DEM Of Difference Research Articles

Volumetric Analysis of the Landslide in Abe Barek, Afghanistan Based on Nonlinear Mapping of Stereo Satellite Imagery-Derived DEMs

On 2 May 2014, a large-scale landslide in Abe Barek, Badakhshan, Afghanistan, produced extensive damage to the buildings and killed hundreds of people. Evaluations of the extent and the volume of the displaced materials are vital for post-disaster management activities. In this study, we present the applicability of a nonlinear geometric correction technique for decreasing the undesired registration errors between pre- and post-event digital elevation models (DEMs) generated from high-resolution stereo pair satellite imagery, identifying landslide affected areas, and quantifying the landslide volume from DEMs of difference (DoD) analysis. The nonlinear mapping method consists of shifting vector generation in subareas of the DEMs, consensus operations, and interpolation of the shifting vectors. The quality assessment confirmed that the method outperformed the simple DoD technique by eliminating a large-scale of geometric errors in an unaffected area. We estimated the volume of the landslide as 1.05 × 106 m3 from the DoD corrected by the nonlinear method, and discussed the relationship between the area and volume compared to those of the previous studies.

The Spatio-Temporal Variability of Frost Blisters in a Perennial Frozen Lake along the Antarctic Coast as Indicator of the Groundwater Supply

Remote sensing, and unmanned aerial vehicles (UAVs) in particular, can be a valid tool for assessing the dynamics of cryotic features as frost blisters and to monitor the surface changes and the sublimation rates on perennially frozen lakes that host important ecosystems. In this paper, through the use of these remote sensing techniques, we aim to understand the type of groundwater supply of an Antarctic perennial frozen lake that encompasses two frost blisters (M1 and M2) through the temporal analysis of the features’ elevation changes (frost blisters and lake ice level). The frozen lake is located at Boulder Clay (northern Victoria Land, Antarctica). We relied on several photogrammetric models, past satellite images and ground pictures to conduct differencing of digital elevation models, areal variations and pixel counting. In addition, in situ measurements of the ice sublimation or snow accumulation were carried out. The two frost blisters showed different elevation trends with M1 higher in the past (1996–2004) than recently (2014–2019), while M2 showed an opposite trend, similarly to the ice level. Indeed, the linear regression between M2 elevation changes and the ice level variation was statistically significant, as well as with the annual thawing degree days, while M1 did not show significant results. From these results we can infer that the groundwater supply of M1 can be related to a sublake open talik (hydraulic system) as confirmed also by pressurized brines found below M1, during a drilling in summer 2019. For M2 the groundwater flow is still not completely clear although the hydrostatic system seems the easiest explanation as well as for the uplift of the lake ice.

Baipenzhu Reservoir Inflow Flood Forecasting Based on a Distributed Hydrological Model

For reservoir basins, complex underlying surface conditions, short flood confluence times, and concentrated water volumes make inflow flood forecasting difficult and cause forecast accuracies to be low. Conventional flood forecasting models can no longer meet the required forecast accuracy values for flood control operations. To give full play to the role of reservoirs in flood control and to maximize the use of reservoir flood resources, high-precision inflow flood forecasting is urgently needed as a support mechanism. In this study, the Baipenzhu Reservoir in Guangdong Province was selected as the study case, and an inflow flood forecast scheme was designed for the reservoir by a physically based distributed hydrological model, the Liuxihe model. The results show that the Liuxihe model has strong applicability for flood forecasting in the studied reservoir basin and that the simulation results are very accurate. This study also found that the use of different Digital Elevation Model (DEM) data sources has a certain impact on the structure of the Liuxihe model, but the constructed models can both simulate the inflow flood process of the Baipenzhu Reservoir well. At the same time, the Liuxihe model can reflect the spatial variation in rainfall well, and using the Particle swarm optimization (PSO) algorithm to optimize the initial model parameters can greatly reduce the uncertainty of the model forecasts. According to China’s hydrological information forecast standards, the Liuxihe model forecast schemes constructed by the two data sources are rated as Grade A and can be used for real-time flood forecasting in the Baipenzhu Reservoir basin.

