Great Sand Dunes National Park is home to the tallest dunes in North America and one of the most complex dune systems in the world. As part of their research and monitoring work, the park’s staff must track how the dunes shift from year to year. But because it is federally designated wilderness area, vehicles are not allowed to drive on the dunes. Any ground measurements must be done entirely on foot, and with the highest dune measuring 755 feet tall, a full ground survey of the sand-packed area is next to impossible. Drone mapping, integrated with civil engineering software, produces accurate geospatial mapping of the sand dunes and provides researchers with high-quality data sets.
Wohnrade Civil Engineers, Inc (WCE) began using drones in their work eighteen months ago. Led by principal engineer Mary Wohnrade, P.E., the company combines DroneDeploy’s deliverables with the Autodesk suite and other engineering software to create accurate aerial surveys. The firm uses this information to provide their clients with a range of professional services, including pre-development site reconnaissance, digital elevation models, topographical mapping and volumetric calculations.
Mary belongs to UAS Colorado, a business league that supports the use of UAVs in professional services. It was through this group that she heard about an opportunity to map the Great Sand Dunes. In October, 2016 Mary and her team used drone mapping to conduct an aerial topographic survey of the dunes.
Drones Provide Access in Difficult Terrain
Like with any sand dunes, those at Great Dunes National Park shift and migrate with the wind. As part of their ongoing research into the geology and ecosystem of the park, researchers keep year-over-year data on this shifting landscape. But, because the difficult terrain makes a full ground survey extremely lengthy, if not impossible, the research team has in the past relied mostly on hand measurements of selected points on the dunes. In 2011, the entire park was mapped from a plane using LiDAR, as part of a larger US Geological Survey project that sought to survey Colorado’s San Luis Valley. But except for this one occasion, year over year the research team has relied on a handheld GPS unit to annually measure twelve index dues scattered around the main dune.
Looking for a way to gather accurate, comprehensive data annually, the park staff reached out to UAS Colorado, who put them in touch with Mary Wohnrade and her engineering firm. WCE mapped a one-square mile section of the dune field, centered on the Star Dune, the tallest of the group.
Regulations usually prohibit flying drones in national parks, but Constantin Diehl at UAS Colorado secured the Scientific Research and Collecting Permit from the NPS to do so. Mary and her team flew the area with a fixed-wing SwiftTrainer that was equipped with a custom platform and flight management system operated by Jack Elston of Black Swift Technologies. They used a 75/75 overlap and obtained 1,755 images. The one square mile area was flown in moderately windy conditions and took two and a half hours to fly. Flight time was longer than usual because their permit specified that they could not take off or land the drone within park boundaries, meaning they had to launch five miles away from the area of interest (AOI).
DroneDeploy Data Integrates with Engineering
After flying the AOI, Mary’s next step was to obtain a point cloud that could be imported into engineering software. Initially, she sent her images and ground control points to two drone mapping services for processing.
Of the two mapping services, she says, “DroneDeploy deliverables turned out the best, so that’s what we gave the National Park Service.”
The high-resolution point cloud generated in DroneDeploy contained over 145 million points, giving her a significant amount of data to work with. Mary used the drone-generated point cloud as the basis for a multi-step workflow that involved several engineering software programs. The final product was an extremely accurate topographic map at a one-foot contour interval, and a georeferenced, high-resolution orthomosaic image. She also delivered a .shp file for park researchers to use in their ArcGIS software.
In addition to the .shp file, park researchers also requested an orthomosaic map, 3D point cloud and 3D model. Mary exported the 3D model directly from her DroneDeploy dashboard.
UAV Surveys Save Clients an Average of 40% Over Traditional Methods
Surveying by ground is always a lengthy process, especially compared to drone technology. Two or three days of data collection by ground can be reduced to just a few hours with the use of drones. Although the Great Sand Dunes have never been surveyed by ground, it is safe to say that, due to the inaccessibility of the terrain, the process would take far longer than just a few days. As it was, it took a two-person crew from WCE eight full hours just to set seven portable ground control points.
Because a full ground survey has never been done at the site, it isn’t possible to calculate the exact cost savings of the drone project. However, WCE has analyzed the cost savings of other surveying projects and determined that their unmanned aerial surveys save clients an average of 40% when compared to traditional survey methods.
Infographic courtesy of Mary Wohnrade, Wohnrade Civil Engineers
High Resolution Results May Enable New Discoveries
The research staff is very pleased with the results, stating, “It is definitely the best aerial imagery of the dunes to date!” Also, “the accuracy numbers from your survey are impressive.”
They will use the digital terrain model as a baseline for monitoring changes to the sand dunes over time. And even in this early stage, they believe some of the other maps Mary gave them may lead to new discoveries. Thanks to the orthomosaic map, researchers have already picked out linear features on the flanks of the dune that they had never noticed before.
As WCE’s civil engineering workflow shows, drone mapping, combined with adequate ground control points, provides the basis for highly accurate aerial surveys. These surveys not only save considerable time and money, but they also allow for data collection in difficult terrain, providing researchers with high-resolution information that is much more detailed than what they can capture on foot.
Where to learn more
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