ICESat Laser Altimetry Data Research Articles

High-Accuracy Laser Altimetry Global Elevation Control Point Dataset for Satellite Topographic Mapping

As the accuracy of satellite laser altimetry is susceptible to real-time atmospheric conditions, with-in footprint topography fluctuation, and detector noise, etc., we proposed a method by comprehensively analyzing the laser ranging error and evaluation labels to extract high-accuracy elevation control points that is suitable for satellite imagery based topographic mapping applications. Using the ICESat laser altimetry data, a global high accuracy laser altimetry dataset including more than 60 million control points, based on the laser altimetry ranging error model and waveform quality analysis is proposed by the paper. For land areas, except for areas of water, snow/ice, and polar ice sheets, the dataset can provide the elevation control points for worldwide satellite topographic mapping using high spatial resolution imageries or other science researches that depend on accurate earth’s elevation information. We further used airborne lidar data from six study areas around the world to carefully validate the dataset’s accuracy. The results showed that, this dataset can meet the accuracy requirement of global mapping using high spatial resolution satellite imageries in terrains with a slope below 25°. Compared to the raw dataset, the proportion of footprint elevations that conform to the accuracy standard (0.5m@ slope<2°, 1.5m@ 2°≤slope<6° and 3m@ 6°≤slope<25°) is increased from 68.24%, 59.97% and 26.52% to 87.58%, 90.04% and 83.91% respectively. This method can assure that its extracted results’ accuracy is either very close to or better than that obtained by the methods proposed in relevant studies, with a much larger number of laser footprints have been reserved.

Recent glacier and lake changes in High Mountain Asia and their relation to precipitation changes

Abstract. We present an updated, spatially resolved estimate of 2003–2008 glacier surface elevation changes for the entire region of High Mountain Asia (HMA) from ICESat laser altimetry data. The results reveal a diverse pattern that is caused by spatially greatly varying glacier sensitivity, in particular to precipitation availability and changes. We introduce a spatially resolved zonation where ICESat samples are grouped into units of similar glacier behaviour, glacier type and topographic settings. In several regions, our new zonation reveals local differences and anomalies that have not been described previously. Glaciers in the Eastern Pamirs, Kunlun Shan and central TP were thickening by 0.1–0.7 m a−1, and the thickening anomaly has a crisp boundary in the Eastern Pamirs that continues just north of the central Karakoram. Glaciers in the south and east of the TP were thinning, with increasing rates towards southeast. We attribute the glacier thickening signal to a stepwise increase in precipitation around ∼1997–2000 on the Tibetan Plateau (TP). The precipitation change is reflected by growth of endorheic lakes in particular in the northern and eastern TP. We estimate lake volume changes through a combination of repeat lake extents from Landsat data and shoreline elevations from ICESat and the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) for over 1300 lakes. The rise in water volume contained in the lakes corresponds to 4–25 mm a−1, when distributed over entire catchments, for the areas where we see glacier thickening. The precipitation increase is also visible in sparse in situ measurements and MERRA-2 climate reanalysis data but less visible in ERA-Interim reanalysis data. Taking into account evaporation loss, the difference between average annual precipitation during the 1990s and 2000s suggested by these datasets is 34–100 mm a−1, depending on region, which can fully explain both lake growth and glacier thickening (Kunlun Shan) or glacier geometry changes such as thinning tongues while upper glacier areas were thickening or stable (eastern TP). The precipitation increase reflected in these glacier changes possibly extended to the northern slopes of the Tarim Basin, where glaciers were nearly in balance in 2003–2008. Along the entire Himalaya, glaciers on the first orographic ridge, which are exposed to abundant precipitation, were thinning less than glaciers in the dryer climate of the inner ranges. Thinning rates in the Tien Shan vary spatially but are rather stronger than in other parts of HMA.