The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance

A recent storm (27th–30th October 2018), named Vaia, hit most part of the Northeast of Italy affecting the geomorphic aspect of almost all mountain catchments of the area. The event triggered new instabilities such as windthrows, landslides and debris flows. At present, few studies dealt with the analysis of the impact of a Large Infrequent Disturbance at large catchment scale. This work provides a focus on the Tegnas Torrent Basin (Belluno Province) and aims at detecting how, where, and how much this storm affected the basin. Moreover, it integrates two different approaches considering both the dynamic and static aspects of the sediment, via DEM of Difference (DoD) and Index of Connectivity (IC), respectively. The Tegnas sub-basins responded contrastingly: the Bordina (volcanic origin and covered by pastures and spruce forests) was mainly affected by windthrows (7% of the sub-basin area) and landslides (0.5%), while the Angheraz (outcropping dolomite rocks), was stricken only by debris flows (1.0%). Morphological changes were clear along the entire channel network, with predominant erosion in the steepest upstream parts (over 2 m of the channel elevation), and deposition in the lower main valley floor (over 3 m of the channel elevation). The IC analysis along the instabilities highlighted that the windthrows occurred mainly in areas of high connectivity, which may be important for future management strategies. Moreover, the proposed integrated approach, based on the combination IC-DoD, permitted a detailed identification of sediment routing and a contemporary estimation of erosion and deposition volumes generated by a high magnitude low-frequency event. Based on these results, cascading processes are expected and further analysis are required to fully consider the impact of a Large Infrequent Disturbance.

A method for estimating sediment budgets of washover deposits using digital terrain models

AbstractWashover fans are located on small barriers in fetch‐limited micro‐tidal coastal environments in Denmark. These washover fans are formed during high‐energy storm events and we present a method to quantify their volumes and to estimate sediment exchanges between washover fans and their adjacent morphologies. We use high resolution digital terrain models (DTMs) based on light detection and ranging (LiDAR) data. We have delineated landforms using known methods of scale analysis and geomorphometric classification. We quantified volumes of the delineated landforms and estimated the related sediment budgets. These computed volumes were compared using different pre‐depositional surfaces. Finally, we assessed the sediment exchange and associated sources of sediments of the washover fans. We applied a scale analysis to determine suitable DTM resolution and focal statistics window size as input to a geomorphometric classification analysis. Landform areas and landforms were delineated using morphometric threshold values, and volumes and sediment budgets of the delineated landforms were computed using different assumptions to define the pre‐depositional surface. Resulting washover fan volumes were validated against digital elevation model (DEM) of difference (DoD) derived volumes. Sediment budgets were derived from representative volumes of the washover fans and adjacent berms. We show that quantification of washover features derived from DTMs, using geomorphometric analysis is feasible and that the presented approach provides estimates of washover deposit volumes with an accuracy between 1% and 28% compared to control volumes. © 2021 John Wiley & Sons, Ltd.

Evaluation of geomorphological approaches combined with digital elevation models for the Nash's instantaneous unit hydrograph

Methodologies for estimation of direct surface runoff (DSR) hydrographs have been proposed, especially for the Unit Hydrograph (UH) and Instantaneous Unit Hydrograph (IUH). The widely used IUH developed by Nash (NIUH) has two main parameters: number of reservoirs (n) and storage coefficient (k), which by definition are determined based on observed rainfall-runoff events. In order to overcome the need for hydrological data for application of NIUH, geomorphological approaches (GAs) have been developed to provide the aforementioned parameters for ungauged watersheds, thus allowing for the estimation of DSR hydrographs using only relief information. The aim of this study was to evaluate the performance of 4 GAs (GA1 - Nash, 1960; GA2 - Wu, 1963; GA3 - Rosso, 1984; GA4 - Bhaskar et al., 1997). Moreover, this study sought to investigate the influence of different digital elevation models (DEMs) (TOPO, ASTER, SRTM-30 and SRTM-90) on the determination of geomorphological attributes necessary for the application of the above-mentioned GAs. Twenty rainfall-runoff events observed in a small Brazilian watershed were used to evaluate the performance of NIUH combined with different GAs and DEMs for estimation of DSR hydrographs. In view of the results found in this study, the main conclusions are: i) the GAs exert more influence on the parameters n and k than the DEMs; ii) GA1 and GA2 outperformed GA3 and GA4; and iii) GA2 coupled with the ASTER DEM results in the best estimates of DSR hydrographs.

