PyTrx : a python-based monoscopic terrestrial photogrammetry toolset for glaciology

This work was affiliated with the CRIOS project (Calving Rates and Impact On Sea Level), which was supported by the Conoco Phillips-Lundin Northern Area Program. PH was funded by a NERC Ph.D. studentship (reference number 1396698).Terrestrial time-lapse photogrammetry is a rapidly growing method for deriving measurements from glacial environments because it provides high spatio-temporal resolution records of change. Currently, however, the potential usefulness of time-lapse data is limited by the unavailability of user-friendly photogrammetry toolsets. Such data are used primarily to calculate ice flow velocities or to serve as qualitative records. PyTrx (available at https://github.com/PennyHow/PyTrx) is presented here as a Python-alternative toolset to widen the range of monoscopic photogrammetry (i.e., from a single viewpoint) toolsets on offer to the glaciology community. The toolset holds core photogrammetric functions for template generation, feature-tracking, camea calibration and optimization, image registration, and georectification (using a planar projective transformation model). In addition, PyTrx facilitates areal and line measurements, which can be detected from imagery using either an automated or manual approach. Examples of PyTrx's applications are demonstrated using time-lapse imagery from Kronebreen and Tunabreen, two tidewater glaciers in Svalbard. Products from these applications include ice flow velocities, surface areas of supraglacial lakes and meltwater plumes, and glacier terminus profiles.Publisher PDFPeer reviewe

Pointcatcher software:analysis of glacial time-lapse photography and integration with multi-temporal digital elevation models

Terrestrial time-lapse photography offers insight into glacial processes through high spatial and temporal resolution imagery. However, oblique camera views complicate measurement in geographic coordinates, and lead to reliance on specific imaging geometries or simplifying assumptions for calculating parameters such as ice velocity. We develop a novel approach that integrates time-lapse imagery with multi-temporal digital elevation models to derive full 3D coordinates for natural features tracked throughout a monoscopic image sequence. This enables daily independent measurement of horizontal (ice flow) and vertical (ice melt) velocities. By combining two terrestrial laser scanner surveys with a 73-day sequence from Sólheimajökull, Iceland, variations in horizontal ice velocity of ~10% were identified over timescales of ~25 days. An overall surface elevation decrease of ~3.0 m showed rate changes asynchronous with the horizontal velocity variations, demonstrating a temporal disconnect between the processes of ice surface lowering and mechanisms of glacier movement. Our software, ‘Pointcatcher’, is freely available for user-friendly interactive processing of general time-lapse sequences and includes Monte Carlo error analysis and uncertainty projection onto DEM surfaces. It is particularly suited for analysis of challenging oblique glacial imagery, and we discuss good features to track, both for correction of camera motion and for deriving ice velocities

Millimetre-wave radar observations of glacier calving at Hansbreen (Svalbard) correlated with TLS, time-lapse camera images and seismic records

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Calving controlled by melt-under-cutting: detailed calving styles revealed through time-lapse observations

Here, we present a highly detailed study of calving dynamics at Tunabreen, a tidewater glacier in Svalbard. A time-lapse camera was trained on the terminus and programmed to capture images every three seconds over a 28-hour period in August 2015, producing a highly detailed record of 34,117 images from which 358 individual calving events were distinguished. Calving activity is characterised by frequent events (12.8 events per hour) that are small relative to the spectrum of calving events observed, demonstrating the prevalence of small-scale calving mechanisms. Five calving styles were observed, with a high proportion of calving events (82%) originating at, or above, the waterline. The tidal cycle plays a key role in the timing of calving events, with 68% occurring on the falling limb of the tide. Calving activity is concentrated where meltwater plumes surface at the glacier front, and a ∼5 m undercut at the base of the glacier suggests that meltwater plumes encourage melt-undercutting. We conclude that frontal ablation at Tunabreen may be paced by submarine melt rates, as suggested from similar observations at glaciers in Svalbard and Alaska. Using submarine melt rate to calculate frontal ablation would greatly simplify estimations of tidewater glacier losses in prognostic models

Ice‐marginal proglacial lakes across Greenland: Present status and a possible future

Ice-marginal lakes can affect glacier dynamics but are ignored in studies of the evolution of the Greenland ice sheet (GrIS) and of peripheral mountain glaciers and ice caps (PGICs). Here we show that lakes occupy 10 % of the GrIS ice margin and occur on 5 % of PGICs. Ice velocity at the GrIS margin is enhanced by ∼ 25 % at lakes versus on land. Mean ice discharge into lakes is ∼ 4.9 Gt.yr, which is ∼1 % of ice discharged through marine termini. We locate thousands of subglacial overdeepenings within which 7,404 km2 of future lakes could form, all of which will be ice-marginal at some time. Future lakes in the west and east will be restricted to the margin of the GrIS and within alpine valleys, respectively. This status and possible future leads us to contend that lakes should be incorporated into projections of Greenland ice loss

Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries

Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P < 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely

Dynamical change at tidewater glaciers examined using time-lapse photogrammetry

Retreating glaciers and ice sheets provide a significant contribution to sea level rise, which will affect future populations and their activities. Accurate sea level projections are needed in order to best inform policy makers, but these projections are limited by our understanding of dynamical change at marine-terminating glaciers. Terrestrial time-lapse photography has proved to be a viable approach for obtaining high-detail observational records, and is used here to examine signals of dynamical change at two tidewater glaciers in Svalbard. Photogrammetric measurements were extracted using PyTrx (`Python Tracking'), a new photogrammetry toolbox that has been developed here for deriving velocities (e.g. glacier surface velocity), surface areas (e.g. supraglacial lake area, surfacing plume area), and line distances (e.g. terminus profiles). PyTrx has been created as a Python-alternative photogrammetry software, and offers additional functionality to the typical monoscopic feature-tracking toolboxes that are currently available. Subglacial hydrology and its relation to basal sliding were examined at Kronebreen, Svalbard. The results revealed a difference in flow efficiency between the north and south regions of the glacier tongue, which influences spatial patterns in surface velocities. Long-term changes in ice flow were concluded to be controlled by the location of effcient and inefficient drainage, and the position of regions where water is stored and released. Changes in terminus conditions and calving processes were examined at Tunabreen, a surge-type tidewater glacier. Observations suggested that atmospheric forcing plays a larger role in terminus stability than previously considered, and it is likely that terminus dynamics at Tunabreen are the product of a unique interplay between oceanic and atmospheric forcing which are shaped by the glacier's surge-type nature. Additionally, calving activity at Tunabreen can be characterised as high-frequency, low-magnitude events, and a high proportion of its long-term calving activity can be attributed to the rate of under-cutting at the terminus. In all, these studies demonstrate that long-term changes in glacier dynamics are dictated by the small changes in basal and terminus conditions, and how they vary from year-to-year. Future research now needs to be directed towards understanding how small-scale processes vary over multiple melt seasons, in order to establish how they operate at longer timescales. PyTrx provides an appropriate basis to continue this work and expand the capabilities of the toolbox