Bas Altena

• Altena, Bas; Haga, ON; Nuth, Christopher & Kääb, Andreas (2019). Monitoring sub-weekly evolution of surface velocity and elevation for a high-latitude surging glacier using sentinel-2. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences.  ISSN 1682-1750.  42(2), s 1723- 1727 . doi: 10.5194/isprs-archives-XLII-2-W13-1723-2019
• Altena, Bas; Scambos, Ted A; Fahnestock, M & Kääb, Andreas (2019). Extracting recent short-term glacier velocity evolution over southern Alaska and the Yukon from a large collection of Landsat data. The Cryosphere.  ISSN 1994-0416.  13(3), s 795- 814 . doi: 10.5194/tc-13-795-2019 Show summary

  The measurement of glacier velocity fields using repeat satellite imagery has become a standard method of cryospheric research. However, the reliable discovery of important glacier velocity variations on a large scale is still problematic because time series span different time intervals and are partly populated with erroneous velocity estimates. In this study we build upon existing glacier velocity products from the GoLIVE dataset (https://nsidc.org/data/golive, last access: 26 February 2019) and compile a multi-temporal stack of velocity data over the Saint Elias Mountains and vicinity. Each layer has a time separation of 32 days, making it possible to observe details such as within-season velocity change over an area of roughly 150 000 km2. Our methodology is robust as it is based upon a fuzzy voting scheme applied in a discrete parameter space and thus is able to filter multiple outliers. The multi-temporal data stack is then smoothed to facilitate interpretation. This results in a spatiotemporal dataset in which one can identify short-term glacier dynamics on a regional scale. The goal is not to improve accuracy or precision but to enhance extraction of the timing and location of ice flow events such as glacier surges. Our implementation is fully automatic and the approach is independent of geographical area or satellite system used. We demonstrate this automatic method on a large glacier area in Alaska and Canada. Within the Saint Elias and Kluane mountain ranges, several surges and their propagation characteristics are identified and tracked through time, as well as more complicated dynamics in the Wrangell Mountains.

• Deschamps-Berger, Cesar; Nuth, Christopher; Van Pelt, Ward; Berthier, Etienne; Kohler, Jack & Altena, Bas (2019). Closing the mass budget of a tidewater glacier: the example of Kronebreen, Svalbard. Journal of Glaciology.  ISSN 0022-1430.  65(249), s 136- 148 . doi: 10.1017/jog.2018.98 Full text in Research Archive.
• Gong, Yongmei; Zwinger, Thomas; Åström, J.; Altena, Bas; Schellenberger, Thomas; Gladstone, Rupert & Moore, John C. (2018). Simulating the roles of crevasse routing of surface water and basal friction on the surge evolution of Basin 3, Austfonna ice cap. The Cryosphere.  ISSN 1994-0416.  12(5), s 1563- 1577 . doi: 10.5194/tc-12-1563-2018
• Altena, Bas & Kääb, Andreas (2017). Elevation change and improved velocity retrieval using orthorectified optical satellite data from different orbits. Remote Sensing.  ISSN 2072-4292.  9(3) . doi: 10.3390/rs9030300 Full text in Research Archive. Show summary