EXPERIMENTAL DEM EXTRACTION FROM ASTER STEREO PAIRS AND 3D REGISTRATION BASED ON ICESAT LASER ALTIMETRY DATA IN UPSTREAM AREA OF LAMBERT GLACIER, ANTARCTICA

Abstract. DEM Extraction from ASTER stereo pairs and three-dimensional registration by reference to ICESat laser altimetry data are carried out in upstream area of Lambert Glacier, East Antarctica. Since the study area is located in inland of East Antarctica where few textures exist, registration between DEM and ICESat data is performed. Firstly, the ASTER DEM generation is based on rational function model (RFM) and the procedure includes: a) rational polynomial coefficient (RPC) computation from ASTER metadata, b) L1A image product de-noise and destriping, c) local histogram equalization and matching, d) artificial collection of tie points and bundle adjustment, and e) coarse-to-fine hierarchical matching of five levels and grid matching. The matching results are filtered semi-automatically. Hereafter, DEM is interpolated using spline method with ground points converted from matching points. Secondly, the generated ASTER DEM is registered to ICESat data in three-dimensional space after Least-squares rigid transformation using singular value decomposition (SVD). The process is stated as: a) correspondence selection of terrain feature points from ICESat and DEM profiles, b) rigid transformation of generated ASTER DEM using selected feature correspondences based on least squares technique. The registration shows a good result that the elevation difference between DEM and ICESat data is low with a mean value less than 2 meters and the standard deviation around 7 meters. This DEM is generated and specially registered in Antarctic typical region without obvious ground rock control points and serves as true terrain input for further radar altimetry simulation.

Analysis of water level variation of lakes and reservoirs in Xinjiang, China using ICESat laser altimetry data (2003-2009).

This study utilizes ICESat Release 33 GLA14 data to analyse water level variation of Xinjiang’s lakes and reservoirs from 2003 to 2009. By using Landsat images, lakes and reservoirs with area larger than 1 km2 are numerically delineated with a software tool. Based on ICESat observations, we analyse the characteristics of water level variation in different geographic environments, as well as investigate the reasons for the variation. Results indicate that climatic warming contributes to rising water levels in lakes in mountainous areas, especially for lakes that are recharged by snow and glacial melting. For lakes in oases, the water levels are affected jointly by human activity and climate change, while the water levels of reservoirs are mainly affected by human activity. Comparing the annual average rates of water levels, those of lakes are higher than those of reservoirs in oasis areas. The main reasons for the decreasing water levels in desert regions are the reduction of recharged runoff and high evaporation. By analysing the variation of water levels and water volume in different geologic environments, it is found that water level and volume increased in mountainous regions, and decreased in oasis regions and desert regions. Finding also demonstrate that decreasing volume is greater than increasing volume, which results in decreasing total volume of Xinjiang lakes and reservoirs.

Validation of satellite altimetry by kinematic GNSS in central East Antarctica

Abstract. Ice-surface elevation profiles of more than 30 000 km in total length are derived from kinematic GNSS (GPS and the Russian GLONASS) observations on sledge convoy vehicles along traverses between Vostok Station and the East Antarctic coast. These profiles have accuracies between 4 and 9 cm. They are used to validate elevation data sets from both radar and laser satellite altimetry as well as four digital elevation models. A crossover analysis with three different processing versions of Envisat radar altimetry elevation profiles yields a clear preference for the relocation method over the direct method of slope correction and for threshold retrackers over functional fit algorithms. The validation of CryoSat-2 low-resolution mode and SARIn mode data sets documents the progress made from baseline B to C elevation products. ICESat laser altimetry data are demonstrated to be accurate to a few decimetres over a wide range of surface slopes. A crossover adjustment in the region of subglacial Lake Vostok combining ICESat elevation data with our GNSS profiles yields a new set of ICESat laser campaign biases and provides new, independent evidence for the stability of the ice-surface elevation above the lake. The evaluation of the digital elevation models reveals the benefits of combining laser and radar altimetry.

