No abstract

[1] We performed three field campaigns in 2004, 2007, and 2010 at the southern margin of the Jakobshavn Isbrae, West Greenland, in order to infer flow velocities and their changes from photogrammetric time-lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface. Area-wide analysis of more than 3000 three-dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events. From 2004 to 2010, the grounding line of Jakobshavn Isbrae retreated 3.5 AE 0.2 km. Considering previously published results, the grounding line retreat amounts to 6 km since 1985. The glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events. Observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m/d can be reached. Moreover, large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event.After the break up, the glacier slowly adjusts to the new boundary conditions within the next 4-5 days. Flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large-scale glacier dynamics.

[1] During the summer of 2004, the front area of the Jakobshavn Isbrae was monitored using a geodetic-photogrammetric survey with temporarily coincident precise observations of local ocean tides in the Disko Bay close to Ilulissat. The geodetic and photogrammetric observations were conducted at the southern margin of the glacier front. The largest observed horizontal flow velocities are in the central part of the front with values up to 45 m/d. This is a factor of 2 greater than the average velocities at the front area observed in the last century. Our new observations confirm previous estimates of an acceleration of glacier flow during the last decade. The photogrammetric survey provided flow trajectories for 4000 surface points with a time resolution of 30 min. These flow trajectories were used to compare the vertical motion of the glacier with the observed tides. The existence of a free-floating glacier tongue in 2004 was confirmed by these data. However, it occupied only a small belt, of at most a few 100 m width, in the central part of the glacier front. Horizontal motion did not appear to depend on the tidal phase, unlike some of the fast-moving ice streams of West Antarctica.

Abstract. This paper presents a comprehensive method for the determination of glacier surface motion vector fields at high spatial and temporal resolution. These vector fields can be derived from monocular terrestrial camera image sequences and are a valuable data source for glaciological analysis of the motion behaviour of glaciers. The measurement concepts for the acquisition of image sequences are presented, and an automated monoscopic image sequence processing chain is developed. Motion vector fields can be derived with high precision by applying automatic subpixel-accuracy image matching techniques on grey value patterns in the image sequences. Well-established matching techniques have been adapted to the special characteristics of the glacier data in order to achieve high reliability in automatic image sequence processing, including the handling of moving shadows as well as motion effects induced by small instabilities in the camera set-up. Suitable geo-referencing techniques were developed to transform image measurements into a reference coordinate system.The result of monoscopic image sequence analysis is a dense raster of glacier surface point trajectories for each image sequence. Each translation vector component in these trajectories can be determined with an accuracy of a few centimetres for points at a distance of several kilometres from the camera. Extensive practical validation experiments have shown that motion vector and trajectory fields derived from monocular image sequences can be used for the determination of high-resolution velocity fields of glaciers, including the analysis of tidal effects on glacier movement, the investigation of a glacier's motion behaviour during calving events, the determination of the position and migration of the grounding line and the detection of subglacial channels during glacier lake outburst floods.

