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Landcover-Dependent Assessment of the Relative Height Accuracy in TanDEM-X DEM Products
Abstract
Digital elevation models (DEMs) are extensively used for a variety of scientific and commercial applications. For the global TanDEM-X DEM, one of the main performance parameters is the relative height accuracy, which is specified to be under 2 m for flat terrain. Land cover types where the radar signal penetrates into a volume, as forest and ice, are excluded from this specification. Knowing the accuracy of the DEM for a specific land cover type is essential for applications relying on it, such as navigation applications, gradient and aspect estimations, and others. This letter is meant to be as add-on the global relative analysis presented in
[1]
. Here, we present a characterization of the relative height accuracy based on the interferometric coherence assessing the performance for every class defined by the CCI Land Cover Maps at a continental and global basis. This characterization raises the awareness of the advantages and limitations of the DEM for each specific application and helps scientists using the TanDEM-X DEM to interpret their results. In addition, an estimation of the relative height accuracy for acquisitions where volume decorrelation is present is locally performed using the high-frequency component of repeat-pass DEMs differences. For two particular land cover types, both methods are compared. The main source of errors in the DEM generation is clearly associated with strong inhomogeneous signal returns, becoming a driving factor on the actual accuracy of the estimated mean phase center height.
... Mission specifications for TanDEM-X target a 10 m absolute vertical accuracy (90% linear error, LE90) and a 2 m relative vertical accuracy for slopes < 20%, or 4 m for slopes > 40% (Wessel, 2016). The vertical accuracy of the final TanDEM-X DEM at 0.4 arc-seconds has been assessed based on comparisons with ICESat points (Rizzoli et al., 2017), height error maps (HEM) (Gonzalez and Rizzoli, 2018) or KGPS, GPS and LIDAR measurements (Wessel et al., 2018). Using ICESat data, Rizzoli et al. (2017) calculated a global LE90 value of 3.49 m, or just 0.88 m when forested or ice cells were not considered (RMSE not reported). ...
... By also comparing TanDEM-X to 23,951 GPS benchmark points taken across the USA and to 3 high-resolution LiDAR based DEMs, Wessel et al. (2018) report RMSE values of 1.1 m for short vegetation, 1.4 m for developed vegetation and 1.8 m for forests. Most recently, Gonzalez and Rizzoli (2018) used Height Error Maps (HEM) maps to estimate relative height accuracy (or random error), reporting a global value of 1.25 m (thus below the mission specification of 2 m), with the relative height error larger in landcover associated with forests. TanDEM-X has also been compared to other global DEMs (except MERIT), with TanDEM-X performing favorably (Grohmann, 2018). ...
... For instance, some landcover classes within the tree-cover category can perform reasonably well (e.g. 'Broadleaved, deciduous, open to closed (15-40%)'), Gonzalez and Rizzoli (2018) who find a similar amount of variation with landcover categories when assessing relative height error. ...
Freely available Global Digital Elevation Models (GDEMs) are essential for many scientific and humanitarian applications. Recently, TanDEM-X 90 has been released with a global coverage at 3 arc sec resolution. Its release is sure to generate keen interest as it provides an alternative to the widely used Shuttle Radar Topography Mission (SRTM) DEM, especially for flood risk management as for low slope floodplains height errors can become particularly significant. Here, we provide a first accuracy assessment of TanDEM-X 90 for selected floodplain sites and compare it to other popular global DEMs – the Shuttle Radar Topography Mission (SRTM) and the error-reduced version of SRTM called Multi-Error-Removed-Improved-Terrain (MERIT) DEM. We characterize vertical height errors by comparing against high resolution LiDAR DEMs for 32 floodplain locations in 6 continents. Results indicate that the average vertical accuracy of TanDEM-X 90 and MERIT are similar and are both a significant improvement on SRTM. We further our analysis by assessing vertical accuracy by landcover, with our results suggesting that TanDEM-X 90 is the most accurate global DEM in all landcover categories tested except short vegetation and tree-covered areas where MERIT is demonstrably more accurate. Lastly, we present the first characterization of the spatial error structure of any TanDEM-X DEM product, and find the spatial error structure is similar to MERIT, with MERIT generally having lower sill values and larger ranges than TanDEM-X 90 and SRTM. Our findings suggest that TanDEM-X 90 has the potential to become the benchmark global DEM in floodplains with careful removal of errors from vegetation, and at this stage should be used alongside MERIT in any flood risk application.
... The two twin satellites TerraSAR-X and TanDEM-X have been flying in close orbit formation since 2010, constituting a single-pass interferometer with variable baselines and acquisition geometries (Krieger et al., 2007;Zink et al., 2021). The main goal of the mission is the generation of a global Digital Elevation Model (DEM) at a spatial resolution of 12 m, which was completed successfully in 2016 (Rizzoli et al., 2017;Gonzalez and Rizzoli, 2018). Beside the nominal DEM product, for each TanDEM-X bistatic acquisition, additional bypass products, such as the calibrated backscatter, the interferometric phase and the interferometric coherence, are available as well. ...
