Surveying an open-pit mine can be a hazardous undertaking. To obtain accurate volume measurements, it is necessary to pick up edges, known in the industry as “toes and crests,” as well as heaps. These are important features, since they provide a way to verify the current shape of a mine; but in light of increasingly stringent safety regulations and penalties, some companies refuse to let the surveyor get too close to such areas. Surveying the site from the air is an effective solution to this challenge.
It’s also a cost-effective solution. Namibian Mining Survey Services (NMSS) estimates that using an unmanned aerial system (UAS) can save more than 95 percent in mobilization costs, that is, bringing in resources from outside the country to conduct a lidar/photogrammetric survey. Believing UAS to be an important part of the future of surveying, NMSS had been investigating the technology for some time, and a recent project provided the perfect opportunity to try it out.
NMSS selected the Gatewing X100 for the job based on a demo at a platinum mine, where the results closely tracked those of a previous lidar survey.
The project was to survey a portion of Abenab Mine, a vanadium-lead mine owned by South West Africa Company and located just west of Tsumeb. The mine had been closed in the 1960s, but feasibility studies were underway to see if it would be viable to reopen the operation. Mine management needed to know volumes of all waste and tailings dumps, slimes, dams, and open-pit excavations. The main pit was roughly circular, about 60 meters deep and 120 meters across. Two smaller pits were covered in fairly thick vegetation but had enough ground showing to provide an accurate shape.
The survey area was approximately 100 hectares. The flying height was set at 150 meters in order to provide a ground separation distance of 5 centimeters. Ground control points (GCPs) were constructed from 1-meter lengths of masonite cut into 10-centimeter-wide strips; painted bright red, the strips were designed to provide 20 x 2 pixel coverage on the images. A total of 10 GCPs were set out in strategic positions covering a wide range of elevations, with points on top of the dumps, on undisturbed ground level, and in the pits. The points were fixed from existing control on the UTM34S coordinate system, by fast static techniques.
Launching the X100
Based on the Gatewing training received, basic photogrammetry principles and a few trials, NMSS determined that 9 a.m. to 3 p.m. was the best time to fly in order to avoid shadow. The flight area, including a previously surveyed area that would serve as a check, covered 140 hectares. Assuming favorable wind conditions, NMSS expected to cover the area on a single flight.
Arriving on site at 7 a.m., Dave Bansemer of NMSS started setting out the GCPs while his colleague performed the fast static survey. By 10 a.m., all GCPs had been placed and fixed. Having identified a suitable take-off and landing spot (a farm road), they proceeded through the pre-flight and flight checklist, and then launched the X100 at 11 a.m.. After completing the flight in around 35 minutes, with some turbulence at the 150-meter flying altitude, the X100 landed safely, albeit short of the goal, in an open area.
Once the data was downloaded, the team returned to Tsumeb to begin the processing. They started with the post-processing of the GCPs, and then moved to the coordinates obtained in the photo-control identification process. NMSS used Gatewing Stretchout Pro software for the photogrammetrical processing.
After specifying the coordinate system and identifying the GCPs, number-crunching began; the processing ran for around seven hours before the final point cloud and orthomosaics were created. The mean horizontal error was 3 centimeters and the vertical error was 9 centimeters, well within the error budget.
The first check was to see if all areas had been covered. NMSS then checked the point cloud against the previous survey. The tie-in was perfect. Some gaps in the point cloud seemed to correspond with tree canopy areas; to ensure complete accuracy, the team resurveyed a few areas using a spatial station.
NMSS learned some important lessons from using UAV technology for survey, which Bansemer lists for the benefit of future users:
- Make sure you have enough control. It is sometimes difficult to place your control points exactly in the corners of your flight and one in the center, as the actual flight is influenced by wind direction and the shape of the flight may change accordingly. Put down more points than recommended.
- Make sure that your ground control point size is relevant to your flying height. You will not be able to identify a 10-centimeter wide strip if you fly at 300 meters.
- Check the completeness of the job before you leave the area.
- Make sure there is sufficient area for a safe landing. Bansemer recommends at least a 300-meter strip, taking obstacles into account in the event of a short landing.)
The fast static techniques described were carried out with Trimble R6 GPS systems. Re-survey was done with the Trimble VX spatial station. The Gateway X100 is manufactured by Trimble.