This study presents the application of a cost-effective, unmanned aerial
vehicle (UAV) to investigate calving dynamics at a major marine-terminating
outlet glacier draining the western sector of the Greenland ice sheet. The
UAV was flown over Store Glacier on three sorties during summer 2013 and
acquired over 2000 overlapping, geotagged images of the calving front at an
Observational and modelling studies have demonstrated that Greenland's
marine outlet glaciers have a complex and potentially non-linear response to
both environmental forcing (e.g. Vieli et al., 2000; Benn et al., 2007;
Holland et al., 2008; Howat et al., 2010; Hubbard, 2011; Joughin et al.,
2012; Walter et al., 2012; Carr et al., 2013) and to changes in front
position (Howat et al., 2007; Luckman et al., 2006; Joughin et al., 2008).
To quantify these processes and feedbacks, regular and accurate
high-resolution measurements are required to capture the key spatio-temporal
linkages between rates of ice calving, flow, surface lowering and frontal
advance/retreat. Despite significant advances in satellite remote sensing,
limitations of spatial resolution (e.g. MODIS) and/or frequency of repeat
imagery (e.g. Landsat or TerraSar-X) render detailed, day-to-day
analysis of calving-front dynamics unfeasible. On the other hand,
acquisition of digital imagery from unmanned aerial
vehicles (UAVs) combined with the development of
stereo-photogrammetry software has enabled the provision of high-resolution
3-D georeferenced data on demand for geoscience applications (e.g.
d'Oleire-Oltmanns et al., 2012; Hugenholtz et al., 2012, 2013; Whitehead et
al., 2013; Lucieer et al., 2014). This represents a cost-effective technique
technique for acquiring aerial data in remote, hazardous and/or
inaccessible regions and recent applications for emerging snow and ice
investigation abound the web (e.g. see the highly informative site
of Matt Nolan;
Attributes of the flight surveys and image acquisition of the UAV.
Between July and August 2013, an off-the-shelf, fixed wing UAV equipped with
a compact digital camera flew three sorties over the calving front of Store
Glacier, West Greenland. The aerial photographs obtained during these
flights were used to produce high-resolution ( detail the UAV, in terms of its payload and camera settings, and its specific deployment to Store
Glacier; describe the techniques used for processing the aerial images and quantifying glaciological
processes; discuss the significance of the data we obtained which includes calving events, the
character, orientation and morphology of crevasses, surface velocities, ice discharge
and changes in thickness and position of the calving front.
Store Glacier is a large marine-terminating (tidewater) outlet glacier located in the Uummannaq district of West Greenland (Fig. 1). The calving front has a width of 5.3 km and an aerial calving front (freeboard) of up to 110 m a.s.l. (Ahn and Box, 2010). Aerial photography from 1948 onwards reveals that Store Glacier's frontal position has remained stable over the last 65 years (Weidick, 1995). Seasonally, the calving front exhibits advance and retreat of up to 400 m (Howat et al., 2010). The study here focuses specifically on glacier dynamics during the melt season under open-water, tidal modulation of ice flow.
Flowchart of the control set-up and picture of the UAV at base camp with the relative novices.
The UAV airframe is an off-the-self Skywalker X8 (
The autopilot is an open-source project called Ardupilot
(
The advantage of this package is that it can be assembled within a day from off-the-shelf parts and is cost effective at less than USD 2000. The X8 is also relatively straightforward to fly, robust, easily repairable and floats, all added bonuses when being deployed in remote areas by potential novices. Furthermore, the Ardupilot firmware is open source and hence can be programmed for specific requirements, for example camera triggering (see below).
Two lightweight digital cameras were tested at the field site: a Panasonic
Lumix DMC-LX5 10.1 megapixel (MP) camera with a 24 mm wide-angle zoom lens
and a 16.1 MP Sony NEX-5N with a 16 mm fixed focal length lens though
results presented here are limited to the former. A SPOT GPS tracking device
was also included in the payload to facilitate recovery should a mission
fail (which it did). The focal length of the Lumix lens was adjusted to
5.1 mm (35 mm equivalent) to allow the widest possible coverage which gave the
camera a 73.7
The open-source software APM Mission Planner (
UAV operations were based out of a field camp with the advantage of a 50 m
area of flat alluvial terrace with relatively boulder and bedrock free
ground for manual remote-control take-off and landing. This location did,
however, require a
Three-dimensional data were extracted from the aerial photos using Agisoft PhotoScan Pro software (Agisoft LLC, 2013). This software's strength lies in its ability to fully automate workflow and enables non-specialists to process aerial images and produce 3-D models which can be exported as georeferenced orthophotos and DEMs (e.g. Figs. 3 and 6). The first stage of processing is image alignment using the structure-from-motion (SfM) technique. SfM allows for the reconstruction of 3-D geometry and camera position from a sequence of two-dimensional images captured from multiple viewpoints (Ullman, 1979). PhotoScan implements SfM algorithms to monitor the movement of features through a sequence of multiple images and is used to estimate the location of high-contrast features (e.g. edges), obtain the relative location of the acquisition positions and produce a sparse 3-D point cloud of those features. The Ardupilot flight logs of the onboard navigation sensors allow the camera positions and the 3-D point cloud to be georeferenced within instrument precision. SfM also enables the camera calibration parameters (e.g. focal length and distortion coefficients) to be automatically refined; hence, there is no need to pre-calibrate the cameras and lens optics (Verhoeven, 2011).