Assessing Orographic Variability in Glacial Thickness Changes at the Tibetan Plateau Using ICESat Laser Altimetry

Monitoring glacier changes is essential for estimating the water mass balance of the Tibetan Plateau. In this study, we exploit ICESat laser altimetry data in combination with the SRTM DEM and the GLIMS glacier mask to estimate trends in change in glacial thickness between 2003 and 2009 on the whole Tibetan Plateau. Considering acquisition conditions of ICESat measurements and terrain surface characteristics, annual glacier elevation trends were estimated for 15 different settings with respect to terrain slope and roughness. In the end, we only included ICESat elevations acquired over terrain with a slope below 20° and a roughness at the footprint scale below 15 m. With this setting, 90 glaciated areas could be distinguished. The results show that most of observed glaciated areas on the Tibetan Plateau are thinning, except for some glaciers in the northwest. In general, glacier elevations on the whole Tibetan Plateau decreased at an average rate of -0.17± 0.47 m per year (m a-1) between 2003 and 2009, taking together glaciers of any size, distribution, and location of the observed glaciated area. Both rate and rate error estimates are obtained by accumulating results from individual regions using least squares techniques. Our results notably show that trends in glacier thickness change indeed strongly depend on the relative position in a mountain range.

High-quality seamless DEM generation blending SRTM-1, ASTER GDEM v2 and ICESat/GLAS observations

The absence of a high-quality seamless global digital elevation model (DEM) dataset has been a challenge for the Earth-related research fields. Recently, the 1-arc-second Shuttle Radar Topography Mission (SRTM-1) data have been released globally, covering over 80% of the Earth’s land surface (60°N–56°S). However, voids and anomalies still exist in some tiles, which has prevented the SRTM-1 dataset from being directly used without further processing. In this paper, we propose a method to generate a seamless DEM dataset blending SRTM-1, ASTER GDEM v2, and ICESat laser altimetry data. The ASTER GDEM v2 data are used as the elevation source for the SRTM void filling. To get a reliable filling source, ICESat GLAS points are incorporated to enhance the accuracy of the ASTER data within the void regions, using an artificial neural network (ANN) model. After correction, the voids in the SRTM-1 data are filled with the corrected ASTER GDEM values. The triangular irregular network based delta surface fill (DSF) method is then employed to eliminate the vertical bias between them. Finally, an adaptive outlier filter is applied to all the data tiles. The final result is a seamless global DEM dataset. ICESat points collected from 2003 to 2009 were used to validate the effectiveness of the proposed method, and to assess the vertical accuracy of the global DEM products in China. Furthermore, channel networks in the Yangtze River Basin were also extracted for the data assessment.

Height changes over subglacial Lake Vostok, East Antarctica: Insights from GNSS observations

AbstractHeight changes of the ice surface above subglacial Lake Vostok, East Antarctica, reflect the integral effect of different processes within the subglacial environment and the ice sheet. Repeated GNSS (Global Navigation Satellite Systems) observations on 56 surface markers in the Lake Vostok region spanning 11 years and continuous GNSS observations at Vostok station over 5 years are used to determine the vertical firn particle movement. Vertical marker velocities are derived with an accuracy of 1 cm/yr or better. Repeated measurements of surface height profiles around Vostok station using kinematic GNSS observations on a snowmobile allow the quantification of surface height changes at 308 crossover points. The height change rate was determined at 1 ± 5 mm/yr, thus indicating a stable ice surface height over the last decade. It is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period. The continuous GNSS observations demonstrate that the particle heights vary linearly with time. Nonlinear height changes do not exceed ±1 cm at Vostok station and constrain the magnitude of spatiotemporal lake‐level variations. ICESat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters. Assuming the ice sheet to be in steady state over the entire lake, estimates for the surface accumulation, on basal accretion/melt rates and on flux divergence, are derived.

Seasonal and abrupt changes in the water level of closed lakes on the Tibetan Plateau and implications for climate impacts

