Mission possible, with the right tools

Mapping of 21 careers in 60 days

The mission was to capture 21 quarries in 60 days. In this time frame, we planned and performed the UAV photogrammetry flights, edited the RAW images, processed the data, extracted the topography using Virtual Surveyor and produced the orthophotos, digital surface model (DSM), digital elevation model (DTM) and contour maps. for each site. The main customer, a government institute, needed extremely accurate and reliable 3D models and maps, as well as data that their engineers could work with in a CAD environment. “There was a lot of pressure due to the short lead time,” says Nuno Santos, owner and CEO of Aerisurvey, a company based in Mangualde, Portugal, specializing in high-precision surveys using unmanned aerial vehicles (UAVs or UAVs). “Drones”).

To get the job done, Aerisurvey used the mdMapper1000DG, a quadcopter drone (that is, it runs on four propellers) produced by Microdrones. This Germany-based company pioneered a reliable quadcopter that was rugged enough for professional applications. The drone has a flight time of up to 45 minutes and covers up to 80 hectares with a single battery charge. For the payload, it carries a 42.4 megapixel Sony RX1R II camera as sensor and an Applanix APX-15-L drone inertial measurement unit (IMU) for georeferencing.

Set industry standards

Survey data sets for the mining industry must be reliable and accurate so that geotechnicians can base their decisions on them. Some companies still feel that only surveyors using total stations can provide the precision needed, and drones and photogrammetry cannot provide the resolution they need. These impressions are changing with the emergence of equipment like the mdMapper1000DG, which Microdrones markets as a complete solution for surveying, construction, mining, precision agriculture and other geospatial applications. Aerisurvey was one of the first to adopt the technology it has been using since its inception in 2017.

Approximately 60-70% of Aerisurvey’s current activities are surveys for clients in the mining industry. For them, the ability to map large and small-scale features efficiently and accurately is key. Traditional surveying using ground-based techniques – such as total stations – is time consuming, requires more surveyors in the field, and can pose safety hazards, as equipment must be installed and work performed very close to ‘a ledge or at the bottom of the pit. Drone surveying can capture hard-to-reach areas with better output quality.

Microdrones UAV systems offer ease of use, lower cost, and better accuracy and repeatability. The software provided on the system ensures that the drone lands safely in its home position, even if the weather conditions change or the battery is low. With the mdCockpit app, operators have a “tap and fly” capability, which enables mission planning and tracking as well as flight and flight path rehearsal. Intuitive post-processing options are available for the collected data. The result is a simpler and more reliable workflow, so that larger amounts of data can be captured and processed more affordably, on a more regular basis.

A gigantic task

Once Aerisurvey was given the green light for the 21 quarry project, it faced a number of challenges. One was the weather, as the work had to be done during the rainy and windy months of December and January. Another problem was vacations, as businesses tend to slow down and workers take vacations this time of year. There was also the customer’s demand for deliverables to be presented the same for all careers. This meant that it would be difficult to work with other companies in the field, as each surveying company has their own processes and presents their results in a slightly different way.

An example of highly accurate orthophotos, MSNs, DTMs, and slope and topographic maps produced for each site using data collected with the mdMapper1000DG.

On the ground and in the air

The location of the quarries and the topography also presented challenges. The sites stretched 900 km from the far north of Portugal (just 8 km from the border with Spain) to near Faro in the far south. Some quarries were large (40 to 50 hectares) and had high vertical walls with drops of up to 170 m. Stronger winds at higher altitudes meant that a mission sometimes had to be postponed, even when ground conditions appeared favorable.

In the end, Santos piloted the missions on his own and did most of the data processing himself. This remarkable achievement made the project a kind of showcase for the power and reliability of the Microdrones system. “Reliability is paramount in drones,” continues Santos. “When I look back, the volume of fieldwork and the data produced was insane. I traveled 12,000 km in 60 days, working 12 to 16 hours a day.

Because the schedule only allowed three days for each career, Aerisurvey really had to rely on the equipment. “Fortunately, we had the Microdrones mdMapper1000DG,” he adds. “We never had a single problem.”