  Optical satellite products are available at different processing levels. Of these products, terrain corrected (i.e., orthorectified) products are the ones mostly used for glacier displacement estimation. For terrain correction, a digital elevation model (DEM) is used that typically stems from various data sources with variable qualities, from dispersed time instances, or with different spatial resolutions. Consequently, terrain representation used for orthorectifying satellite images is often in disagreement with reality at image acquisition. Normally, the lateral orthoprojection offsets resulting from vertical DEM errors are taken into account in the geolocation error budget of the corrected images, or may even be neglected. The largest offsets of this type are often found over glaciers, as these may show strong elevation changes over time and thus large elevation errors in the reference DEM with respect to image acquisition. The detection and correction of such orthorectification offsets is further complicated by ice flow which adds a second offset component to the displacement vectors between orthorectified data. Vice versa, measurement of glacier flow is complicated by the inherent superposition of ice movement vectors and orthorectification offset vectors. In this study, we try to estimate these orthorectification offsets in the presence of terrain movement and translate them to elevation biases in the reference surface. We demonstrate our method using three different sites which include very dynamic glaciers. For the Oriental Glacier, an outlet of the Southern Patagonian icefield, Landsat 7 and 8 data from different orbits enabled the identification of trends related to elevation change. For the Aletsch Glacier, Swiss Alps, we assess the terrain offsets of both Landsat 8 and Sentinel-2A: a superior DEM appears to be used for Landsat in comparison to Sentinel-2, however a systematic bias is observed in the snow covered areas. Lastly, we demonstrate our methodology in a pipeline structure; displacement estimates for the Helheim-glacier, in Greenland, are mapped and corrected for orthorectification offsets between data from different orbits, which enables a twice as dense a temporal resolution of velocity data, as compared to the standard method of measuring velocities from repeat-orbit data only. In addition, we introduce and implement a novel matching method which uses image triplets. By formulating the three image displacements as a convolution, a geometric constraint can be exploited. Such a constraint enhances the reliability of the displacement estimations. Furthermore the implementation is simple and computationally swift.

• Altena, Bas & Kääb, Andreas (2017). Weekly glacier flow estimation from dense satellite time series using adapted optical flow technology. Frontiers in Earth Science.  ISSN 2296-6463.  5 . doi: 10.3389/feart.2017.00053 Full text in Research Archive.
• Altena, Bas; Mousivand, A; Mascaro, J & Kääb, Andreas (2017). Potential and limitations of photometric reconstruction through a flock of dove cubesats. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences.  ISSN 1682-1750.  42(3), s 7- 11 . doi: 10.5194/isprs-archives-XLII-3-W3-7-2017 Full text in Research Archive.
• Kääb, Andreas; Altena, Bas & Mascaro, Joseph (2017). Coseismic displacements of the 14 November 2016 Mw 7.8 Kaikoura, New Zealand, earthquake using the Planet optical cubesat constellation. Natural hazards and earth system sciences.  ISSN 1561-8633.  17, s 627- 639 . doi: 10.5194/nhess-17-627-2017 Full text in Research Archive. Show summary

  Satellite measurements of coseismic displacements are typically based on synthetic aperture radar (SAR) interferometry or amplitude tracking, or based on optical data such as from Landsat, Sentinel-2, SPOT, ASTER, very high-resolution satellites, or air photos. Here, we evaluate a new class of optical satellite images for this purpose – data from cubesats. More specific, we investigate the PlanetScope cubesat constellation for horizontal surface displacements by the 14 November 2016 Mw 7.8 Kaikoura, New Zealand, earthquake. Single PlanetScope scenes are 2–4 m-resolution visible and near-infrared frame images of approximately 20–30 km  ×  9–15 km in size, acquired in continuous sequence along an orbit of approximately 375–475 km height. From single scenes or mosaics from before and after the earthquake, we observe surface displacements of up to almost 10 m and estimate matching accuracies from PlanetScope data between ±0.25 and ±0.7 pixels (∼ ±0.75 to ±2.0 m), depending on time interval and image product type. Thereby, the most optimistic accuracy estimate of ±0.25 pixels might actually be typical for the final, sun-synchronous, and near-polar-orbit PlanetScope constellation when unrectified data are used for matching. This accuracy, the daily revisit anticipated for the PlanetScope constellation for the entire land surface of Earth, and a number of other features, together offer new possibilities for investigating coseismic and other Earth surface displacements and managing related hazards and disasters, and complement existing SAR and optical methods. For comparison and for a better regional overview we also match the coseismic displacements by the 2016 Kaikoura earthquake using Landsat 8 and Sentinel-2 data.