Summary Using ICESat laser altimetry data, we examine seasonal and abrupt changes in the water level of 105 closed lakes on the Tibetan Plateau (TP) from 2003 to 2009. The cluster analysis method is applied to categorize different temporal evolution patterns of lake level, and the links between abrupt lake-level variations of the different clusters in specific seasons and key climatic variables during 2003–2009 based on 12 weather stations are further analyzed. The results show that seasonal lake-level variations were featured by strong spatio-temporal heterogeneity. Most lakes, especially in south Tibet, the central and northeastern Tibetan Plateau, showed large water-level increases (0.307 ± 0.301 m/year) in warm seasons (March–October), while showed declines or minor fluctuations (−0.091 ± 0.202 m/year) in cold seasons (November–February). Many small lakes in the Changtang Plateau and northern TP showed negative water budgets in warm seasons due to low precipitation and strong evaporation, but positive water budgets depending on seasonal snow meltwater supply in cold seasons. These lakes can be partitioned into eight clusters according to the common characteristics of seasonal and consistent abrupt lake-level variations. Lakes of Clusters 4 and 5 are not analyzed in detail because of their scattered distributions. The abrupt lake-level rises or declines for Clusters 1, 2, 3, 7 were inferred to be closely associated with dramatic changes in precipitation and evaporation. For example, most lakes in Cluster 1 experienced the substantial water-level rises (∼0.99 m on average) in the warm season of 2005, which were largely attributable to the high precipitation and low evaporation in this season. The abrupt changes of water level for lakes in Clusters 6 and 8 (in the Changtang Plateau and northern TP) were probably associated with more snow meltwater supply. Time series of GRACE-observed terrestrial water storage changes confirm that the abrupt lake-level changes in specific seasons are associated with abnormal hydro-climatological conditions. Besides, the limitations of spatial pattern analysis of lake variations classification based on cluster analysis and the possible primary causes of lake level fluctuations are discussed.

Mass loss of Greenland's glaciers and ice caps 2003–2008 revealed from ICESat laser altimetry data

AbstractThe recently finalized inventory of Greenland's glaciers and ice caps (GIC) allows for the first time to determine the mass changes of the GIC separately from the ice sheet using space‐borne laser altimetry data. Corrections for firn compaction and density that are based on climatic conditions are applied for the conversion from volume to mass changes. The GIC which are clearly separable from the icesheet (i.e., have a distinct ice divide or no connection) lost 27.9 ± 10.7 Gt a−1 or 0.08 ± 0.03 mm a−1 sea‐level equivalent (SLE) between October 2003 and March 2008. All GIC (including those with strong but hydrologically separable connections) lost 40.9 ± 16.5 Gt a−1 (0.12 ± 0.05 mm a−1 SLE). This is a significant fraction (~14 or 20%) of the reported overall mass loss of Greenland and up to 10% of the estimated contribution from the world's GIC to sea level rise. The loss was highest in southeastern and lowest in northern Greenland.

Acceleration of the Greenland ice sheet mass loss as observed by GRACE: Confidence and sensitivity

We examine the scale and spatial distribution of the mass change acceleration in Greenland and its statistical significance, using processed gravimetric data from the GRACE mission for the period 2002–2011. Three different data products – the CNES/GRGS, DMT-1b and GGFC GRACE solutions – have been used, all revealing an accelerating mass loss in Greenland, though with significant local differences between the three datasets. Compensating for leakage effects, we obtain acceleration values of −18.6Gt/yr2 for CNES/GRGS, −8.8Gt/yr2 for DMT-1b, and −14.8Gt/yr2 for GGFC.We find considerable mass loss acceleration in the Canadian Arctic Archipelago, some of which will leak into the values for Greenland, depending on the approach used, and for our computations the leakage has been estimated at up to −4.7Gt/yr2.The length of the time series of the GRACE data makes a huge difference in establishing an acceleration of the data. For both 10-day and monthly GRACE solutions, an observed acceleration on the order of 10–20Gt/yr2 is shown to require more than 5 yrs of data to establish with statistical significance.In order to provide an independent evaluation, ICESat laser altimetry data have been smoothed to match the resolution of the GRACE solutions. This gives us an estimated upper bound for the acceleration of about −29.7Gt/yr2 for the period 2003–2009, consistent with the acceleration values and corresponding confidence intervals found with GRACE data.