Deep learning models have shown encouraging capabilities for mapping accurately forests at medium resolution with TanDEM-X interferometric SAR data. Such models, as most of current state-of-the-art deep learning techniques in remote sensing, are trained in a fully-supervised way, which requires a large amount of labeled data for training and validation. In this work, our aim is to exploit the high-resolution capabilities of the TanDEM-X mission to map forests at 6 m. The goal is to overcome the intrinsic limitations posed by midresolution products, which affect, e.g., the detection of narrow roads within vegetated areas and the precise delineation of forested regions contours. To cope with the lack of extended reliable reference datasets at such a high resolution, we investigate self-supervised learning techniques for extracting highly informative representations from the input features, followed by a supervised training step with a significantly smaller number of reliable labels. A 1 m resolution forest/non-forest reference map over Pennsylvania, USA, allows for comparing different training approaches for the development of an effective forest mapping framework with limited labeled samples. We select the best-performing approach over this test region and apply it in a real-case forest mapping scenario over the Amazon rainforest, where only very few labeled data at high resolution are available. In this challenging scenario, the proposed self-supervised framework significantly enhances the classification accuracy with respect to fully-supervised methods, trained using the same amount of labeled data, representing an extremely promising starting point for large-scale, very high-resolution forest mapping with TanDEM-X data.
... As apparent from many studies based on both active and passive-sensor produced surface models, DEMs, the vertical accuracy performance of the end product is highly correlated with the land cover type and topographic uncertainties within the target during the actual image acquisition (Shortridge 2006;Wechsler and Kroll 2006;Hebeler and Purves 2009;Altunel 2018;Gonzalez and Rizzoli 2018;). Although smaller in caliber in terms of investment, coverage and know-how, aerial stereo image capture, today, is not entirely different from those of the satellite-based ones. ...
Elevation, vertical accuracy of any topographic Earth representation, e. g. stereo surface models, topo maps, DEMs, etc., is important if such data will be the base of further projects or development plans. The main form of these types of data in Türkiye is “1:25000” scaled quad maps. The third generation such maps were produced via digital stereo air-photo capture and photogrammetry capabilities as opposed to the previous two analogue based releases. Through this long-adapted scale, land cover types, hydrological formations, surface features, down to house rooftops, can be depicted in these maps. Elevation integration are also provided
through the contour lines drawn in 10 m elevation difference showing intervals. They are the most frequently addressed topographic data type in forestry education as well as in profession. With the establishment of county-wide active GNSS network, very high precision elevation verification has become available for multitude of purposes. In this study, four dam reservoirs intensively surveyed using CORS-GPS were used to assess the vertical accuracies of the corresponding quad-map based DEMs produced in different resolutions. RMSEs ranged from 5.49 m to 14.22 m when the entire quad sheets were used while they ranged from 2.58 m to 8.95 m when the quads were purposely cut. Canopy closure apparently worsened the
results.
... The main goal of the mission was the generation of a global digital elevation model (DEM) at a spatial resolution of 12 m. The two global acquisitions, acquired between 2011 and 2016, were exploited together with additional acquisitions over difficult terrain to generate a DEM with unprecedented accuracy [15,16]. Besides the nominal DEM product, for each TanDEM-X bistatic acquisition, characterized by the absence of temporal decorrelation, additional quantities, such as the absolutely calibrated backscatter, the interferometric phase and the interferometric coherence, are available. ...
The TanDEM-X synthetic aperture radar (SAR) system allows for the recording of bistatic interferometric SAR (InSAR) acquisitions, which provide additional information to the common amplitude images acquired by monostatic SAR systems. More concretely, the volume decorrelation factor, which can be derived from the bistatic interferometric coherence, is a reliable indicator of the presence of vegetation and it was used as main input feature for the generation of the global TanDEM-X forest/non-forest map, by means of a clustering algorithm. In this work, we investigate the capabilities of deep Convolutional Neural Networks (CNNs) for mapping tropical forests at large-scale using TanDEM-X InSAR data. For this purpose, we rely on a U-Net architecture, which takes as input a set of feature maps selected on the basis of previous preparatory works. Moreover, we design an ad hoc training strategy, aimed at developing a robust model for global mapping purposes, which has to properly manage the large variety of different acquisition geometries characterizing the TanDEM-X global data set. In addition to detecting forest/non-forest areas, the CNN has also been trained to detect water surfaces, which are typically characterized by low values of coherence. By applying the proposed method on single TanDEM-X images, we achieved a significant performance improvement with respect to the baseline clustering approach, with an average F-score increase of 0.13. We then applied such a model for mapping the entire Amazon rainforest, as well as the other tropical forests in Central Africa and South-East Asia, in order to test its robustness and generalization capabilities, and we observed that forests are typically well detected as contour closed regions and that water classification is reliable, too. Finally, the generated maps show a great potential for mapping temporal changes occurring over forested areas and can be used for generating large-scale maps of deforestation.
... The absolute vertical accuracy of 10 m (90% linear error, LE90) and relative height accuracy of 2 m have been reported by mission specification for slopes <20%, or 4 m for slopes >40% [75]. The vertical accuracy of the 12 m TanDEM-X DEM product has been assessed using ICESat points [70], GPS ground truth values, and LiDAR observed values [76]-or a height error map (HEM) [77] for reference. However, to date, very few studies [78] have been conducted to assess the vertical accuracy of the TanDEM-X 90 m DEM and, particularly, its comparison with other freely available global DEMs [74,79,80]. ...
