 5 Results... 5 Verification... 6 Conclusion... 7 Appendix... 8 Per flight RMSE Results... 8 Independent Verification Results... 8 Flight Data... 9 QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 1

2 Glossary DSM Digital Surface Model GCP Ground Control Point GIS Geo-Information System GLONASS Globalnaya Navigatsionnaya Sputnikovaya Sistema (transliteration) GNSS Global Navigation Satellite System GPS Global Positioning System GSD Ground Sampling Distance OS Ordnance Survey PPK Post Processing Kinematic RMSE Root Mean Square Error RTK Real Time Kinematic UAV Unmanned Aerial Vehicle WGS84 World Geodetic System 1984 Introduction Objectives In September 2016, QuestUAV released its first PPK enabled surveying drone allowing users to generate highly-accurate topographic maps and 3D models from aerial photography without the need for physical GCPs. The objective of this whitepaper is to validate the spatial accuracy of the new PPK enabled DATAhawk PPK aircraft on the basis of independently measured verification points (GCPs) and repeated aerial surveys of a test site in the UK. QuestUAV has continued the product development and successfully integrated the PPK technology to its compact mapping drone, the DATAhawk PPK. A dual band GPS/GLONASS receiver on board the DATAhawk PPK produces spatial accuracies in the centimetre range for both 2D maps and 3D models. QuestUAV s simple and automated PPK workflow saves users hours of mission time since there is a minimal requirement for walking the survey site and measure GCPs and working with GCPs in the postprocessing phase. All GNSS positioning data are stored on board the DATAhawk PPK eliminating the need for a real-time data link with a fixed reference station while still guaranteeing RTK cm-level position accuracy. PPK coordinates can be more accurate than RTK ones, since the processing can be more refined. Figure 1: Test site for DATAhawk Accuracy Assessment QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 2

3 Claims QuestUAV claims the following accuracies on the test area using the DATAhawk PPK system without the use of GCPs. Q-100 DATAhawk PPK Absolute RMSE Accuracies Study Area X Y Z 3.0cm 3.3cm 3.9cm camera positions are initially recorded in the global coordinate system World Geodetic System (WGS) 1984 and then transformed to the local coordinate system OSGB 1936 / British National Grid during the post processing phase. The corrected image positions are recorded in a comma-separated values (.csv) file which are then imported into photogrammetric software packages such as Pix4Dmapper or Agisoft Photoscan. The test site is located on the North East coast of England (Low Hauxley) and covers 100 hectares of agricultural land, coastal dunes and beach. By being very close to the North Sea, weather conditions are windy and changeable, which aids to performing stress tests for both the QuestUAV aircraft and image data quality. Equipment and Workflow The DATAhawk PPK aircraft is equipped with a dual-frequency L1/L2 receiver tracking both GPS and GLONASS signals. 132 hardware channels allow the system to simultaneously track all visible GPS/GLONASS satellites. In parallel, the receiver records the shutter events of the camera and logs the information during flight on the on-board SD card. Camera locations are corrected after the flight during the post-processing phase, when GNSS receiver data is combined with base station data to calculate the exact position of each camera exposure. All Figure 2 DATAhawk PPK Miniline launch The sensor on board the DATAhawk PPK is the Sony RX100 (mk2) The aircraft is launched with either the standard QuestUAV mini-launch line, or the AirDock (Figure 2) and can be landed via parachute or belly landing. Methodology The accuracy of the DATAhawk PPK system is validated by comparing the results of repeated aerial surveys with precisely measured verification points (GCPs). The verification workflow includes the following steps: QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 3

4 Reference Data Mark verification points on the ground Measure verification points using an independent surveying company Reference Base Station Setup a reference base station Aerial Surveys Conduct aerial surveys over the test site with the Q-100 DATAhawk PPK Post-processing Post-process camera locations and process image to create point clouds, othormosaics and DEMs Statistics Extract target points and calculate accuracy statistics Verification Provide datasets to Survey Control Services and Oakley Aerial Surveys for independent verification. Reference Data A total of 30 target points previously distributed over the 100ha test site and measured by an independent surveying company were used. The majority of the points were placed on tarmac and permanent structures such as rocks and stable stone walls in order to avoid any movements for the duration of the accuracy study. arms) or yellow marker boards with black diagonals elsewhere. These markers are clearly visible on the aerial imagery and form the basis for assessing the spatial accuracy of Orthomosaic and Digital Surface Models (DSM). Prior to the flight test commencing all points were measured independently in two separate sessions using Trimble GPS/GLONASS receivers taking RTK corrections from a base station. For any point the reading spread within a visit was no more than 4mm in xy, 7mm in z. The final accepted value for each reference point was the averaged values of the individual observations. The base station was located on one of the reference points and run for an eight hour static fix. Baselines were computed from six of the UK Ordnance Survey active stations, giving residuals of 4mm in plan and 12mm in height. Figure 3: Surveyor measuring the reference points on the ground. The reference points consist of white crosses painted on hard surfaces (approx. 15cm Flight Reference Station The PPK workflow does not need a real-time data link with a base station during a flight, however the GNSS information has to be recorded by a base station throughout the flight duration, at one second interval, to allow for positioning corrections to be applied during the post-processing phase. Within the UK, the public domain data released by the Ordnance Survey is at a 30 QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 4