Assessing of topographic changes in the Fairy Stream, Phan Thiet by 3D topographic mapping method

Currently, the Fairy Stream is a breathtaking tourist destination for domestic and foreign tourists due to the natural unspoiled beauty of the area. However, this landscape is vulnerable to human impacts such as littering, carving words on sand walls, climbing on sand walls causing slips and falls. Therefore, the requirements for management and monitoring of topographic changes are essential to preserving the beauty of this place. Unmanned aerial vehicle (UAV) technology is being widely applied in the world with much researches on applications in the fields of agriculture, mining, construction, etc. Thanks to the development of the "Structure from Motion" (SfM) method to generate a digital surface model (DSM) from UAV images and orthomosaic, which helps to accurately and quickly assess the topographic changes of the area. The author uses UAVs in landscape assessment, researches topographic changes in the Fairy Stream, Binh Thuan tourist, and 3D models at three different times about every 4 months. DEM of Difference (DoD) method is applied to analyze the change in elevation of the area and the Digital Shoreline Analysis System (DSAS) method to calculate the accretion and erosion interval over time. Flow velocity and material composition analysis demonstrate that the flow velocity exceeds the maximum erosion resistance velocities of fine and medium grain materials. Calculation by DSAS shows that erosion rate is 1.95m/8 months and accretion rate is 1.33m/8 months. After assessing, the author concludes that the fluctuation mechanism of the Fairy Stream is affected by the Northeast and Southwest monsoons, combined with the tourist factor. The study has demonstrated the potential of using UAVs in terrain change assessment to supports authorities in decision-making, provide better management measures such as zoning sensitive areas, placing signs, restricting visitors, etc.

TanDEM-X:Deriving InSAR Height Changes and Velocity Dynamics of Great Aletsch Glacier

Great Aletsch Glacier (Grosser Aletschgletscher), the largest glacier in the European Alps, contains 20% of the entire Swiss ice mass. Therefore, it has been selected as a super-testsite for the TanDEM-X satellite mission. Dense time series with a repeat interval down to 11 d were acquired between 2011 and 2019 using two polarizations (HH and VV) and across-track baselines of 0-1 km. To evaluate the use of interferometric single-pass synthetic aperture radar mission for glaciological applications, we implemented a processing pipeline in interactive data language (IDL) and computed 130 digital elevation models (DEMs) from bistatic radar interferograms. We present a method to circumvent a common pitfall during orthorectification of radar DEM differences. Regression analysis of DEM time series shows a height loss of up to 8 m a$^{-1}$ on the tongue and 1.5 m a$^{-1}$ when averaged over the whole glacier area. In spring 2013, we observed X-band penetration depths of 4±2 m in the accumulation area. For strongly crevassed areas, the coherence drops already to 0.5 for across-track baselines <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> > 200 m. With patch-based incoherent offset tracking, we obtained an almost complete 200 m resolution velocity map. Velocities reach up to 0.8 m d$^{-1}$, show a seasonal variability of ±0.05 m d$^{-1}$, and agree within 0.04 m d$^{-1}$(root mean square error) with field measurements. Copolar phase differences suggest an approximation of snow accumulation. We demonstrate orthorectification of the backscatter intensity using simultaneously acquired TanDEM-X interferograms, which allows for the decoupling of horizontal velocity estimates from phase-center height changes due to penetration and ice melt.

A Short-Time Repeat TLS Survey to Estimate Rates of Glacier Retreat and Patterns of Forefield Development (Case Study: Scottbreen, SW Svalbard)

