Abstract
Unmanned Aerial Vehicles (UAVs) have revolutionised the availability of high resolution topographic data in many disciplines due to their relatively low-cost and ease of deployment. Consumer-grade Real Time Kinematic Global Navigation Satellite System (RTK-GNSS) equipped UAVs offer potential to reduce or eliminate ground control points (GCPs) from SfM photogrammetry surveys, removing time-consuming target deployment. Despite this, the removal of ground control can substantially reduce the georeferencing accuracy of SfM photogrammetry outputs. Here, a DJI Phantom 4 RTK UAV is deployed to survey a 2 × 0.5 km reach of the braided River Feshie, Scotland that has local channel-bar relief of c.1 m and median grain size c.60 mm. Five rectangular adjacent blocks were flown, with images collected at 20° from the nadir across a double grid, with strips flown in opposing directions to achieve locally convergent imagery geometry. Check point errors for seven scenarios with varying configurations of GCPs were tested. Results show that, contrary to some published Direct Georeferencing UAV investigations, GCPs are not essential for accurate kilometre-scale topographic modelling. Using no GCPs, 3300 independent spatially-distributed RTK-GNSS surveyed check points have mean z-axis error −0.010 m (RMSE = 0.066 m). Using 5 GCPs gave 0.016 m (RMSE = 0.072 m). Our check point results do not show vertical systematic errors, such as doming, using either 0 or 5 GCPs. However, acquiring spatially distributed independent check points to check for systematic errors is recommended. Our results imply that an RTK-GNSS UAV can produce acceptable errors with no ground control, alongside spatially distributed independent check points, demonstrating that the technique is versatile for rapid kilometre-scale topographic survey in a range of geomorphic environments.
Highlights
Topographic surveys of dynamic landforms, for scientific or management applications such as flow modelling [1,2], topographic change detection [3,4,5,6] and geomorphic unit mapping [7,8,9] typically cover areas of up to a few km2
We have performed a rigorous assessment of the impact of using ground control points (GCPs) when producing topographic data using real time kinematic (RTK)-Global Navigation Satellite System (GNSS) Unmanned Aerial Vehicles (UAVs)
Users should consider flight plan design prior to the completion of an RTK-GNSS UAV survey, as suboptimal flight plans can be a cause of systematic errors in SfM outputs
Summary
Topographic surveys of dynamic landforms, for scientific or management applications such as flow modelling [1,2], topographic change detection [3,4,5,6] and geomorphic unit mapping [7,8,9] typically cover areas of up to a few km. The challenges for applying these technologies in dynamic environments compared to alternative techniques, such as static [19] and dynamic [20] terrestrial laser scanning, are: survey logistics including flight permissions, flight duration, weather and access for GCP placement [11,21,22,23,24]; imagery acquisition to mitigate structural errors [25,26]; and, postprocessing of point clouds to reconstruct accurate submerged topography [27,28] and classify vegetation [29]. The recent development of Direct Georeferencing (DG) of imagery captured by UAVs [30,31,32] potentially offers the opportunity to reduce the logistical challenge and time-consuming nature of GCP field operations [33]. Direct Georeferencing involves camera positioning using a UAV mounted
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