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EETE MAY 2015

Navigation & geolocation Unmanned aerial solutions come down to Earth By Phil Sawarynski Advances in airborne photography and image processing have produced new solutions well suited for applications in open pit mining and large earthwork grading projects. Mine engineers and geologists use digital terrain models (DTM) to determine production volumes and manage the movement of ore and overburden. Because the mine landscape changes constantly, the best results come when measurements used to create the terrain (or surface) models are taken at regular intervals. The work to obtain this data is complicated by sheer size (large pit mines can be several kilometers across), difficult terrain, weather and safety concerns. The problems can be addressed by systems comprised of compact, low-cost unmanned aerial systems (UAS) together with new approaches for image processing and photogrammetry. The measurements needed by mine engineers are traditionally obtained by terrestrial surveys or aerial imagery. Terrestrial measurements produce accurate results, but the technique is slow and the presence of surveyors on the ground can interrupt normal mine operations. Aerial imagery is less disruptive, but longer lead times and higher costs can force mines to take measurements only once or twice per year, which results in suboptimal data. A better solution comes in the form of small UAS, which can be dispatched on short notice and are less susceptible to weather. A fixed-wing UAS such as the Trimble® UX5 Unmanned Aircraft System carries a high-resolution digital camera, GNSS navigation system, flight controls and communication equipment. Weighing roughly 2.5 kg and with a wingspan of one meter, the battery-powered UX5 can be carried and operated by one person. When on site, the operator defines and loads the flight plan into the aircraft’s onboard systems and then deploys and monitors the flight. Flight planning is based on requirements for scale and resolution of the final data, which controls parameters including flight altitude above ground level (AGL) as well as along- and cross-path image overlap. Launched from a small catapult, the aircraft autonomously executes the flight plan and collects images along specified flight lines before landing at a designated location. For projects that require multiple flights, the turnaround time to download images, upload a new fight plan, replace the battery and relaunch is roughly 10 minutes. The aircraft can operate for up to 50 minutes, with a cruise speed of 80km/h and a range of 60km. Typical flight altitude is 75 to 200m AGL, based on project requirements and subject to local air safety regulations. While the UX5 operates autonomously, it is always in sight and radio communication with the operator. Automatic or user-initiated failsafe procedures can divert the UX5 to a safe landing in the event of changing conditions or loss of communications. The aircraft is equipped with a 24MP digital camera operating in RGB spectra. For applications in forestry or agriculture, it can be configured to operate in near-IR wavelengths. The camera produces ground resolution of 2.6 cm at 100 m AGL. Digital images from the UX5 are processed using Trimble Business Center (TBC) software. Traditional photogrammetry calculations are based on the known location of the camera. But because the aircraft is moving rapidly during data capture, the raw images may contain significant geometric errors. To correct this, the software adjusts the photo stations to remove error in location, orientation and camera calibration. The software first uses computer vision algorithms to identify common tie points in overlapping images. Then the photo stations undergo a simultaneous adjustment to produce a best fit over the complete project area. With the photo stations resolved, the next stage is to connect the photographic models to the ground. Mines typically use a coordinate system that is tied to geodetic horizontal and vertical reference frames. While the UX5 onboard GNSS system provides navigation and flight control, it is not sufficient for precise geo-referencing. Prior to the flights, ground control points are placed in locations where they can be readily identified in the photos. They are then observed using precise GNSS or optical survey instruments and methods. These measurements are processed and adjusted in the Trimble Business Center to produce accurate coordinates in the designated geographic reference frame. Once the ground control targets are identified and registered in the photos, the software performs a final adjustment to produce georeferenced solutions for the photo stations. For quality control, a subset of ground control targets is purposely omitted from the adjustment. The targets’ known positions can be compared to their positions derived from the imagery to confirm the overall accuracy of the solution. Using a desktop computer, the image adjustment is completed automatically and requires roughly 10 seconds per image, including refinement of the camera photo stations. The average re-projection error is roughly 1 pixel. With the adjustment complete, the software uses conventional photogrammetric algorithms to compute the 3D coordinates of any pixel in an image. Typical deliverables include orthomosiacs, 3D digital surface models and point clouds. The surface models and point clouds are produced automatically Phil Sawarynski is Business Area Director of the Survey and Engineering Geospatial Division at at Trimble – www.trimble.com The Trimble Business Center software produces orthographic images, point clouds and photorealistic 3D surface models. Integrated processing of aerial and terrestrial imagery combined with GNSS and optical terrestrial data produces georeferenced models for design and analysis. 34 Electronic Engineering Times Europe May 2015 www.electronics-eetimes.com


EETE MAY 2015
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