The study presents findings from comparative analyses of high-resolution differential digital elevation models (DEM of Difference—DoD) based on terrestrial laser scanning (TLS) surveys. The research was conducted on the 0.2 km2 Scottbreen valley glacier foreland located in the north-western part of Wedel-Jarlsberg Land (Svalbard) in August of 2013. The comparison between DTMs at 3-week intervals made it possible to identify erosion and depositional areas, as well as the volume of the melting glacier’s terminus. It showed a considerable recession rate of the Scottbreen (20 m year−1) while its forefield was being reshaped by the proglacial Scott River. A study area of 205,389 m2, 31% of which is occupied by the glacier (clear ice zone), was included in the repeated TLS survey, which was performed from five permanent scan station points (registered on the basis of five target points—TP). The resultant point clouds with a density ranging from 91 to 336 pt m−2 were converted into DEMs (at a spacing of 0.1 m). They were then put together to identify erosion and depositional areas using Geomorphic Change Detection Software (GCD). During the 3-week interval, the retreat of the glacier’s snout ranged from 3 to 9 m (mean of 5 m), which was accompanied by an average lowering of the surface by up to 0.86 m (±0.03 m) and a decrease of ice volume by 53,475 m3 (±1761 m3). The deglaciated area increased by 4549 m2 (~5%) as a result of the recession, which resulted in an extensive reshaping of the recently deglaciated area. The DEM of Difference (DoD) analyses showed the following: (i) lowering of the glacial surface by melting and ii) predominance of deposition in the glacier’s marginal zone. In fact, 17,570 m3 (±1172 m3) of sediments were deposited in the glacier forefield (41,451 m2). Also, the erosion of sediment layers having a volume of 11,974 m3 (±1313 m3) covered an area equal to 46,429 m2 (53%). This occurrence was primarily based on the washing away of banks and the deepening of proglacial stream beds, as well as the washing away of the lower parts of moraine hillocks and outwash fans.

More dynamic than expected: an updated survey of surging glaciers in the Pamir

Abstract. The investigation of surging glaciers using remote sensing has recently seen a strong increase as freely available satellite data and digital elevation models (DEMs) can provide detailed information about surges that often take place in remote and inaccessible regions. Apart from analysing individual surges, satellite information is increasingly used to collect valuable data on surging glaciers. Related inventories have recently been published for several regions in High Mountain Asia including the Karakoram or parts of the Pamir and western Kunlun Shan, but information for the entire Pamir is solely available from a historic database listing about 80 glaciers with confirmed surges. Here we present an updated inventory of confirmed glacier surges for the Pamir that considers results from earlier studies and is largely based on a systematic analysis of Landsat image time series (1988 to 2018), very high-resolution imagery (Corona, Hexagon, Bing Maps, Google Earth) and DEM differences. Actively surging glaciers (e.g. with advancing termini) were identified from animations and flicker images and the typical elevation change patterns (lowering in an upper reservoir zone and thickening further down in a receiving zone). In total, we identified 206 spatially distinct surges within 186 glacier bodies mostly clustered in the northern and western part of the Pamir. Where possible, minimum and maximum glacier extents were digitised, but often interacting tributaries made a clear separation challenging. Most surging glaciers (n=70) are found in the larger size classes (&gt;10 km2), but two of them are very small (&lt;0.5 km2). We also found several surges where the length of the glacier increased by more than 100 %. The created datasets are available at: https://doi.org/10.1594/PANGAEA.914150 (Goerlich et al., 2020).

Spatio-temporal variability of surface mass balance in the accumulation zone of the Mer de Glace, French Alps, from multitemporal terrestrial LiDAR measurements

AbstractSpatio-temporal variability of the winter surface mass balance is a major uncertainty in the modelling of annual surface mass balance. Moreover, its measurement at high spatio-temporal resolution (sub-200 m) is very useful to force, calibrate or validate models. This study presents the results of year-round field campaigns to study the evolution of the surface mass balance in a ~2 km2 portion of the accumulation zone of the Mer de Glace (France). It is based on repeated LiDAR acquisitions, submergence-velocity measurements and meteorological records. The two methods used to quantify submergence velocities show good agreement. They present a linear temporal evolution without significant seasonal changes but display significant spatial variability. We conclude that a dense network of submergence velocity measurements is required to reduce the uncertainties when computing winter and annual surface mass balance from digital elevation model differencing. Finally, a hight spatio-temporal variability of the winter surface mass balance is highlighted (e.g., a std dev. of 0.92 m in April) even though the topography is homogeneous (std dev. of 25 m). Attempts to relate this variability to different morpho-topographic variables and wind-related indexes show the need for studies conducted at the snowfall event scale to obtain a better understanding of the variability in mass balance at the glacier scale.

