Now is a special time for geospatial service providers. Many call it as important a time in the history of geospatial technology as the introduction of geographic information systems (GIS). Arguably, unmanned aerial systems (UAS) and 3D data acquisition tools are undergoing some of the most notable innovations, impacting aerial surveyors and mappers most.
Plane, Helicopter or Drone?
“Drones are a huge factor,” says Mike Tully, CP, GISP, president and CEO of Aerial Services Inc. in Cedar Falls, Iowa. “Drones will begin competing with traditional manned aircraft projects, especially as the federal regulations allow beyond-visual-line-of-sight. Today they don’t, for the most part. Tomorrow they will,” Tully says. When it comes to choosing which aircraft to use for aerial data acquisition, helicopters are often selected for low, slow-flying projects including mapping, patrol and inspection. While the platform used to be used for film projects, he says it is now usually used for dense LiDAR collection.
Deciding whether to use an airplane or a drone can be a very difficult choice. For large-area collection, an airplane is the best option; for one, FAA regulations restrict drone flights over people. Manned flights have no such restrictions. However, even if such flights were permitted by the FAA, an airplane would still be the better choice because it is less costly. For small, local projects or for work that a manned aircraft cannot perform, drones make the most sense. However, even small projects can be best served with an airplane if the travel distance for the pilot is significant.
As for survey-grade data, Day — who defines this as products meeting the ASPRS horizontal and vertical accuracy specifications by using pixel size of imagery — says it can easily be achieved by imaging and LiDAR systems mounted to manned aircraft systems. “But from Keystone’s published studies, we have determined that survey-grade data can be generated by a UAS in very limited and controlled situations that are not really cost effective,” he says. “In general, we believe UAS data is best suited for products where accuracy needs to achieve three to four times the pixel size.”
In the end, a photogrammetry project is the same for any type of aircraft or sensor, Day says. The area must be covered with a user-specified amount of overlap between strips. “Even navigating the airspace is similar for manned and unmanned,” he points out.
Photogrammetry or LiDAR?
Similar to aircraft, clients either directly dictate which sensor systems will be used, or their needs — type of imagery and level of accuracy — guide the choices made by the user and service provider.
Tully says most remote sensing and mapping requires imagery and some kind of point cloud. Photogrammetric techniques can be used to derive a point cloud from the photography, or the point cloud can be collected directly using a LiDAR sensor.
Aside from cost, a key difference between photogrammetrically-derived point clouds and LiDAR is the ground. Because LiDAR uses active sensor technology, it can push through vegetation and reflect off the ground beneath. “Photogrammetrically derived point clouds are simply going to show the surface of things,” Tully says. “So LiDAR gives us a distinct advantage by giving us both the surface of things, which is very important, but also the bare earth elevations that photogrammetry alone just can’t give us. … LiDAR will generally give us a leg up and give us much more accurate models of the earth’s surface, which is, frequently, what’s needed.”
Since LiDAR covers the ground and the surface of things, and photogrammetry only gets to the surface, as LiDAR sensors become less costly, Tully says he can see LiDAR dominating 3D modeling of the earth’s surface. “I can see a day when building 3D models of things under the sensor can’t be produced as efficiently using photos only because our LiDAR sensors will be powerful enough and cheap enough to make the photos-only method of remote sensing unnecessary.”
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