Publicly available Digital Elevation Models (DEM) derived from various space-based platforms (Satellite/Space Shuttle Endeavour) have had a tremendous impact on the quantification of landscape characteristics, and the related processes and products. The accuracy of elevation data from six major public domain satellite-derived Digital Elevation Models (a 30 m grid size—ASTER GDEM version 3 (Ast30), SRTM version 3 (Srt30), CartoDEM version V3R1 (Crt30)—and 90 m grid size—SRTM version 4.1 (Srt90), MERIT (MRT90), and TanDEM-X (TDX90)), as well as the improvement in accuracy achieved by applying a correction (linear fit) using Differential Global Positioning System (DGPS) estimates at Ground Control Points (GCPs) is examined in detail. The study area is a hard rock terrain that overall is flat-like with undulating and uneven surfaces (IIT (ISM) Campus and its environs) where the statistical analysis (corrected and uncorrected DEMs), correlation statistics and statistical tests (for elevation and slope), the impact of resampling methods, and the optimum number of GCPs for reduction of error in order to use it in further applications have been presented in detail. As the application of DGPS data at GCPs helps in the substantial reduction of bias by the removal of systematic error, it is recommended that DEMs may be corrected using DGPS before being used in any scientific studies.
... 3) ЦМР Copernicus DEM glo-30 и glo-90 [Strobl, 2020] (разрешение 1" и 3"), в основе которых лежит коммерческая квазиглобальная ЦМР WorldDEM [Rizzoli et al., 2017] с разрешением 0,4". Эта модель, полученная радарграмметрическим методом по данным съемки КА TanDEM-X, является самой точной квазиглобальной ЦМР [Rizzoli et al., 2017;Gonzalez, Rizzoli, 2018 Эти методы применяются с целью создания моделей прогнозного картографирования почв, растительности и др., в которых одной из групп предикторов (ковариат обучающих выборок) являются цифровые модели морфометрических величин, рассчитываемые по ЦМР. ...
Topography is the most important component of the geographical shell, one of the main elements of geosystems, and the framework of a landscape. geomorphometry is a science, the subject of which is modeling and analyzing the topography and the relationships between topography and other components of geosystems. Currently, the apparatus of geomorphometry is widely used to solve various multi-scale problems of the Earth sciences. As part of the RFBR competition “Expansion”, we present an analytical review of the development of theory, methods, and applications of geomorphometry for the period of 2016–2021. For the analysis, we used a sample of 485 of the strongest and most original papers published in international journals belonging to the JCR Web of Science Core Collection quartile I and II (Q1–Q2), as well as monographs from leading international publishers. We analyze factors caused a progress in geomorphometry in recent years. These include widespread use of unmanned aerial survey and digital photogrammetry, development of tools and methods for survey of submarine topography, emergence of new publicly available digital elevation models (DEMs), development of new methods of DEM preprocessing for their filtering and noise suppression, development of methods of two-dimensional and three-dimensional visualization of DEMs, introduction of machine learning techniques, etc. We consider some aspects of the geomorphometric theory developed in 2016–2021. We discuss new computational methods for calculating morphometric models from DEM, as well as the problems facing the developers and users of such methods. We consider application of geomorphometry for solving multiscale problems of geomorphology, hydrology, soil science, geology, glaciology, speleology, plant science and forestry, zoogeography, oceanology, planetology, landslide studies, remote sensing, urban studies, and archaeology.
... This conclusion is supported > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 14 by recent works such as that by Chimitdorzhiev, et al. [38], who found an error in canopy height, concluding that radar interferometric measurements underestimate the actual forest height by an average value of 5.5 m for high coverage and by 2-4 m for medium coverage. On the other hand, Gonzalez and Rizzoli [57] summarized the global relative height accuracy of the global TanDEM-X DEM for all continents and cover types. In our case, for Europe, the authors set accuracy levels of 1.64 m for forests, of 0.94 m for short vegetation and of 0.96 for shrubland, which account for most of our cover categories. ...
Information about forest structures is becoming crucial to Earths global carbon cycle, forest habitats and biodiversity. The Global Ecosystem Dynamics Investigation (GEDI) provides 25-m diameter footprints of the surface for 3D structure measurements. The main goal of this study is to compare 12 949 footprints of GEDI data with other airborne and spaceborne Digital Elevation Models (DEMs) for Southwest Spain. Ground elevation differences (ELM) are analyzed by comparing GEDI measurements with ALS LiDAR- and TanDEM-X-derived DEMs. The vertical structure (RH100) is compared to the ALS LiDAR measurement. Ten zones are analyzed, considering different degrees of coverage and slopes. We achieved an RMSE of 6.13 m for the ELM when comparing GEDI and LiDAR data and an RMSE of 7.14 m when comparing GEDI and TanDEM-X data. For some of the studied areas, these values were considerably smaller, with RMSE values even lower than 1 m. For the RH100 metric, an RMSE of 7.03 m was achieved when comparing GEDI and LiDAR data, but again with a minimum value of 2.09 m for one zone. The results show a clear relation to coverage and slope, especially for the latter. This work also evaluates the positional uncertainty of GEDI footprints, shifting them 10 m along and across the track of the satellite orbit. The outcomes reveal a strong tendency to obtain better results in the ELM when setting the footprint to 270 and displacing it within 10 m of its positional uncertainty in comparison with the LiDAR and TanDEM-X data.