Long-Term Changes of Morphodynamics on Little Ice Age Lateral Moraines and the Resulting Sediment Transfer into Mountain Streams in the Upper Kauner Valley, Austria

Since the end of the Little Ice Age (LIA), formerly glaciated areas have undergone considerable changes in their morphodynamics due to external forces and system-internal dynamics. Using multi-temporal high-resolution digital elevation models (DEMs) from different remote sensing techniques such as historical digital aerial images and light detection and ranging (LiDAR), and the resulting DEMs of difference (DoD), spatial erosion and accumulation patterns can be analyzed in proglacial areas over several decades. In this study, several morphological sediment budgets of different test sites on lateral moraines and different long-term periods were determined, covering a total period of 49 years. The test sites show high ongoing morphodynamics, and therefore low vegetation development. A decrease as well as an increase of the mean annual erosion volume could be demonstrated at the different test sites. All test sites show a slope–channel coupling and a decrease in the efficiency of sediment transport from slopes to channels. These developments are generally subject to conditions of increasing temperature, decreasing short-term precipitation patterns and increasing runoff from adjacent mountain streams. Finally, the study shows that sediment is still available on the investigated test sites and the paraglacial adjustment process is still in progress even after several decades of deglaciation (~133 years).

Quantification of mudcracks-driven erosion using terrestrial laser scanning in laboratory runoff experiment

Understanding rill development and erosion rates is crucial for understanding changes in hillslope morphology and soil erosion. Mudcracks, a widely distributed feature on the land surface, play an important role in erosion, but the quantitative relationship between mudcracks and erosion has seldom been reported. Thanks to advances in terrestrial laser scanning (TLS, also known as terrestrial LiDAR), topographic changes can be observed at high resolution. This study is designed to quantify the effects of mudcracks on erosion. Mudstone specimens with and without mudcracks were analyzed. Using TLS, we quantified the millimeter-scale effects of mudcracks on slope erosion and rill development in laboratory experiments with well-controlled samples and water discharge. The point cloud data collected by TLS were then converted to digital elevation models (DEMs) for DEM of difference (DoD) analysis. The results show that mudcracks facilitate rill development. In the early stage of the experiment, the erosion depth of the cracked mudstone is twice that of the surface without cracks, and the total erosion amount is more than twice. During the experiments, the dominant erosion pattern changes from sheet erosion to rill erosion and then lateral erosion. Moreover, our study highlights the potential of TLS and DoD for high-resolution spatio-temporal topographic surveys to identify and characterize micro-relief-induced erosion.

UAV-based evaluation of morphological changes induced by extreme rainfall events in meandering rivers.

Morphological changes, caused by the erosion and deposition processes due to water discharge and sediment flux occur, in the banks along the river channels and in the estuaries. Flow rate is one of the most important factors that can change river morphology. The geometric shapes of the meanders and the river flow parameters are crucial components in the areas where erosion or deposition occurs in the meandering rivers. Extreme precipitation triggers erosion on the slopes, which causes significant morphological changes in large areas during and after the event. The flow and sediment amount observed in a river basin with extreme precipitation increases and exceeds the long-term average value. Hereby, erosion severity can be determined by performing spatial analyses on remotely sensed imagery acquired before and after an extreme precipitation event. Changes of erosion and deposition along the river channels and overspill channels can be examined by comparing multi-temporal Unmanned Aerial Vehicle (UAV) based Digital Surface Model (DSM) data. In this study, morphological changes in the Büyük Menderes River located in the western Turkey, were monitored with pre-flood (June 2018), during flood (January 2019), and post-flood (September 2019) UAV surveys, and the spatial and volumetric changes of eroded/deposited sediment were quantified. For this purpose, the DSAS (Digital Shoreline Analysis System) method and the DEM of Difference (DoD) method were used to determine the changes on the riverbank and to compare the periodic volumetric morphological changes. Hereby, Structure from Motion (SfM) photogrammetry technique was exploited to a low-cost UAV derived imagery to achieve riverbank, areal and volumetric changes following the extreme rainfall events extracted from the time series of Tropical Rainfall Measuring Mission (TRMM) satellite data. The change analyses were performed to figure out the periodic morphodynamic variations and the impact of the flood on the selected meandering structures. In conclusion, although the river water level increased by 0.4–5.9 meters with the flood occurred in January 2019, the sediment deposition areas reformed after the flood event, as the water level decreased. Two-year monitoring revealed that the sinuosity index (SI) values changed during the flood approached the pre-flood values over time. Moreover, it was observed that the amount of the deposited sediments in September 2019 approached that of June 2018.