Once the photos have been aligned, a multi-view reconstruction algorithm is
applied to produce a 3-D polygon mesh which operates on pixel values rather
than features and enables the fine details of the 3-D geometry to be
constructed (Verhoeven, 2011). The user determines the precision of the
final 3-D model based on image resolution and pixel footprint. A medium
quality setting was chosen yielding DEMs with between 38 and 40 cm/pixel
ground sampling resolution (GSD), which were resampled to a Cartesian 50 cm
grid to enable intercomparison (Table 1). Higher resolutions (
Two problems of accuracy were encountered in DEM production: (1) PhotoScan
failed to reconstruct a flat sea level of constant elevation, and (2)
relative positional errors between the DEMs constructed from different
sorties were up to 17.12 m horizontally and 11.38 m vertically. Positional
errors were due to the specified limits of the onboard L1 GPS of
Changes in calving-front positions were obtained from these data combined with a Landsat 8 panchromatic image obtained on 12 June (Fig. 3b). Each calving-front position was digitised according to the procedure outlined by Moon and Joughin (2008), whereby a polygon of the calving-front retreat or advance is digitised and divided by the width of the glacier. This method has been used in previous studies (e.g. Howat et al., 2010; Schild and Hamilton, 2013) and enables intercomparison of results. Surface elevation change was calculated from the residual difference of the DEMs (Fig. 3a).
Ice flow across the terminus region was calculated by feature tracking
performed on successive DEMs using the ENVI Cosi-CORR software module (Fig. 4b).
These velocities were then used to estimate ice flux through the
calving front for the same period under the assumption of plug flow (uniform
velocity profile with depth) and using a calving-front cross section
obtained from Xu et al. (2013) and modified by single and multi-beam echo
sounder bathymetry obtained by S/V
To investigate the distribution and patterns of crevassing, each DEM was Gaussian filtered at 200 pixels (100 m) in ArcGIS and subtracted from the original DEM to yield the pattern of negative surface anomalies. These anomalies were converted into polygons to map and hence quantify crevasse distribution and character (Fig. 6a). The resulting polygons were enclosed by a minimum bounding rectangle, which allowed the orientation, width, length and depth of crevasses to be extracted (Fig. 6a, Table 2). Water-filled crevasses were automatically located in the ENVI package using the supervised maximum likelihood classification (MLC) method. Representative training samples for water-filled areas were chosen from the colour composite orthophoto (Fig. 6b). The trained tool then classifies pixels that are interpreted as water into the desired class. The resulting raster image was converted into a shapefile and used to mask and define the area of the water-filled crevasses across the terminus. These procedures allow thousands of crevasses in multiple orthoimages and DEMs to be quantified easily without the difficulties and dangers associated with direct field measurements.
The relative horizontal uncertainties between the DEMs were investigated by
feature tracking the stationary bedrock at the sides of the glacier. The root
mean square (rms)
horizontal displacement was
Due to the lack of reflected light from deep crevasse recesses, the DEM generation process cannot quantify the narrowest sections of all fractures and resultant crevasse depths are therefore a minimum estimate. The technique is also clearly limited to line of sight precluding narrow fractures which extend for tens of centimetres horizontally and potentially up to a few metres vertically (Hambrey and Lawson, 2000; Mottram and Benn, 2009).
Finally, there are a number of practical difficulties when operating an autonomous aircraft in remote and inaccessible environments. Mission planning is critical; knowledge of the local weather conditions, as well as up-to-date satellite imagery and DEMs are a prerequisite.
Three successful UAV sorties were flown over Store Glacier calving front providing imagery, orthophotos and DEMs on 1 and 2 July and the 23 August, herein referred to as flights and associated products 1 to 3, respectively (Table 1). The interval between flights 1 and 2 was 19 hours and comparison between these outputs enables identification of processes operating over a daily (short) timescale, be it a very specific snapshot. The third sortie was flown 52 days later and comparison between these outputs enables investigation of late-seasonal change. The footprint of the four cross-glacier transects flown extends just over 1 km upstream from the calving front and herein is referred to as the terminus.