• Kääb, Andreas; Winsvold, Solveig Havstad; Altena, Bas; Nuth, Christopher; Nagler, Thomas & Wuite, Jan (2016). Glacier remote sensing using Sentinel-2. Part I: Radiometric and geometric performance, and application to ice velocity. Remote Sensing.  ISSN 2072-4292.  8(7) . doi: 10.3390/rs8070598 Full text in Research Archive.
• Altena, Bas; Kääb, Andreas & Nuth, Christopher (2015). Robust glacier displacements using knowledge-based image matching, In  Analysis of multitemporal Remote sensing images (Multi-Temp), 2015 8th International workshop on the....  IEEE Press.  ISBN 978-1-4673-7119-3.  Unummerert.
• Nuth, Christopher; Schuler, Thomas; Kohler, Jack; Altena, Bas & Hagen, Jon Ove Methlie (2012). Estimating the long-term calving flux of Kronebreen, Svalbard, from geodetic elevation changes and mass-balance modelling. Journal of Glaciology.  ISSN 0022-1430.  58(207), s 119- 133 . doi: 10.3189/2012JoG11J036

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• Altena, Bas (2019). Observing the ice of our planet with daily cubesat imagery. Institute for Interdisciplinary Mountain Research (IGF) of the Austrian Academy of Sciences (ÖAW).  ISBN 978-3-7001-8565-9.  2 s.

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• Altena, Bas; Armstrong, William & Kääb, Andreas (2019). On the origin of ogives.
• Altena, Bas & Kääb, Andreas (2019). River ice velocity from Sentinel-twogether and PROBA-VELOCITY. Show summary

  Most optical satellite systems are set in a sun-synchronous orbit. Because of such similar configurations, orbits-of-opportunity arise. These are combinations of satellites that sense near-simultaneously. Consequently, more information can be generated from a single satellite mission when it is orchestrated with another. Hence, one tactic is to let a small-satellite piggyback with a monitoring program. In this study we demonstrate such a strategy where we make use of the PROBA-V small-satellite in combination with Sentinel-2. The inclination of both satellites are very similar, as well as, their overpass time. This makes large scale extraction of displacements possible. One such application that can make use of this recording sequence is the monitoring of river ice break-up. In spring the melting season sets in and Arctic rivers increase in flow discharge. Eventually the water is able to break the ice cover, which will then float along. However, congestion of this ice results in jamming, associated flooding and ice run, causing damage to infrastructure and river banks. The long stretch of rivers make monitoring difficult, thus satellite remote sensing can be a means for identifying bottlenecks. The resolution of Proba-V is 100 meters, but the time separation results in drift of the floating ice in the order of 1 km. Hence, from data of this satellite pair we are able to generate a dense velocity field over the Lena River in Russia, which stretches more than 300 km. This study demonstrates the potential of the Sentinels as an umbrella for other smaller missions. This near-simultaneous configuration has not been envisioned as a mission objective, but is coincidence. However for a future small-satellite design through clever orbit configurations can easily enhance a satellites product portfolio.

• Altena, Bas; Schellenberger, Thomas & Kääb, Andreas (2019). Bringing together remote sensing data to produce region-wide glacier velocities. Show summary

  The recent increase of medium resolution satellite programs that consistently senses the Earth surface has brought great potential for glaciology. Satellites like the Sentinel-1&2 tandems, Landsat 7&8 duo and Rapideye constellation sense mountain ranges almost without missing a day. Consequently, high resolution time-series can be constructed, for snow-cover or crack propagation, or other single scene information. Properties derived from combinations of acquisitions, such as, glacier velocity is more challenging. Velocity estimation is based upon image matching, which can be hindered by coherence loss or cloud cover. Consequently, generated velocity data have gaps and have scattered coverage at different time intervals. This steady data stream has resulted in several initiatives to use image matching techniques in batch processing mode. Resulting in glacier velocity services being set-up (e.g.: cryoportal, GoLIVE, Geodetic data portal), but these initiatives are satellite specific. If such products are merged it is possible to increase temporal and spatial coverage and increase the reliability of the velocity estimates. We developed such a workflow that merges glacier velocity products of different time spans and type. This combined dataset covers several mountain ranges and has a regular time step at a monthly pace. This consistent time-series make it possible to observe glacier dynamics that are of short nature. As the satellite recordings already span several years, the dynamics of tidewater glaciers or (partial) surges can be observed and analyzed. This study therefor shows the great potential of bringing together different satellite systems.