... Moreover, it is specified with an unprecedented relative height accuracy for a global DEM product, which is under 2 m over flat areas at a 90% confidence level. Remarkably, the actual performance is far better than the specification, showing that 50% of the delivered geocells (extending by 1 • × 1 • latitude/longitude) are characterized by a relative height accuracy under 1 m [10,11]. No further editing process has been applied, which means that water bodies appear noisy and residual gaps, caused, e.g., by missing data or geometric distortions, have not been filled. ...
The spaceborne mission TanDEM-X successfully acquired and processed a global Digital Elevation Model (DEM) from interferometric bistatic SAR data at X band. The product has been delivered in 2016 and is characterized by an unprecedented vertical accuracy. It is provided at 12 m, 30 m, and 90 m sampling and can be accessed by the scientific community via a standard announcement of opportunity process and the submission of a scientific proposal. The 90 m version is freely available for scientific purposes. The DEM is unedited, which means that it is the pure result of the interferometric SAR processing and subsequent mosaicking. Residual gaps, resulting, e.g., from unprocessable data, are still present and water surfaces appear noisy. This paper reports on the algorithms developed at DLR's Microwaves and Radar Institute for a fully automatic editing of the global TanDEM-X DEM comprising gap filling and water editing. The result is a new global gap-free DEM product at 30 m sampling, which can be used for a large variety of scientific applications. It also serves as a reference for processing the upcoming TanDEM-X Change DEM layer.
This chapter overviews the development of digital terrain analysis and geomorphometry. First, we briefly point out the role of topography as one of the most important components of the environment. Second, we present a short historical overview of the progress of digital terrain modeling, or geomorphometry from the mid-1950s to the mid-2010s. Third, we present a review of the development of theory, methods, and applications of geomorphometry in 2016–2024. In particular, we analyze factors caused a progress in geomorphometry in recent years. These include widespread use of unmanned aerial survey and photogrammetry, development of tools and methods for survey of submarine topography, emergence of new publicly available digital elevation models, and implementation of machine learning techniques. Finally, we discuss the recent application of geomorphometry for solving multiscale problems of geomorphology, hydrology, soil science, geology, glaciology, speleology, geobotany, forestry, zoogeography, oceanology, planetology, landslide science, avalanche and fire hazards, remote sensing, urban studies, and archaeology.
Depending upon the technological advancements, many differently scaled topographical maps were manufactured and put into service in Türkiye. In this particular study, the elevation values of 1:25.000 scaled topographic maps produced with analogue means in 1992-1993 period were compared to later produced, digital means integrated 2009-2010 period maps through precisely measured CORS-GPS ground control points over three different land cover types (agriculture, partial-forest, and forest). 615, 3688 and 1739 systematic ground control points were respectively used inside, agriculture, partial forest and forest designated study sites. Comparisons were made over four different surface models produced from the same topographic maps: purpose-cut topographic sheets (KS), entire topographic sheets (TP), 10 m and 30 m resampled entire topographic sheets (R10 and R30). The results showed that digitizing the map production means and techniques really improved the elevation accuracies of 1:25.000 scaled topographic maps. Elevation accuracies between analogue and digital means produced maps were distinct in agriculture-designated site however, they were not as easily identifiable in partial-forest and forest designated sites. Besides, it was obvious that a resampling algorithm applied to the raster surface models produced using these maps would certainly improve their elevation accuracies.
This study focuses on the quality evaluation of two of the best 1 arc-second public global digital elevation models (DEMs), Copernicus GLO-30 DEM and ALOS AW3D30 DSM, from the perspective of their capability to represent the terrain fine-scale morphology of a complex alpine landscape, located in the Italian Trentino Province. The analysis is performed on an area of 6210 km², considering a reference DEM derived from a high resolution and accurate airborne Lidar survey. The quality assessment goes beyond a conventional approach based on elevation differences statistics, computed on a pixels-by-pixel basis. An ad hoc approach for evaluating the capability to represent fine-scale morphology, including surface roughness, is adopted. Moreover, the quality analysis is performed considering the influence of local morphology and of the different land covers. The findings show that although the two global DEMs have comparable overall quality, their relative performances change according to local landscape characteristics. Copernicus DEM performance is on average better than ALOS in correspondence of urbanized areas as well as in areas without vegetation cover, with gentle slopes and relatively low short-range roughness. Meanwhile, ALOS DEM performance is slightly better than Copernicus in rougher terrain and steeper slopes. In general, both DEMs have poor performances in steep slopes, with a limited capability to describe fine-scale morphology. The adoption of these global DEMs for terrain analysis and modelling of earth surface processes should be performed carefully, considering the impact of different land covers and of local morphology, including surface roughness.