Residual elevation change between 1 and 2 July (Fig. 3a) reveals that the
front retreated in two sections by up to 50 and 80 m, respectively. The more
northerly calving event (A) resulted in a 450 m wide section of the terminus
retreating by between 20 and 50 m, whilst event B produced between 20 and
80 m of retreat across a 400 m section (Fig. 3a). In addition to these two
calving events (which are discussed in section 3.6), the central 4.5 km
frontal section advanced between 12 to 16 m (Fig. 3a). At its lateral
margins, the calving front shows no discernible systematic change though
there are isolated, small calving events, for example, within 50 m of the
southern flank (Fig. 3a). Upstream of the calving front, there is no net
change in mean surface elevation away from the front and the dappled pattern
of residual elevation change is a result of the advection of crevasses and
seracs. Successive long profiles of the terminus between the 1 and 2 July
reveal specific down-glacier crevasse advection with flow (Fig. 6) at a rate
of 5 and 16 m d
Attributes of mean crevasse width, length and orientation in
each zone labelled in Fig. 5. Orientations are measured along the
long axis of each crevasse with respect to the direction of flow which
is 0
Over the entire melt season, larger fluctuations in calving-front position
are observed (Fig. 3b). Over the 19-day period from 12 June to 1 July, mean
frontal retreat was 160 m (Fig. 3c) and between 2 July and 23 August, the
calving front advanced by an average of
The deepest sector of the calving front is located 1 km south of the
centreline and exceeds 540 m below sea level (Fig. 5a). This 200 m wide
sector also corresponds to the greatest thickness of
Maximum surface-flow velocities of 16 m d
Seasonal flow patterns were not obtainable between 2 July and 23 August as the majority of any matching features within the study area required for tracking had already calved into the ocean. Furthermore, it is likely that the morphology of many crevasses and seracs will have changed significantly through melt and deformation and would not be recognised by the cross-correlation procedure.
The morphology and orientation of crevasses varies markedly across the
terminus (Fig. 6). The largest crevasses occur in a sector south of the
glacier centre line in zone 4 (Fig. 6, Table 2). Here, crevasses have mean
minimum depths of 18 m, lengths of 68 m and widths of 31 m. The largest
crevasses are up to 30 m deep, over 500 m long and nearly 200 m wide but no
crevasses that penetrated below sea level were identified. Most crevasses in
this region are arcuate with limbs pointing towards the calving front and
are orientated obliquely to the direction of ice flow (Fig. 6). This arcuate
morphology of crevasses continues across the central 3 km of the terminus in
zone 3 (Fig. 6). Here, crevasses have mean a depth of 10.5 m, length of 50 m
and widths of 18 m (Table 2). In zone 2, 300 to 500 m from the northern
flank, crevasses are aligned obliquely to the direction of ice flow (30–45
Water-filled crevasses were clustered in zone 4, coinciding with the sector
of larger crevasses (Fig. 6b). Water-filled crevasses covered 12 000 m
Successive profiles of the terminus from 1 and 2 July demonstrate how the
UAV surveys are capable of capturing the displacement of crevasses, which
advect downstream at a rate of 5 and 16 m d
The two calving events identified between 1 and 2 July appear to take place under contrasting conditions. Event A consisted of the calving of multiple, relatively small ice blocks with the glacier failing along two main crevasses located 30 and 50 m behind the calving front. These crevasses were between 8 and 10 m deep, respectively, and in this instance, the crevasses located closest to the front were the ones that failed. Event B appears to be a single large event caused by the fracturing of a series of parallel crevasses which were up to 14 m deep and 60 m behind the calving front. Unlike, calving event A, the crevasses that failed in event B were not the closest to the calving front. Indeed, there were other crevasses that were deeper and located nearer to the front, yet did not calve. Water was not observed in any of the crevasses along which calving took place.
The orientation of crevasses suggests that lateral drag is an important resistive stress on Store Glacier. The lateral margins of Store Glacier are characterised by crevasses that are orientated parallel to the direction of flow which suggests that they have formed in response to simple shear stresses associated with the drag of the fjord walls (Fig. 6) (Benn and Evans, 2010). The importance of lateral drag is further demonstrated by the morphology of crevasses found near the glacier flow line (Fig. 6). Their arcuate nature indicates that the principal tensile stresses operating on the ice have been rotated by lateral gradients in ice velocity. These gradients are caused by the simple shear stress between the fjord walls and the margins of the glacier which cause the ice to flow slower (Fig. 4b) (Benn and Evans, 2010).