• Altena, Bas (2018). Daily cubesat imagery to observe and assess processes in the cryosphere.
• Altena, Bas (2018). From satellite snapshots towards snow and ice services.
• Altena, Bas (2018). Knowledge on the ground, through observed ice flow from space.
• Altena, Bas (2018). Should we send Hans Brinker to the Greenland Ice Sheet?. Tvergastein : interdisciplinary journal of the environment.  11, s 14- 23
• Altena, Bas (2018, 01. september). researching glaciers with satellite imagery. [Internett].  Beryllium newsletter.
• Altena, Bas & Kääb, Andreas (2018). High-resolution surface and upper fjord circulation of Greenland fjords from optical remote sensing. Show summary

  Water within Greenland's fjords functions as conduit between the ice sheet and the Atlantic ocean. Exchange of energy and mass in the proglacial fjords is mainly driven by tides, wind, density fluctuations, glacial discharge and calving. All these forces change or alternate at different time scales and/or places. Measuring related (sub)surface fluxes in these fjords is complicated as fjord-ice hampers logistics and iceberg migration is able to damage measurement installations. However, an increasing fleet of optical satellites are sensing the Earth surface and can in this way observe Greenland fjords at multiple times a day. In this study we introduce a novel type of surface velocity extraction over even short time scales (minutes and hours apart), which exploits the multitude of satellites images taken daily over specific sites and is able to separate the surface velocities from the flows occurring lower in the water column. The resulting velocity fields are able to capture small scale features (±10m) over the full range of large fjords (±100km). Imagery from the Planet constellation, Sentinel-2 and Landsat 8 are used to extract the (sub)surface flow regime of Sermilik fjord. This can be estimated for several days within a week, making it possible to capture whole fjord variability and the mode of fjord circulation. Such detailed and complete circulation information will help to complement in-situ measurements and improve the modelling of ice-ocean interaction.

• Altena, Bas & Kääb, Andreas (2018). Doves & displacements.
• Altena, Bas & Kääb, Andreas (2018). ICEFLOW: short-term movements in the cryosphere.
• Altena, Bas & Kääb, Andreas (2018). Kinematics in the Cryosphere from space.
• Altena, Bas & Kääb, Andreas (2018). Observing change in glacier flow from space. Frontiers for Young Minds.  6, s 1- 9 . doi: 10.3389/frym.2018.00009
• Altena, Bas & Kääb, Andreas (2018). Sensing a moving Arctic from space.
• Altena, Bas; Kääb, Andreas; Leclercq, Paul Willem & Nuth, Christopher (2018). Observing change in glacier flow by using optical satellites. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 1977. Show summary

  In the last couple of years many Earth observation satellites with optical instruments have been set in space. These satellites generate an enormous amount of data and give us a image of different landforms on Earth. The data are available for researchers in Earth Science, though efficiently transforming this imagery data to glaciological information has been a challenge. The work in this dissertation presents modern day techniques to extract glacier velocity information from the satellite imagery. Now it is possible to extract reliable displacement measurements from any satellite independent of its flight path. In this way extracting reliable decadal changes of glacier velocity is finally possible. Moreover, by recent development in technology and clever algorithms developed in this PhD work, extracting short term velocity changes are one of the possibilities. So the timing of sliding of a glacier due to melt water can be observed and located. Lastly, methods for data reduction of big data volumes are exploited to develop a discovery tool that is able to observe glacier dynamics over several large mountain ranges. This research might be the first step towards transforming large data volumes into useful information for worldwide glacier monitoring.

