High-accuracy UAV photogrammetry of ice sheet dynamics with no ground control

Abstract

Abstract. Unmanned aerial vehicles (UAVs) and structure from motion with multi-view stereo (SfM–MVS) photogrammetry are increasingly common tools for geoscience applications, but final product accuracy can be significantly diminished in the absence of a dense and well-distributed network of ground control points (GCPs). This is problematic in inaccessible or hazardous field environments, including highly crevassed glaciers, where implementing suitable GCP networks would be logistically difficult if not impossible. To overcome this challenge, we present an alternative geolocation approach known as GNSS-supported aerial triangulation (GNSS-AT). Here, an on-board carrier-phase GNSS receiver is used to determine the location of photo acquisitions using kinematic differential carrier-phase positioning. The camera positions can be used as the geospatial input to the photogrammetry process. We describe the implementation of this method in a low-cost, custom-built UAV and apply the method in a glaciological setting at Store Glacier in western Greenland. We validate the technique at the calving front, achieving topographic uncertainties of ±0.12 m horizontally (∼1.1× the ground sampling distance) and ±0.14 m vertically (∼1.3× the ground sampling distance), when flying at an altitude of ∼ 450 m above ground level. This compares favourably with previous GCP-derived uncertainties in glacial environments and allows us to apply the SfM–MVS photogrammetry at an inland study site where ice flows at 2 m day−1 and stable ground control is not available. Here, we were able to produce, without the use of GCPs, the first UAV-derived velocity fields of an ice sheet interior. Given the growing use of UAVs and SfM–MVS in glaciology and the geosciences, GNSS-AT will be of interest to those wishing to use UAV photogrammetry to obtain high-precision measurements of topographic change in contexts where GCP collection is logistically constrained.

High-Endurance UAV for Monitoring Calving Glaciers: Application to the Inglefield Bredning and Eqip Sermia, Greenland

“…In July 2018 at Eqip glacier, we followed Chudley et al (2019) and used an extra differential carrier-phase GNSS receiver to record camera positions. We used two similar singlefrequency Emlid Reach receivers (https://emlid.com/reach/), which are low-cost, light-weight, and log carrier-phase data: one fixed on the ground (called "base station") to provide a stationary reference point and one (called "rover") installed on the UAV and connected to the hot-shoe adaptor of our on-board camera.…”

“…• tidewater glacier ice flow and iceberg calving (Ryan et al, 2015;Jouvet et al, 2017;Chudley et al, 2019), • surface ice motion of mountain glaciers (Immerzeel et al, 2014;Gindraux et al, 2017), • glacial structures (Tonkin et al, 2016;Ely et al, 2017;Jones et al, 2018), • distribution of organic matter (Hodson et al, 2007;Stibal et al, 2017;Ryan et al, 2018), • spatial albedo patterns (Ryan et al, 2017Podgorny et al, 2018), • supraglacial drainage networks (Rippin et al, 2015), • ice surface melt (Bash et al, 2018).…”

“…In response to these challenges, Ryan et al (2015) built a dedicated fixed-wing UAV platform based on an open-source autopilot to perform structure-from-motion multi-view stereo (SfM-MVS) photogrammetrical mapping of calving glaciers in Greenland. Recently, Chudley et al (2019) added an on-board differential carrier-phase GNSS receiver to this platform which enabled the acquisition of georeferenced photogrammetrical products with high accuracy (in the range of one pixel size) without the use of GCPs.…”

“…In this paper, we detail an open-source, low-cost, fixed-wing UAV designed for monitoring glacial processes similar to the one presented in Ryan et al (2015), Jouvet et al (2017), Chudley et al (2019), , but specifically optimized for flight endurance to survey remote areas. We demonstrate the potential of our platform with 21 long-distance surveys over six fast-flowing glaciers of the Inglefield Bredning, northwest Greenland, and Eqip Sermia, west Greenland.…”

Innovative methods to monitor rock and mountain slope deformation

“…UAS-P allows for a high level of accuracy validated against extensometer and dGNSS data, with a lower displacement detection limit of 0.05 m/a. There is still room for improvement of the lower level of detection using lower elevation flight plans, real-time-kinematics for geographical correction, and improved flight planning software [76]. The potential to retrieve high resolution ground sampling of 1 to 2 cm is another advantage, especially in unstable or inaccessible terrain where the installation of fixed monitoring systems might be difficult or dangerous.…”