Understanding your UAV elevation survey and GPS accuracy
This section provides a brief and simplified overview to get you started.
UAV’s use photogrammetric methods to capture elevation data. This is similar to how we perceive distance, due to parallax. Yep, basic triangulation. UAV’s use a method called Structure from Motion (SfM). We can park SfM right there - it's a bit complicated and it works. Your UAV RGB images have a lot of overlap, like the distance between your eyes. The distance from your UAV to each of the photos is both accurate and precise. The first thing your UAV and software does is make a surface based on a vast number of distance measurements. OK. We have a distance surface with smooth transitions from photo to photo (due to the overlap). As the distances are well known we also have a detailed measurement of the physical dimension of each photo, regarded as the Ground Sampling Distance (GSD). With UAV’s the GSD is usually less than 0.01 m.
UAV’s operating standalone with the GPS constellation of satellites vary several meters horizontally relative to the real position. The variation in z will be about twice that of x and y. Good for basic mapping horizontally but insufficient for elevation work. In fact, the actual variation in elevation from moment to moment along a flight path is much smaller. True elevation remains ambiguous due to too few satellites in view, and/or their configuration in the sky. Here’s the take home message: the GPS data do not define your surface during data collection. The GPS data are merely an attribute. If the elevation of each cell in your surface was defined by Z during data collection there would be error, as described above. In smooth terrain this would manifest as adjacent cells with sometimes abrupt changes in elevation. This would be a step backwards because we started this process with a very good continuous and smooth distance surface!
Improving the accuracy of your elevation model to survey grade requires tapping into an existing survey network. There are several ways to do this. Survey grade data can be derived from benchmarks established by government, from a Virtual Reference Network (VRN), and/or by using a RTK GPS base station. Each of these options will provide survey grade vertical control (< 3 – 5 cm), or of a range smaller than than the clumps of mud on your field. If a government benchmark elevation is near your site a simple rod and level survey will suffice. If farther away, the benchmark can be used with an RTK base station to move a known elevation a few kilometers to your site. If either of these two approaches is not an option, because existing benchmarks are too far away to be practical, or your area of interest is not in an area of reliable cell phone service, then Post-Processing Kinematic methods (PPK) will be used. To establish local vertical control using the PPK method involves occupying one point on your land (for not less than 2 hours) with a RTK GPS base station. Lastly, real time survey grade UAV’s, like our eBee Plus RTK, can receive corrected positions directly from the VRN during flight, or from a local base station. Note, however, that VRN position reception is affected by distance and topography. RTK missions flown using a GPS base station typically extend farther than the extent of UAV missions, and can be located to accommodate terrain effects.
If you opt to use Ground Control Points (GCP’s), such as Aeropoints, to achieve quasi-survey grade data there are additional time and space requirements. The UAV ground control staff will need to traverse most of your site twice, and post-processing of your data to survey grade data occurs after the survey, which adds time. GCP’s, like Aeropoints, cannot produce survey grade data outside of a VRN network. I’ve regarded GCP grade elevation data as “quasi-survey grade” because that step can be done well, or otherwise if the effort or experience required is lacking. Some sites are simply unsuitable for the GCP approach to vertical control due to terrain, safety considerations, or if site operations cannot be shut down temporarily.
Aeropoints are a trademark of Propeller.