• Altena, Bas; Scambos, Ted A; Fahnestock, M & Kääb, Andreas (2018). Automatic extraction of velocity time-series at mountain range scale from Landsat 8..
• Altena, Bas (2017). Observing processes in the cryosphere through historical and modern optical data.
• Altena, Bas (2017). small cube-sats and big ice-cubes.
• Altena, Bas & Dakka, Jumana (2017). Daily river ice monitoring from space.
• Altena, Bas & Kääb, Andreas (2017). Glacier ice loss monitored through the Planet cubesat constellation. Show summary

  The Planet cubesat constellation is a developing earth observation constellation, that is and will continue to sense the whole earth surface on a daily scale at high resolution. In this contribution, we exploit this datastream to estimate the iceberg production of an outlet glacier of the Southern Patagonian icefield, called Perito Moreno. We demonstrate an automatic pipeline that takes into account the displacement due to glacier motion and the ad-hoc coverage of the glacier, due to different observation angles and orbits. With such a robust and adaptive pipeline, constructing a high resolution and temporal dense dataset is possible to be obtained. Which is of great value to understand the complex behavior of calving.

• Gong, Yongmei; Zwinger, Thomas; Åström, Jan A.; Altena, Bas; Schellenberger, Thomas; Gladstone, Rupert M & Moore, John (2017). Basal friction evolution and crevasse distribution during the surge in Basin-3, Austfonna ice-cap - offline coupling between a continuum ice dynamic model and a discrete element model.
• Leclercq, Paul Willem; Aalstad, Kristoffer; Altena, Bas & Elvehøy, Hallgeir (2017). Modelling of glacier surface mass balance with assimilation of glacier mass balance and snow cover observations from remote sensing.
• Altena, Bas & Kääb, Andreas (2016). Estimating the seasonal glacier flow-field evolution of Kronebreen from the SPOT5-take5 campaign.
• Altena, Bas & Kääb, Andreas (2016). Glacier photogrammetry.
• Altena, Bas; Leclercq, Paul Willem & Kääb, Andreas (2016). Automatic monitoring of the transient snowline on mountain glaciers.
• Kääb, Andreas & Altena, Bas (2016). An icy Planet.
• Kääb, Andreas; Treichler, Désirée Silvana; Girod, Luc Maurice Ramuntcho; Lefeuvre, Pierre-Marie; Schellenberger, Thomas; Altena, Bas; Jonassen, Vetle; Nuth, Christopher; McNabb, Robert Whitfield & Gilbert, Adrien (2016). On the Karakoram anomaly.
• Kääb, Andreas; Winsvold, Solveig Havstad; Altena, Bas & Nuth, Christopher (2016). The potential of Sentinel-2 for investigating glaciers, permafrost and related natural hazards.
• Leclercq, Paul Willem; Aalstad, Kristoffer; Elvehøy, Hallgeir & Altena, Bas (2016). Assimilation of glacier mass balance and snow cover fraction observations in a glacier surface mass balance model.
• Winsvold, Solveig Havstad; Kääb, Andreas; Altena, Bas & Nuth, Christopher (2016). Radiometrisk og geometrisk ytelse av Sentinel-2 relatert til brekartlegging.
• Altena, Bas (2016). Surges of St. Elias glaciers, recent and blasts from the past.
• Altena, Bas; Dunse, Thorben & Schellenberger, Thomas (2016). Crevasse evolution of Basin-3 during its speed-up.
• Altena, Bas & Kääb, Andreas (2016). Exploiting orthorectification offsets in optical satellite data for glacier elevation change and velocity.
• Altena, Bas & Kääb, Andreas (2016). Why is my orthophoto distorted?.
• Kääb, Andreas & Altena, Bas (2016). Microsatellite constellations for monitoring cryospheric processes and related natural hazards.
• Leclercq, Paul Willem; Aalstad, Kristoffer; Elvehøy, Hallgeir & Altena, Bas (2016). Assimilation of glacier mass balance and snow cover fraction observations in a glacier surface mass balance model.
• Winsvold, Solveig Havstad; Kääb, Andreas; Altena, Bas & Nuth, Christopher (2016). The potential of Sentinel-2 for investigating glaciers and glacier lakes.
• Winsvold, Solveig Havstad; Kääb, Andreas; Altena, Bas & Nuth, Christopher (2016). Using a new optical satellite for investigating glaciers and Jøkulhlaups.
• Winsvold, Solveig Havstad; Kääb, Andreas; Nuth, Christopher & Altena, Bas (2016). Fusion of Optical and SAR Time-series for Glacier Mapping Applications.
• Winsvold, Solveig Havstad; Kääb, Andreas; Nuth, Christopher & Altena, Bas (2016). Landsat time-series analysis opens new approaches for regional glacier mapping.
• Altena, Bas & Kääb, Andreas (2015). Increasing information content of multi-spectral imagery for glacier mapping. Show summary