Blue ice areas and their topographical properties in the Lambert glacier, Amery Iceshelf system using Landsat ETM+, ICESat laser altimetry and ASTER GDEM data

AbstractBlue ice extent and its geographical distribution during the three years from December 1999–January 2003 have been mapped together with rock exposure areas for the entire Lambert-Amery glacial basin at an unprecedented level of spatial detail using Landsat ETM+ images. Various geometric and shape attributes for each blue ice patch have been derived. The total area of blue ice is estimated to be 20 422 km2, accounting for 1.48% of the glacial basin. We also found that the image texture information is helpful for distinguishing different types of blue ice: rough blue ice, smooth blue ice, and level blue ice. Rough blue ice areas are mostly associated with glacial dynamics and located in relatively low elevation regions, smooth blue ice areas are often related to nunataks and mountains, steep slopes and wind blows, and level blue ice areas are formed by melt-induced lakes in the margin of the ice shelf and melt-induced ponds on the ice shelf in low elevations. The elevation and surface slope properties of different types of blue ice area are characterized through regional topographical analysis with ICESat laser altimetry data. The effect of mountain height on the blue ice extent is also examined with local topographic profile analysis based on the ASTER global digital elevation model. The wide variation of the ratio of blue ice area length to mountain height indicates that the factors controlling the formation and extent of blue ice are more complicated than we previously thought.

Accuracy Enhancement of ASTER Global Digital Elevation Models Using ICESat Data

Global Digital Elevation Models (GDEM) are considered very attractive for current research and application areas due to their free and wide range accessibility. The ASTER Global Digital Elevation Model exhibits the highest spatial resolution data of all global DEMs and it is generated for almost the whole globe. Unfortunately, ASTERGDEM data include many artifacts and height errors that decrease the quality and elevation accuracy significantly. This study provides a method for quality improvement of the ASTER GDEM data by correcting systematic height errors using ICESat laser altimetry data and removing artifacts and anomalies based on a segment-based outlier detection and elimination algorithm. Additionally, elevation errors within water bodies are corrected using a water mask produced from a high-resolution shoreline data set. Results indicate that the accuracy of the corrected ASTER GDEM is significantly improved and most artifacts are appropriately eliminated. Nevertheless, artifacts containing lower height values with respect to the neighboring ground pixels are not entirely eliminated due to confusion with some real non-terrain 3D objects. The proposed method is particularly useful for areas where other high quality DEMs such as SRTM are not available.

Mass change detection in Antarctic ice sheet using ICESat block analysis techniques from 2003-2008

In this paper,the ICESat laser altimetry data is used to obtain an estimate of the mass balance of Antarctic ice sheet from February 2003 to March 2008.The time series of elevation change in Antarctic ice sheet are derived by the block crossover analysis using the ICESat nadir ground track,and the calculation of the campaign basis is discussed.A least square regression of crossover difference is applied to calculate the average elevation change trend and the seasonal cycle,and then the mass changes of Antarctic ice sheet are estimated by combining the elevation change rate with the surface firn density model.The result shows that seasonal cycle signals are obvious in Antarctic ice sheet height changes,and the average annual amplitude is about 2.21 cm.On the coast of the Antarctica continent,there are significant thinning and thickening, especially near the Amundsen Sea embayment of west Antarctic and Antarctic Peninsula. Considering the influence of GIA(three public GIA models),our best estimate of the mass change in Antarctic ice sheet is about -82～-73Gt/yr.For the ICESat,the ice sheet surface firn density model and the GIA model are the main factors in the mass change estimates.

Surface elevation changes at the front of the Ross Ice Shelf: Implications for basal melting

[1] Rapid melting beneath the Ross Ice Shelf (RIS) occurs near the ice front, likely in response to a secondary buoyant plume with tidal mixing, and is sensitive to seasonal water temperatures in front of the ice shelf. The front of the RIS is examined using GLAS ICESat laser altimetry data. Spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front. Melt rates (b) increase exponentially as the front is approached, from approximately zero at 40 km from the front to an average of 2.8 ± 1.0 m a−1 within the front kilometer. Melt estimates within the front 60 km are best fit by the relationship b = 2.0 × exp(−x/11900) m a−1, where x denotes distance from the front. Frontal melt totals approximately 16 km3 a−1 in the front 40 km, which accounts for between 10% and 40% of current published estimates of total melt beneath the RIS. Spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved. Frontal melt is modeled as a one-dimensional buoyant plume with tidal currents included. These model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front.