5 second interval and cannot be used directly to process data from the DATAhawk PPK aircraft. QuestUAV therefore, set up a permanent reference station about 3.5km from the study site recording GNSS correction data during a flight. Subsequent position corrections during the test phase will be related to this reference station. Aerial Survey The test site was flown with the DATAhawk PPK aircraft repeatedly over a fivemonth period while testing different flight and sensor settings, including: Flight altitude: 400ft Image overlap: 69m/60m/53m lag distance Flight path: Parallel/perpendicular grid lines Camera settings: Auto/ aperture priority/ shutter priority/ manual Camera lens: 10.4mm wide angle lens. Approximately half of the flights were carried out in sunny weather while the other half were flown in overcast and rainy conditions. Wind speeds were high for the majority of flights with up to 35kts at operational height. During the flight hours, there was no significant geomagnetic activity recorded. Post-Processing After flight, camera positions were corrected by combining GNSS recordings of the aircraft with the corresponding information from the flight reference station on the ground to provide more precise cm-level PPK values. These corrected locations are then transformed into a local coordinate system and stored in a csv file for use during photogrammetric processing. Photogrammetric processing was carried out with the Agisoft PhotoScan software, including the generation of orthomosaic and digital elevation model and their export as GeoTiff files. Statistics Data analysis was performed inside the open-source GIS software QGIS. For each flight orthomosaic and DSM were imported into QGIS and the 3D image coordinates of the target points extracted. The image coordinates were compared with the coordinates measured during the ground survey and their accuracies were determined by using the RMSE. The RMSE is the standard error measure to estimate geo-spatial precision. It represents the sample standard deviation of the differences between the coordinates measured in flight (p) and the observed coordinates during the ground survey (a). Results During the workflow optimisation process, 27 flights were performed for adjustment and refinement purposes to obtain optimal camera settings, aerial positions and flight parameters (referred to as flights PPK1 - PPK27). Results were achieved with the Sony RX100 camera. Results were further improved by using manual camera configurations dependent on light and weather conditions. After the optimisation process the accuracy assessment was carried out on a further 10 flights and processed without using any ground control points as known positions, then compared with known reference points to determine accuracies (referred to as flights PPK28 PPK37). The table below summarises the results of the validation. QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 5

6 Table 1: Accuracy assessment on the basis of 10 flights with 30 check points each (ground reference). X (m) Y (m) Z (m) XY (m) XYZ (m) Mean Error Standard Deviation RMSE RMSE best RMSE worst Verification Independent verification was carried out by a professional surveying company with long-term experience in the provision of ground control. Two flights were randomly selected by the independent verifiers for this independent verification. The raw data and imagery for these two flights were reprocessed using Agisoft Photoscan and validated using the point cloud output in Trimble RealWorks. The independent processed data generally agreed with the QuestUAV processing, with some small variations, typically within 1cm. Verification Results (Flights PPK32 & PPK35) Mean Error Standard Deviation RMSE Max Positive Error Max Negative Error QuestUAV Independent Difference X Y Z X Y Z X Y Z X Y Z X Y Z Notably, Flight PPK32 contained an anomaly which resulted in an unusually poor Max Negative Error of (see appendix). This point was more than triple the error of the 2nd worst point ( ) on this particular flight but was still included when calculating the RMSE. This increased the Y RMSE and resulted in the only flight to contain a Y RMSE worse than the Z RMSE. This anomaly was detected both by ourselves and the independent verifiers which further strengthens the accuracy and integrity of our claims. QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 6

7 Conclusion Results show that the accuracies which can be achieved by the DATAhawk PPK are 1 x GSD (planar) and 2 x GSD (vertical) without the use of any ground reference information. Independent verification reveals that the DATAhawk PPK provides good positioning, enabling results close or equal to those provided using ground control points. During normal operations, it is advised that some discrete ground control points are provided to provide assurance that the processing systems have performed correctly. The DATAhawk PPK was stress-tested in real-life scenarios, including challenging coastal weather conditions and non-ideal light conditions. System performance was consistent over the whole test site. The QuestUAV test series have shown that the new DATAhawk PPK aircraft is a robust and reliable system for high accuracy aerial surveys without the need of physical GCPs. The PPK workflow allows for the easy combination of high-resolution aerial images with highprecision GNSS data during the post-processing phase to create high-quality orthomosaics and digital surface models. QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 7

8 Appendix Per flight RMSE Results RMSE PPK29 PPK30 PPK31 PPK32 PPK33 PPK34 PPK35 PPK36 PPK37 PPK38 X Y Z xyz XYZ Independent Verification Results Verification Results Mean Max Positive Error Max Negative Error RMSE Standard Deviation Mean (Absolute Residuals) PPK32 PPK35 Average QuestUAV Independent Difference QuestUAV Independent Difference QuestUAV Independent Difference X Y Z X Y Z X Y Z X Y Z X Y Z X Y Z QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 8

9 Flight Data Ground Survey PPK29 PPK30 PPK31 PPK32 PPK33 PPK34 PPK35 PPK36 PPK37 PPK38 Point ID Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height Easting Northing Height GROUND SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY FLIGHT SURVEY QuestUAV Ltd. No publication, presentation or reprocessing without permission of QuestUAV Ltd. 9