The simple shearing caused by velocity gradients is further demonstrated by the differing relationship between velocity and depths between the north and south side of the glacier (Fig. 5c, d). On the north side, the velocity increases gradually from the fjord wall to the centre of the glacier, reflecting the gradual deepening of bathymetry and the resulting decrease of basal and lateral drag. On the south side, the velocities are higher than the north side for given depths and distances from the lateral margins (Fig. 5c, d). We hypothesise that, because the deepest part of the glacier is situated 1 km south of the centreline, the ice on south side is more influenced by faster flowing ice which exerts a simple shear stress on the shallower, adjacent ice (250–400 m thick). This causes the shallow ice to flow faster than ice with similar thicknesses and distance from the lateral margins on the north side (Fig. 5c).
The mass flux through the calving front was calculated at 3.8
The lack of variation in the position of the lateral margins of the glacier shows that a balance is maintained between the ice flux input and submarine melting and calving output in this zone throughout the melt season. The balance could be explained by the mechanism of calving events. At the lateral margins calving is characterised by small, regular events such as calving event A (Fig. 3a). The regularity of these small events means that any small advance or retreat is regulated almost instantly by changes in calving rate which returns the lateral margins of the glacier to the same position. Calving rate could also be moderated by changes in the bathymetry. When the lateral margins advance, calving rates increase due to the abrupt deepening of the bathymetry seaward of the lateral margins of the glacier which cause basal drag to be reduced. Ice-flow acceleration can lead to increased longitudinal stretching and deeper crevassing, thereby increasing calving rate and leading to retreat to its original, bathymetrically pinned position.
The centre of the calving front is much more active with calving and
submarine melt rates that vary on a seasonal timescale. We propose that the
main cause of variability is due to calving rates which are highly irregular
throughout the melt season (Jung et al., 2010). Our observations also
support the suggestion that calving rates are dominated by major calving
events which have a time interval of around 28 days (e.g. Jung et al.,
2010). If the calving front advances for 28 days at 16 m d
Towards the end of the melt season (23 August), a widespread surface
deflation of 0.12 m d
Another important observation is the order of magnitude reduction of the
area of water-filled crevasses between early July and late August (Fig. 6).
Surface air temperatures directly influence the extent of water-filled
crevasses. AWS data reveal that mean daily air temperature was
A UAV equipped with a commercial digital camera enabled us to obtain high-resolution DEMs and orthophotos of the calving front of a major tidewater glacier at an affordable price. Airborne lidar currently presents the only alternative method for acquiring DEMs with comparable accuracy and precision. However, to fly consecutive sorties in a remote environment is likely to be prohibitively expensive and with sufficient ground control points the digital photogrammetry approach may also exceed the accuracy of this technique.
The three sorties flown enabled key glaciological parameters to be
quantified at sufficient detail to reveal that the terminus of Store Glacier
is a complex system with large variations in crevasse patterns surface
velocities, calving processes, surface elevations and front positions at a
daily and seasonal timescale. Surface velocities vary across the terminus
and are influenced by both basal and lateral drag (Figs. 4b and 5c, d). The
oblique orientation and arcuate nature of crevasses suggests that the
principal extending strain rate is orientated obliquely to the direction of
flow and we therefore propose that resistive stresses at the terminus of
Store Glacier are dominated by lateral drag (Fig. 6). With this in mind, the
retreat of Store Glacier into a wider trough could significantly increase the ice
discharge. We estimated that the ice flux through the calving front of Store
Glacier
was 13.9 Gt a
Future studies, with more frequent sorties could be used to compare and
investigate further glaciological changes over a more continuous timespan.
There is also the possibility of more sophisticated payloads with radiation,
albedo and other multi-band sensors as well as radar and laser altimetry.
There are many potential cryospheric applications for investigation, such as
sea ice, marine and terrestrial-terminating glaciers and, with increased
range, ice sheets, that can be achieved with the use of repeat UAV surveys.
We have demonstrated that for calving outlet glaciers, a UAV carrying a high-resolution digital camera would be sufficient to investigate the following
projects:
analysis of the thickness and back stress exerted by the ice mélange
during the winter and the effect of its break out on glacier flow, calving rate and
character; seasonal changes in the depth, density, orientation and nature of crevassing and their impact on calving rate and
character; the influence of daily to seasonal melt and supraglacial lake drainage on downstream dynamics and
calving; analysis of daily to seasonal fluctuations in calving flux, terminus position and impact on upstream dynamics and thinning.
We thank Matt Nolan, Doug Benn and Mauri Pelto for their thorough and
insightful reviews, and Anders Damsgaard for his short comments: all of
which greatly improved the manuscript. Funding for the fieldwork was made
possible by the UK Natural Environmental Research Council (NERC) grant
NE/K005871/1 (Subglacial Access and Fast Ice Research Experiment (SAFIRE):
Resolving the Basal Control on Ice Flow and Calving in Greenland). NERC also
funded