![]() The traditional approach is by surveying characteristic breaklines or by planimetry, where the volume is calculated by interpolating and summing up cross sections/profiles along an axis. Stockpile volumes can be determined using different methods. ![]() With modern 2 frequency receivers and differential correction signals it is possible to determine survey points up to an accuracy of few centimetres. GNSS is a general term for global navigation satellite systems that include, among others, the navigation systems GPS (USA), GLONASS (Russia), Compass (China) and Galileo (Europe). They are used both to reference the photogrammetric model with an established local or national coordinate system as well as improve the overall accuracy of the model. Ground control points (GCP) are well identifiable landmarks on the ground of known location. The value depends mainly on the camera sensor resolution, focal length and flying height. For example in an image with 5 cm GSD, a pixel corresponds to 5 cm on the ground. The ground sample distance (GSD) is a term from remote sensing to describe the pixel resolution measured on the ground. Time investment for all 3 methods were equivalent due to the small area surveyed (above 2 to 5 hectares, using UAV technology will significantly improve efficiency and reduce time investment). With this type of scanner and at a scanning distance up to several 100 m, the relative point accuracy is generally expected to be at or below 5 mm. For the LIDAR survey, a Riegl VZ400 laser scanner was employed on site B, with two survey stations that were georeferenced by means of GNSS survey points. In total, some 500 ground points were sampled. To measure the photogrammetric ground control points (GCPs) and terrestrial verification points a surveyor grade dual frequency RTK GNSS receiver was used attaining elevation precision of 3 to 5 cm. A total of 86 images were acquired using an overlap of 93%.For site B, 212 images with an overlap of 93% were acquired, achieving a GSD of 5 cm with a flight at 120 to 140 meters above ground. Flights for site A (gravel stockpile) were at an altitude of 130 to 150 m above ground, achieving a ground sample distance (GSD) of 5 cm. This site was used for the comparison with a terrestrial LIDAR scan as well as for crosschecking with GNSS survey points.īoth flights were performed with senseFly fixed wing UAVs equipped with 16 megapixel consumer grade Canon Ixus cameras. Site B contains a sparsely vegetated earth protection dam against rockfall, several gravel stockpiles and an asphalt road. ![]() It was used for comparison with terrestrial GNSS survey points. This site possesses good photogrammetric underground textures. The stockpiles show trapezoidal cross sections and have heights between 7 to 10m. ![]() The case study was performed on two different sites: Site A contains several gravel / sand stockpiles as well as an asphalt road and some farmland with sparse vegetation. In addition, results are comparable to traditional photogrammetry results yielding 2 to 3 GSD all the while using lower quality UAV imagery and a fully automatic workflow. The obtained results prove it: UAV’s are ideally suited for stockpile volume measurements with a mean difference of only some centimetres when compared to the surface obtained with terrestrial LiDAR or GNSS test points. Reason why Pix4D set up a stockpile test case to give first answers when comparing the use of Pix4D’s UAV image processing software (Pix4Dmapper) with terrestrial LIDAR scan and GNSS point surveys. Though this common question looks simple at first sight, it is not trivial to answer. One of the first questions asked when considering the use of UAV surveying methods for stockpile volumetric is about expected accuracy. A comparison between Pix4D UAV photogrammetry software and GNSS / terrestrial LIDAR scan surveys. How accurate is UAV surveying? Testing stockpile volumetrics to get your answer.
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