Rock glaciers and transitional ice‐debris complexes predominate the Central Andean landform assemblage, yet regional studies on their state of activity and their kinematics remain sparse. Here we utilize the national glacier inventory of Argentina to quantify surface velocity fields of 244 rock glaciers and 51 ice‐debris complexes, located in the Cordón del Plata range, Argentina. Applying a feature‐tracking approach to repeated RapidEye satellite imagery acquired between 2010 and 2017/18, we find mean displacement rates between 0.37 and 2.61 m year−1 for 149 landforms, while for the remaining 146 features, surface movement remains below our level of detection. We compare our satellite‐derived velocity fields with ground‐truth data from two local field sites and find closely matching results in magnitude and spatial distribution. With average displacement of one‐third of the active rock glaciers and ice‐debris complexes exceeding 1 m year−1, the region hosts an exceptional number of fast‐flowing periglacial landforms, compared to other mountain belts. Using a random forest model, we test the predictive power of 25 morphometric and topoclimatic candidate predictors for modelling the state of activity of rock glaciers and ice‐debris complexes on two different scales. For entire landforms and individual landform segments, constructed along displacement centrelines, we can predict the state of activity with overall accuracies of 70.08% (mean AUROC = 0.785) and 74.86% (mean AUROC = 0.753), respectively. While topoclimatic parameters such as solar radiation and elevation are most important for entire landforms, geometric parameters become more important at the scale of landform segments. Despite tentative correlations between local slope and surface kinematics, our results point to factors integrating slope and distance to the source to govern local deformation. We conclude that feature tracking in optical imagery is feasible for regional studies in remote regions and provides valuable insight into the current state of the Andean cryosphere. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Hemispheric image processing with the goal of solar radiation determination from ground-based fisheye images is a valuable tool for silvicultural analysis in forest ecosystems. The basic idea of the technique is taking a hemispheric crown image with a camera equipped with a 180°fisheye lens, segmenting the image in order to identify solar radiation relevant open sky areas, and then merging the open sky area with a radiation and sun-path model in order to compute the total annual or seasonal solar radiation for a plant. The results of hemispheric image processing can be used to quantitatively evaluate the growth chances of ground vegetation (e.g., tree regeneration) in forest ecosystems. This paper shows steps towards the operationalization and optimization of the method. As a prerequisite to support geometric handling and georeferencing of hemispheric images, an equi-angular camera model is shown to describe the imaging geometry of fisheye lenses. The model is extended by a set of additional parameters to handle deviations from the ideal model. In practical tests, a precision potential of 0.1 pixels could be obtained with off-the-shelf fisheye lenses. In addition, a method for handling the effects of chromatic aberration, which may amount to several pixels in fisheye lens systems, is discussed. The central topic of the paper is the development of a versatile method for segmenting hemispheric forest crown images. The method is based on linear segmentoriented classification on radial profiles. It combines global thresholding techniques with local image analysis to ensure a reliable segmentation in different types of forest under various cloud conditions. Sub-pixel classification is incorporated to optimize the accuracy of the method. The performance of the developed method is validated in a number of practical tests.

Glaciar San Rafael in the Northern Patagonian Icefield, with a length of 46 km and an ice area of 722 km², is the lowest latitude tidewater outlet glacier in the world and one of the fastest and most productive glaciers in southern South America in terms of iceberg flux. In a joint project of the TU Dresden and CECS, spatio-temporal velocity fields in the region of the glacier front were determined in a campaign in austral spring of 2009. Monoscopic terrestrial image sequences were recorded with an intervallometer mode high resolution digital camera over several days. In these image sequences, a large number of glacier surface points were tracked by subpixel accuracy feature tracking techniques. Scaling and georeferencing of the trajectories obtained from image space tracking was performed via a multi-station GPS-supported photogrammetric network. <br><br> The technique allows for tracking hundreds of glacier surface points at a measurement accuracy in the order of one decimeter and an almost arbitrarily high temporary resolution. The results show velocities of up to 16 m per day. No significant tidal signals could be observed. Our velocities are in agreement with earlier measurements from theodolite and satellite interferometry performed in 1986–1994, suggesting that the current thinning of 3.5 m/y at the front is not due to dynamic thinning but rather by enhanced melting

Abstract. This paper presents a comprehensive method for the determination of motion vector fields of glaciers at high spatial and temporal resolution. These vector fields can be derived from monocular terrestrial camera image sequences and are a valuable data source for glaciological analysis of the motion behaviour of glaciers. The measurement concepts for the acquisition of image sequences are presented, and an automated monoscopic image sequence processing chain is developed. Motion vector fields can be derived with high precision by applying automatic sub-pixel-accuracy image matching techniques on grey value patterns in the image sequences. Well-established matching techniques have been adapted to the special characteristics of the glacier data in order to achieve high reliability in automatic image sequence processing, including the handling of moving shadows as well as motion effects induced by small instabilities in the camera setup. Suitable geo-referencing techniques were developed to transform image measurements into a reference coordinate system. The result of the monoscopic image sequence analysis is a dense raster of glacier surface point trajectories for each image sequence. Each translation vector component in these trajectories can be determined with an accuracy of some centimetres for points at a distance of several kilometres from the camera. Extensive practical validation experiments show that motion vector and trajectory fields derived from monocular image sequences can be used for the determination of high resolution velocity fields of glaciers, for the analysis of the effects of tides on glacier movement, for the investigation of a glacier's motion behaviour during calving events, for the determination of the position and migration of the grounding line and for the detection of sub glacial channels during glacier lake outburst floods.