  Optical image matching is a powerful technique to extract glacier velocities. This technique is able to work in a highly automatic fashion, and is thus a valuable tool for large scale glacier analysis. Image matching relies on the principle of persistent surface features which can be identified in an image pair. For glacier velocity estimation, these features are typically crevasses, meltwater channels, debris, dust patterns, etc. Thus in order to successfully match two images, such features should be the most dominant objects in the image, in respect to other phenomena which are short lived. To reach such an optimum, several spectral bands can be chosen, or the higher resolution panchromatic band. However, every option has its pro and con’s, and this study aims at combining these aspects to come to an optimal image. Our method drives on the linearity and cross-spectral correlation between spectral bands. The panchromatic band is the backbone of the image, and additional information is included from the near infrared (NIR) and short wave infrared (SWIR). Consequently, saturation is detected and adjusted. All this can be done in an automatic fashion, and is precluded against snow properties of which the SWIR is highly reactive to. We assess our methodology qualitatively by looking at the top of the correlation score, but also the spread of this peak through Gaussian fitting. The correlation score is related to the reliability of a measurement, while the spread is a proxy for its precision. Matching is done over a large sample set of glaciers, which change in, season, glacier type, and lithology. Furthermore, from our theory we can explain why principle component analysis (PCA) is a powerful pre-processing step. Nevertheless our method seems to perform better, and from our theory we can identify weaknesses in PCA. Furthermore, our method is tested against a higher spatial- and radiometric-resolution acquisition. This gives more confidence in our claims, and justifies our methodological decisions.

• Altena, Bas; Kääb, Andreas & Nuth, Christopher (2015). Robust glacier displacements using knowledge-based image matching. Show summary

  Matching of repeat optical satellite images is a powerful tool for the analysis of glacier movement. Due to the rapid growth of earth observation data archives there is an increasing potential for regional glacier flow studies on a decadal scale. However, exploitation of these image collections is hindered by the currently manual pre- and post-processing steps required. Automating these steps, and using content from all available imagery will help tap the wealth of information present within the data. In this study we explore this possibility with the aim to extract velocity fields that are both accurate and reliable. We propose a bottom-up approach in which simple image matching estimates from three images are pierced together, to give rise to the far more complex time-dependent glacier flow. Taking advantage of our knowledge of glacier behavior, rather then crude statistical inferences. We first loosen the threshold of the matching, estimating dispersions for every candidate match. Then by implementing the procedure for multiple time periods we constrain the solution making it possible to apply probabilistic testing. Our approach is alternative to common top-down implementations and instead derives from first principles. In summary, we introduce the concept of precision and reliability of image matching which enhances the analysis of velocity fields with hypothesis model testing to investigate its driving forces.

• Nuth, Christopher; Kohler, Jack; Köhler, Andreas; Schuler, Thomas; Chapuis, Anne; Altena, Bas & Hagen, Jon Ove Methlie (2012). Observing the Kronebreen glacier system from the ground, by air and space, and through seismic waves.
• Kääb, Andreas Max; Altena, Bas; Karstensen, Jonas & Narama, Chiyuki (2009). Spaceborne remote sensing of glacier mass changes and dynamics.
• Kääb, Andreas Max; Altena, Bas; Karstensen, Jonas; Nuth, Christopher & Narama, Chiyuki (2009). Spaceborne remote sensing of glacier mass changes and dynamics.

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