This study focuses on the quality evaluation of two of the best 1 arc-second public global digital elevation models (DEMs), Copernicus GLO-30 DEM and ALOS AW3D30 DSM, from the perspective of their capability to represent the terrain morphology of a complex alpine landscape, located in the alpine Trentino Province, in the Italian Alps. The analysis is performed on an area of 6210 km ² , considering a reference DEM derived from a high resolution and accurate airborne Lidar DEM. The quality assessment includes, in addition to a conventional analysis of error statistics on a pixels-by-pixel basis, an ad-hoc analysis on the capability to represent the fine-scale morphology and local roughness. The quality analysis is performed considering the influence of local morphology and of the different land covers. The findings show that the two global DEMs have comparable overall quality, but the relative performances change according to local landscape characteristics. Copernicus DEM performance is on average better than ALOS in correspondence of urbanized areas as well in areas without vegetation cover, with gentle slopes and relatively low short-range roughness. ALOS DEM performance is slightly better than Copernicus in rougher terrain and steeper slopes. In general, both DEMs have poor performances in steep slopes, with a limited capability to describe correctly local morphology. The adoption of these global DEMs for terrain analysis and modelling of earth surface processes should be performed carefully, taking into account the impact of different land covers and of local morphology, including surface roughness.
The generation of a truly global DEM with enhanced accuracy, resolution and coverage was the main objective of the TanDEM-X mission. The mission is innovative for many reasons and as a result an abundance of research about its technology has been published so far. This paper aims to fill the literature gap between the scientific use of the TerraSAR-X and TanDEM-X data and the practical use of the final TanDEM-X DEM. A review of the DEM is performed over non trivial areas of the globe, some of which are test sites of international scientific societies. LIDAR data and DSMs are used as reference elevation information and other widely used global DEMs are employed. The visual inspection and the accuracy analysis over 14 sites in Europe, USA and Antarctica show the clear superiority of the TanDEM-X DEM in the family of global DEMs, regardless of the latitude, the terrain type and the land cover. The TanDEM-X DEM is more morphologically detailed than the AW3D30, the ASTER and the SRTM global DEMs, which agrees with its much finer pixel spacing. It is also more complete than the other global DEMs, especially over the polar test sites. The accuracy analysis shows that the TanDEM-X DEM is more accurate than the other global DEMs with RMSEs ranging from 1.0 to 8.7 m, while the RMSEs of the AW3D30, the ASTER, the 1 arcsec SRTM and the 3 arcsec SRTM range from 2.6 to 49.0 m, from 8.3 to 80.2 m, from 3.0 to 8.7 m and from 7.3 to 20.5 m, respectively. The accuracy analysis also shows that the TanDEM-X DEM has better geolocation accuracy, and the application of the computed 3D bias with respect to the reference data leaves the RMSE practically unaffected. All in all, it seems that the TanDEM-X DEM paves the way for a new generation of global DEMs and re-introduces SAR as a state-of-the art technology for global topographic, and other, applications.
This IAG-IDEMS bibliography contains selected papers on global DEMs (digital elevation models), bathymetry and ice products, global composite models (combinations of elevations, bathymetry and ice), and underlying modelling techniques. The listed papers are meant to give an overview on the main characteristics of modern DEM products such as from the SRTM, ASTER, ALOS/PRISM and TanDEM-X missions. Some of the studies assess the quality of the DEM data, e.g., in terms of DEM accuracy and possible caveats (e.g., artefacts, shifts, voids) relevant for geoscience applications. In some cases, the research papers focus on more than one global DEM (e.g. validation studies). The literature is subdivided into the groups: Part A-Overview Papers Part B-Global DEMs 1. SRTM-based DEMs (V2.1, V4.1, V3, MERIT), 30-90 m resolution 2. ASTER-GDEM1/2/3 DEMs, 30 m resolution 3. TanDEM-X DEMs, 12-90 m resolution 4. ALOS World 3D (AW3D) DEM, 5-30 m resolution 5. DEM validation and assessment studies 6. DEM-derived global products Part C-Global Bathymetry and Ice Models 7. Global bathymetry and digital depth models (DDM) 8. Models of Ice Sheets of Greenland/Antarctica Part D-Combined or composite global relief models Part E-Methodologies, issues (e.g., fusion, void-filling, artefacts) If you have suggestions for other papers that could be a good addition to this literature compilation, please feel free to contact me via Christian Hirt, TU Munich, 08 th May 2019
Updated in December 2018. New references in blue for a faster overview This IAG-IDEMS bibliography contains selected papers on global DEMs (digital elevation models), bathymetry and ice products, global composite models (combinations of elevations, bathymetry and ice), and underlying modelling techniques. The listed papers are meant to give an overview on the main characteristics of modern DEM products such as from the SRTM, ASTER, ALOS/PRISM and TanDEM-X missions. Some of the studies assess the quality of the DEM data, e.g., in terms of DEM accuracy and possible caveats (e.g., artefacts, shifts, voids) relevant for geoscience applications. In some cases, the research papers focus on more than one global DEM (e.g. validation studies). The literature is subdivided into the groups: Part A-Overview Papers Part B-Global DEMs 1. SRTM-based DEMs (V2.1, V4.1, V3, MERIT), 30-90 m resolution 2. ASTER-GDEM1/2/3 DEMs, 30 m resolution 3. TanDEM-X DEMs, 12-90 m resolution 4. ALOS World 3D (AW3D) DEM, 5-30 m resolution Part C-Global Bathymetry and Ice Models 5. Global bathymetry and digital depth models (DDM) 6. Models of Ice Sheets of Greenland/Antarctica Part D-Combined or composite global relief models Part E-Methodologies, issues (e.g., fusion, void-filling, artefacts) If you have suggestions for other papers that could be a good addition to this literature compilation, please feel free to contact me via Christian Hirt, TU Munich, 14 th December 2018
The primary objective of the TanDEM-X mission is the generation of a global, consistent, and high-resolution digital elevation model (DEM) with unprecedented global accuracy. The goal is achieved by exploiting the interferometric capabilities of the two twin SAR satellites TerraSAR-X and TanDEM-X, which fly in a close orbit formation, acting as an X-band single-pass interferometer. Between December 2010 and early 2015 all land surfaces have been acquired at least twice, difficult terrain up to seven or eight times. The acquisition strategy, data processing, and DEM calibration and mosaicking have been systematically monitored and optimized throughout the entire mission duration, in order to fulfill the specification. The processing of all data has finally been completed in September 2016 and this paper reports on the final performance of the TanDEM-X global DEM and presents the acquisition and processing strategy which allowed to obtain the final DEM quality. The results confirm the outstanding global accuracy of the delivered product, which can be now utilized for both scientific and commercial applications.