Analysis of velocity field, mass balance, and basal melt of the Lambert Glacier–Amery Ice Shelf system by incorporating Radarsat SAR interferometry and ICESat laser altimetry measurements

By incorporating recently available remote sensing data, we investigated the mass balance for all individual tributary glacial basins of the Lambert Glacier–Amery Ice Shelf system, East Antarctica. On the basis of the ice flow information derived from SAR interferometry and ICESat laser altimetry, we have determined the spatial configuration of eight tributary drainage basins of the Lambert‐Amery glacial system. By combining the coherence information from SAR interferometry and the texture information from SAR and MODIS images, we have interpreted and refined the grounding line position. We calculated ice volume flux of each tributary glacial basin based on the ice velocity field derived from Radarsat three‐pass interferometry together with ice thickness data interpolated from Australian and Russian airborne radio echo sounding (RES) surveys and inferred from ICESat laser altimetry data. Our analysis reveals that three tributary basins have a significant net positive imbalance, while five other subbasins are slightly positive or close to zero balance. Overall, in contrast to previous studies, we find that the grounded ice in Lambert Glacier–Amery Ice Shelf system has a positive mass imbalance of 22.9 ± 4.4 Gt a−1. The net basal melting for the entire Amery Ice Shelf is estimated to be 27.0 ± 7.0 Gt a−1. The melting rate decreases rapidly from the grounding zone to the ice shelf front. Significant basal refreezing is detected in the downstream section of the ice shelf. The mass balance estimates for both the grounded ice sheet and the ice shelf mass differ substantially from other recent estimates.

Definition of ICESat Selection Criteria for Their Use as Height References for TanDEM-X

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The TanDEM-X satellite synthetic aperture radar (SAR) mission, which is the result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, has the goal to deliver a high-precision global digital elevation model (DEM). The X-band SAR interferometry-derived DEMs contain absolute and relative height errors that have to be minimized with the help of height references in order to achieve the specified accuracies. ICESat laser altimetry data are suited for this task, due to their accuracy and global distribution. In order to gain experience in the comparison between a radar-derived DEM and ICESat GLA14 elevation data, an X-band DEM was acquired over a test region with the experimental airborne radar system of DLR in Oberpfaffenhofen. Additionally, a laser DEM of the area was used to verify the height accuracy claimed by previously published ICESat studies over different terrain types and after applying different selection threshold criteria. The analyses described in this paper are the basis for the definition of a suitable global ICESat selection strategy and include the computation of the density of selected ICESat samples over the Earth. These aspects are crucial for a successful TanDEM-X DEM generation. </para>

Positional Accuracy Evaluation of Declassified Hexagon KH-9 Mapping Camera Imagery

This paper examines the positional accuracy of the declassified KH-9 Hexagon imagery and derived DEM. Aimed at geodesy and mapmaking, the KH-9 program (1973 to 1980) resulted in an image archive with worldwide stereo coverage at 6 to 9 m. We used six KH-9 images acquired in 1980 over two testfields in Central Asia. Using reseau marks on the scanned KH- 9 frames, we found and corrected image distortions. In bundle orientation with Ground Control Points (GCPS) from QuickBird images, we achieved horizontal accuracies below 6 m for a flat terrain testfield and approximately 10 m for a mountainous terrain testfield. With three GCPS the image orientation horizontal accuracy degraded by only 20 percent. We generated a DEM from the KH-9 images and estimated its vertical accuracy using IceSAT laser altimetry data and an additional DEM from 1:25 000 topographic maps. The DEM RMSE was 6.18 m over flat terrain and 20.0 m over mountainous terrain.

Static grounding lines and dynamic ice streams: Evidence from the Siple Coast, West Antarctica

Grounding line dynamics are key to our understanding of marine ice sheets such as the West Antarctic Ice Sheet (WAIS). We present an accurate and rapid method for locating grounding lines using GLAS ICESat laser altimetry data. We then apply this technique to the Siple Coast region of the WAIS. Our technique exploits the high surface slope at the grounding line relative to the adjacent flat ice streams and ice shelf. The technique is validated by ground‐based observations. Comparisons between the current grounding line and earlier measured locations demonstrate that the Siple Coast's grounding line has been largely static over the last two decades. This is important as the ice streams have been undergoing considerable variations over this period. We conclude that ice stream flow speeds can vary while the grounding line remains stable. We suggest that grounding lines may migrate abruptly and then stabilize rather than undergoing continuous retreat.