The mdMapper1000DG is programmed to land safely in its home position, even if the weather changes or the battery is low.

Precision commitment

One of Aerisurvey’s core values ​​is data accuracy. “We would never provide a survey with an RMS error greater than one pixel,” Santos says. Although the company used Control Points and Ground Control Points (GCP) for the career study project for its own quality control purposes, the mdMapper1000DG offers the option of Direct Georeferencing (DG), which eliminates the need to install GCPs.

To test the functionality, Aerisurvey conducted a test with a ground sampling distance (GSD) of 2.5 cm and achieved an accuracy of 2.5 cm without GCP. Anyone would consider a mean square error (RMS) equal to GSD a good result, according to Santos, so it’s an impressive result. In this case, using GCP reduced the error to 1.3cm. “We don’t see this kind of precision and accuracy in any other system,” he comments. “Businesses need to see the data to believe it. Once they see the data, they are more than convinced.

Bringing out the best in photogrammetry

Aerisurvey is one of the few companies to date to implement advanced work processes and capabilities to get the most out of photogrammetry for survey deliverables. On this subject, he collaborated with the University of Coimbra, which is an academic leader in photogrammetry applications in Portugal, with more than 80 articles published on the subject.

For example, Aerisurvey always captures images in RAW format. These require more computing power to be processed, but the efficiency is infinitely higher. “When you shoot only in JPEG, you lose data,” says Santos. Data loss is a particular problem in quarries, as deep shadows cast by walls often appear in the same frame as brightly lit areas such as storage. Using a good full-frame sensor allows you to recover both shadows and highlights on images captured in RAW format.

a.

b.

The same scene captured in JPEG (a) and captured in RAW format and edited (b). When shooting in JPEG format, data is lost. By using a good full frame sensor, shadows and highlights can be recovered from images captured in RAW format.

Editing photos individually to create layers is another good practice for producing high quality orthophoto maps. To ensure that the white balance is perfect and not greenish or yellowish, Aerisurvey uses a technique derived from professional photography: a simple color card. By taking a photo of the color map before and after each flight (for a record of the current lighting conditions), the images can be edited later to match what the eye sees visually.

Using a simple color card ensures perfect white balance. By taking a photo of the color map before and after each flight (to record the current lighting conditions), the images can be edited later to match what the eye sees visually.

Deliverables as a point of pride

Such high-precision, color-correct output is worth seeing, not only by engineering departments, but also by CEOs and departments that don’t work with raw data. The presentation of project deliverables is another area where Aerisurvey is at the forefront of industry standards. For careers – as for all Aerisurvey customers – the company produced hardcover printed reports, 3D models and aerial footage videos, as well as an A0 poster of each career site on fine photographic paper. art.

Aerisurvey produced an A0 poster of each career site. These large, very precise and detailed prints give operational staff a good idea of ​​what they are doing.

The poster, in particular, is attracting positive reviews. These voluminous, highly precise and detailed prints find unexpected uses, according to Santos. Standing around a table and looking at a correct impression gives the operational staff a good idea of ​​what they are doing. It can also lead to improved work processes within the customer organization.

[Streamers]

“We have strived to provide a standard, which has become a standard. Now we have even more confidence that we can do it. “

“We are a small business. We have to choose the right tools, like the mdMapper1000DG. Even if that means fewer tools, the best tools give our customers the best results. Quality is always the way to go.

Project specifications

  • Flight plan coverage: 633 ha
  • Treated area: 346 ha
  • Flight distance: 91.5 km
  • Flight time: 6h15
  • Number of flights: 21
  • Images captured: 7,755
  • Superposition of images perpendicular to the flight plan: 95%
  • Superimposition of images parallel to the flight plan: 65%
  • Spatial resolution (GSD): 2.46 cm / px on average
  • 3D points generated: 1 295 564 406
  • Dot density per m³: 270 on average
  • Photogrammetric Support Points (GCP): 152
  • Product files: 639,822 (1.74 TB)
  • Land distance covered: 12,699 km
  • Base stations: 40 (2 in each quarry)
  • RMS error: subpixel

About Debra D. Johnson

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