This paper presents for the first time a detailed study on information content of X-band single-pass interferometric spaceborne SAR data with respect to snow facies characterization. An approach for classifying different snow facies of the Greenland Ice Sheet by exploiting X-band TanDEM-X interferometric synthetic aperture radar acquisitions is firstly detailed. Large-scale mosaics of radar backscatter and volume correlation factor, derived from quicklook images of the interferometric coherence, represent the starting point for applying an unsupervised classification method based on the c-means fuzzy clustering algorithm. The data was acquired during winter 2010/2011. A partition of four different snow facies was chosen and interpreted using reference melt data, snow density, and in situ measurements. The variations in the stratification and micro-structure of firn, such as the variations of density with depth and the presence of percolation features, are identified as relevant parameters for explaining the significant differences in the observed interferometric signatures among different snow facies. Moreover, a statistical analysis of backscatter and volume correlation factor provided useful parameters for characterizing the snow facies behavior and analyzing their dependency on the acquisition geometry. Finally, knowing the location and characterization of the different snow facies, the two-way X-band penetration depth over the whole Ice Sheet was estimated. The obtained mean values vary from 2.3 m for the outer snow facies up to 4.18 m for the inner one. The presented approach represents a starting point for a long-term monitoring of ice sheet dynamics, by acquiring time-series, and is of high relevance for the design of future SAR missions as well.
Global land cover is a key variable in the earth system with feedbacks on
climate, biodiversity and natural resources. However, global land cover
data sets presently fall short of user needs in providing detailed spatial and
thematic information that is consistently mapped over time and easily
transferable to the requirements of earth system models. In 2009, the
European Space Agency launched the Climate Change Initiative (CCI), with land
cover (LC_CCI) as 1 of 13 essential climate variables targeted for
research development. The LC_CCI was implemented in three phases: first
responding to a survey of user needs; developing a global, moderate-resolution land cover data set for three time periods, or epochs (2000, 2005,
and 2010); and the last phase resulting in a user tool for converting land
cover to plant functional type equivalents. Here we present the results of
the LC_CCI project with a focus on the mapping approach used to convert
the United Nations Land Cover Classification System to plant functional types
(PFTs). The translation was performed as part of consultative process among
map producers and users, and resulted in an open-source conversion tool. A
comparison with existing PFT maps used by three earth system modeling teams
shows significant differences between the LC_CCI PFT data set and those
currently used in earth system models with likely consequences for modeling
terrestrial biogeochemistry and land–atmosphere interactions. The main
difference between the new LC_CCI product and PFT data sets used currently
by three different dynamic global vegetation modeling teams is a reduction in
high-latitude grassland cover, a reduction in tropical tree cover and an
expansion in temperate forest cover in Europe. The LC_CCI tool is flexible
for users to modify land cover to PFT conversions and will evolve as phase 2
of the European Space Agency CCI program continues.
Since 2010, TanDEM-X and its twin satellite TerraSAR-X fly in a close orbit formation and form a single-pass synthetic aperture radar (SAR) interferometer. The formation was established to acquire a global high-precision digital elevation model (DEM) using SAR interferometry (InSAR). In order to achieve the required height accuracy of the TanDEM-X DEM, at least two global coverages have to be acquired. However, in difficult and mountainous terrain, up to five coverages are present. Here, acquisitions from ascending and descending orbits are needed to fill gaps and to overcome geometric limitations. Therefore, a strategy to properly combine the available height estimates is mandatory. The objective of this paper is the presentation of the operational TanDEM-X DEM mosaicking approach. In general, multiple InSAR DEM heights are combined by means of a weighted average with the height error as weight. Apart from this widely used mosaicking approach, one big challenge remains with the handling of larger height discrepancies between the input data, which are mainly caused by phase unwrapping errors, but also by temporal changes between acquisitions. In the case of inconsistencies, the TanDEM-X mosaicking approach performs a grouping into height levels. A priority concept is set up to evaluate the different groups of heights considering the number of DEMs and several InSAR quality parameters: the height error, the phase unwrapping method, and the height of ambiguity. This allows the identification of the most reliable height level for mosaicking. This fusion concept is verified on different test areas affected by phase unwrapping errors in flat and mountainous terrain as well as by height discrepancies in forests. The results show that the quality of the final TanDEM-X DEM mosaic benefits a lot from this mosaicking approach.
