ERM LLC recently tested the Parrot Bebop 2 small unmanned aircraft system (sUAS) for photogrammetry and had mixed results. The Bebop 2 is supported by the Pix4D Capture app and sports a very wide angle, or fisheye, lens. A wide angle lens provides more surface coverage in the image compared to an unaltered lens at the same distance to the subject.
This translates into a sUAS’s ability to cover a larger area at a certain altitude or to fly lower and still cover the same area compared to a 94° field-of-view camera. Indeed, the Pix4D Capture app shows that the Bebop 2 can cover almost 4x the area of DJI quadcopters at the same altitude. Another benefit can be faster photogrammetry processing because less images with the requisite overlap are needed to cover a particular area. A wide-field of view camera can also potentially create a better Z-estimate because cameras further away from each other can see the same point.
The downside of a wide-angle lens is that the pixels are spread over a larger area, so pixel resolution is reduced, or in other words, the ground sampling distance (GSD) becomes larger. ERM wanted to see if the lower pixel resolution expected from the Bobop 2 could be offset by flying at lower altitude.
ERMs tests revealed that the Bebop 2 could not provide the detail necessary for ERM’s NADIR photogrammetry surveys. A NADIR survey at 100 feet (ft) AGL provided a GSD of 23 cm. The Bebop ‘s fixed camera angle captured too much sky and far-off imaging in the NADIR survey. Approximately 1/2 of the pixels in the image below are unusable for photogrammetry. This issue could be compensated for by flying lower, but ERM seldom finds situations where flying a grid at under 100 feet is safe.
The Bebop 2 provided much better results in a 50 ft AGL circular survey with a GSD 1.58 cm. ERM believes that careful flights close to and around a 3D object could lead to a good model.
Since the majority of ERM’s work is with NADIR surveys, we decided to pass on the Bebop 2. Tests with ERM’s newest sUAS, a DJI Phantom 4 will be posted soon.