Lawrence, M. 2009. Common tree definitions for national forest inventories in Europe. Silva Fennica 43(2): 303–321. At the international level, various definitions have been established for the compilation and publication of forest resources assessment results over the last decade. These international definitions frequently rely on terms that are not precisely specified for inventory purposes and do not completely cover the requirements arising from the application of National Forest Inventory (NFI) data. Also, with respect to conventional topics such as forest area and growing stock estimation, several terms and expressions referring to individual trees are not, or are only vaguely, defined until now. Since the individual tree is the basic element of any forest resources assessment, the clarification of tree-related terms is an important part of COST Action E43 to harmonise common reporting of National Forest Inventories. Based on a review of existing definitions and on the requirements for harmonised reporting, common tree-related defini-tions are established. One objective of this study is to refine and enhance the applicability of available tree and shrub definitions, in particular with regard to the distinction between trees and shrubs. The study also focuses on the parts or "elements" of trees and on the distinction between these elements as they are of particular importance in growing stock and biomass definitions. Furthermore, several definitions for tree characteristics such as "living" and "stand-ing", as well as tree variables such as height, length, diameter at breast height, and crown projection area are adjusted with respect to NFI purposes. A concluding discussion reflects upon the reviewed, refined and newly established definitions. The definitions presented in this paper provide a firm basis for a common set of harmonised reference definitions developed by COST Action E43 and contribute to the precise and consistent use of terms.
This paper analyses the potential of TanDEM-X to acquire highly accurate digital elevation models (DEMs) on a global scale. For this, an appropriate mission concept will be introduced. This concept is based on an add-on satellite almost identical to the TerraSAR-X satellite, orbiting in close formation with it. Radar images of these two satellites allow for the generation of a world-wide DEM according to the emerging HRTI-3 standard within less than three years. The achievable height accuracy will be derived from a detailed performance analysis taking into account major system and scene parameters. Critical issues will be identified on both the system and mission level. Furthermore, additional applications as demanded by user requirements will include regional DEMs according to HRTI-4 standard or new techniques such as digital beamforming, bi-static observations or polarimetric interferometric SAR. A mission scenario is presented, which fulfils these requirements within the three year mission time.
TanDEM-X (TerraSAR-X add-on for digital elevation measurements) is an innovative spaceborne radar interferometer that is based on two TerraSAR-X radar satellites flying in close formation. The primary objective of the TanDEM-X mission is the generation of a consistent global digital elevation model (DEM) with an unprecedented accuracy, which is equaling or surpassing the HRTI-3 specification. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new radar imaging techniques and applications. This paper gives a detailed overview of the TanDEM-X mission concept which is based on the systematic combination of several innovative technologies. The key elements are the bistatic data acquisition employing an innovative phase synchronization link, a novel satellite formation flying concept allowing for the collection of bistatic data with short along-track baselines, as well as the use of new interferometric modes for system verification and DEM calibration. The interferometric performance is analyzed in detail, taking into account the peculiarities of the bistatic operation. Based on this analysis, an optimized DEM data acquisition plan is derived which employs the combination of multiple data takes with different baselines. Finally, a collection of instructive examples illustrates the capabilities of TanDEM-X for the development and demonstration of new remote sensing applications.
This paper presents the algorithms and analysis results for delineating snow zones using active and passive microwave satellite remote sensing data. With a high-resolution Radarsat synthetic aperture radar (SAR) image mosaic, dry snow zones, percolation zones, wet snow zones, and blue ice patches for the Antarctic continent have been successfully delineated. A competing region growing and merging algorithm is used to initially segment the SAR images into a series of homogeneous regions. Based on the backscatter characteristics and texture property, these image regions are classified into different snow zones. The higher level of knowledge about the areal size of and adjacency relationship between snow zones is incorporated into the algorithms to correct classification errors caused by the SAR image noise and relief-induced radiometric distortions. Mathematical morphology operations and a line-tracing algorithm are designed to extract a vector line representation of snow-zone boundaries. With the daily passive microwave Special Sensor Microwave/Imager (SSM/I) data, dry and melt snow zones were derived using a multiscale wavelet-transform-based method. The analysis results respectively derived from Radarsat SAR and SSM/I data were compared and correlated. The complementary nature and comparative advantages of frequently repeated passive microwave data and spatially detailed radar imagery for detecting and characterizing snow zones were demonstrated
Digital elevation models (DEMs) are of fundamental importance for a broad range of commercial and scientific applications. The relative height accuracy of the DEM is one of the most important quality parameters as it refers to the local differences among adjacent elevation values. A direct comparison against a reliable reference DEM is only possible for some single regions where an accurate model similar in resolution is already available. DEMs generated from interferometric synthetic aperture radar (InSAR) can be analyzed with respect to the relative height accuracy based on the data itself. This paper introduces a novel approach to estimate the relative height accuracy. The analysis is based on the well-known statistical characteristics of SAR interferograms and is designed as a specification check for global DEM generation. First results of evaluation of the relative height accuracy are presented for TanDEM-X data.
Data from spaceborne light detection and ranging (lidar) opens the
possibility to map forest vertical structure globally. We present a
wall-to-wall, global map of canopy height at 1-km spatial resolution,
using 2005 data from the Geoscience Laser Altimeter System (GLAS) aboard
ICESat (Ice, Cloud, and land Elevation Satellite). A challenge in the
use of GLAS data for global vegetation studies is the sparse coverage of
lidar shots (mean = 121 data points/degree2 for the L3C
campaign). However, GLAS-derived canopy height (RH100) values were
highly correlated with other, more spatially dense, ancillary variables
available globally, which allowed us to model global RH100 from forest
type, tree cover, elevation, and climatology maps. The difference
between the model predicted RH100 and footprint level lidar-derived
RH100 values showed that error increased in closed broadleaved forests
such as the Amazon, underscoring the challenges in mapping tall (>40
m) canopies. The resulting map was validated with field measurements
from 66 FLUXNET sites. The modeled RH100 versus in situ canopy height
error (RMSE = 6.1 m, R2 = 0.5; or, RMSE = 4.4 m,
R2 = 0.7 without 7 outliers) is conservative as it also
includes measurement uncertainty and sub pixel variability within the
1-km pixels. Our results were compared against a recently published
canopy height map. We found our values to be in general taller and more
strongly correlated with FLUXNET data. Our map reveals a global
latitudinal gradient in canopy height, increasing towards the equator,
as well as coarse forest disturbance patterns.
The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission comprises two nearly identical satellites: TerraSAR-X (TSX, launched in 2007), and TanDEM-X (TDX, launched in June 2010). The primary objective of the mission is to generate a worldwide and consistent digital elevation model (DEM) with an unprecedented accuracy. During the first 3 months after its launch, the TDX satellite was tested and calibrated in monostatic configuration with both satellites flying in 20 km along-track distance, and it was proven that the system and acquisition performance is almost identical to TSX. Both satellites were then brought into close formation of a few hundred meters distance to begin the bistatic commissioning phase. Since then, TSX and TDX have acted as a large single-pass radar interferometer, which overcomes the limitations imposed by repeat-pass interferometry and allow the acquisition of highly accurate cross- and along-track interferograms. In December 2010, TanDEM-X began with operational global acquisition: bistatic and monostatic SAR images are simultaneously acquired in stripmap mode and processed to interferograms, from which a global DEM is derived. The key parameter in estimating interferometric performance is the coherence, which is deeply evaluated in this paper. The impact of different decorrelation sources as well as the performance stability over time is investigated by means of statistical analyses and dedicated acquisitions on defined test sites, demonstrating the outstanding interferometric capabilities of the TanDEM-X mission.
The primary objective of the TanDEM-X mission is the generation of a global high resolution digital elevation model (DEM) with single-pass SAR interferometry. Within the mission, the Earth's land masses will be mapped at least twice in order to achieve relative vertical accuracies in the order of two meters. This paper presents an analysis approach of the mission performance in terms of relative height error showing the first results obtained from TanDEM-X interferometric data. For critical areas characterized by strong volume decorrelation phenomena or mountainous terrain, different approaches to improve the nal height error are investigated as well.
This paper assesses the accuracy and reliability of tree height retrieval over coniferous plantations using X-band interferometry. Factors such as crown shape, density, tree height, incidence angle, and slope have been assessed and quantified using a simple polarimetric radar interferometry simulator to determine their impact on height retrieval. Results from model simulation show that the most important factors are: crown shape, plantation density, and tree height. Variation in viewing angle and small slopes (<30°) appear to have only small effects. These results appear to be in reasonably good agreement with the retrieved tree height from airborne X-band Intermap data over coniferous plantations in the U.K.
In dual- or multiple-channel synthetic aperture radar (SAR)
imaging modes, cross-channel correlation is a potential source of
information. The sample coherence magnitude is calculated over a moving
window to generate a coherence magnitude map. High-resolution coherence
maps may be useful to discriminate fine structures. Coarser resolution
is needed for a more accurate estimation of the coherence magnitude. In
this study, the accuracy of coherence estimation is investigated as a
function of the coherence map resolution. It is shown that the
space-averaged coherence magnitude is biased toward higher values. The
accuracy of the coherence magnitude estimate obtained is a function of
the number of pixels averaged and the number of independent samples per
pixel (i.e., the coherence map resolution). A method is proposed to
remove the bias from the space-averaged sample coherence magnitude.
Coherence magnitude estimation from complex (magnitude and phase)
coherence maps is also considered. It is established that the magnitude
of the averaged sample coherence estimate is slightly biased for
high-resolution coherence maps and that the bias reduces with coarser
resolution. Finally, coherence estimation for nonstationary targets is
discussed. It is shown that the averaged sample coherence obtained from
complex coherence maps or coherence magnitude maps is suitable for
estimation of nonstationary coherence. The averaged sample (complex)
coherence permits the calculation of an unbiased coherence estimate,
provided that the original signals can be assumed to be locally
stationary over a sufficiently coarse resolution cell
A radar interferometric technique for topographic mapping of
surfaces, implemented utilizing a single synthetic aperture radar (SAR)
system in a nearly repeating orbit, is discussed. The authors
characterize the various sources contributing to the echo correlation
statistics, and isolate the term which most closely describes surficial
change. They then examine the application of this approach to
topographic mapping of vegetated surfaces which may be expected to
possess varying backscatter over time. It is found that there is
decorrelation increasing with time but that digital terrain model
generation remains feasible. The authors present such a map of a
forested area in Oregon which also includes some nearly unvegetated lava
flows. Such a technique could provide a global digital terrain map
Land Cover CCI-Product User Guide-Version 2
- G Kirches