Different Paths for Developing Terrestrial LiDAR Data for Comparative Analyses of Topographic Surface Changes

High resolution terrestrial laser scanning data (TLS; terrestrial LiDAR) provide an excellent background for quantitative resource estimation through the comparative analysis of topographic surface changes. However, unlike airborne LiDAR data, which is usually provided as classified and contains a class of ground points, raw TLS data include all of the points of the scanned space within the specified scanner range. In effect, utilizing the latter data to estimate the volume of the resource by the differential analysis of digital elevation models (DEMs) requires the data to be specially prepared, i.e., separating from the point cloud only the data that represent the relevant class. In the case of natural resources, e.g., mineral resources, the class is represented by ground points. This paper presents the results that were obtained by differential analysis of high resolution DEMs (DEM of difference (DoD) method) using TLS data that were processed both manually (operator noise removal) and with the use of the automatic Cloth Simulation Filter (CSF) algorithm. Three different time pairs of DoD data were analyzed for a potential gravel-cobble deposit area of 45,444 m2, which was located at the bottom of the mouth section of the Scott River in south-east Svalbard. It was found that the applied method of ground point classification had very little influence on the errors in the range of estimating volumetric parameters of the mineral resources and measurement uncertainty. Moreover, it was shown that the point cloud density had an influence on the CSF filtering efficiency and spatial distribution of errors.

Using historical aerial photographs to measure earthquake deformation: Testing the effects of scan resolution

Historical aerial photographs are a valuable source of data for investigating past earthquakes, volcanic activities, ice mass balance and land changes in the pre-modern era. Scan resolution for digitising the historical archives is an important factor in photogrammetric processes, but the effects have not been quantified yet. In this paper, we use the 1978 Ms 7.4 Tabas-e-Golshan earthquake in eastern Iran as a case study to test how scan resolution affects digitisation and hence the measurements of deformation. We scanned 6 aerial photographs that were acquired in 1956 with 150, 300, 600 and 1200 dots per inch (dpi), and generated four sets of pre-earthquake digital elevation model (DEM) covering the Tabas fold. By differencing the pre-earthquake DEMs and the post-earthquake DEM derived from SPOT-6 data acquired in 2013, we made four measurements of slip on the Tabas fold. We compared our measurements to the published model which was derived from matching the pre-earthquake Hexagon Keyhole-9 US image and the post-earthquake SPOT-2 and SPOT-6 images, and found that the 600 dpi scans yield the most consistent result among the four different resolutions, with a misfit of 0.7 m. A different resolution might well be better for a different set of aerial photographs, depending on the quality and photographic scale of the images. We scanned 20 aerial photographs that cover the Kurit fold to the south of the Tabas fold with 600 dpi and used them to investigate the slip on that segment. Similarly, DEMs were extracted first to produce the DEM of difference (DoD). Five swath profiles taken from the DoD show clear discontinuities across two fault strands, i.e. the main Kurit fault (the western strand) and a back thrust (the eastern strand). Based on the profile measurements, we implemented Monte Carlo modelling of elastic dislocation and obtained a slip of 1.4 m and 2.2 m on the western and eastern strands respectively. By combining all the information about the earthquake and postseismic slip, we inferred a coseismic slip of 3.5 m on the detachment fault by which most of the seismic moment was released. This slip enhanced the applied load on part of the Kurit fault, which is partly responsible for the long-lasting postseismic deformation in the region.

Using Multi-Mission Satellite Elevation Data for Delineation of Gilgit Watershed in Pakistan in Geographical Information Technology Environment

The hydro-climatological variations in Gilgit watershed of Upper Indus basin is less scientifically understood due to diverse geography, remoteness of the region and larger variations in climatic conditions. Extraction of catchments at multiple scales is an important task in undertaking the watershed management studies. Satellite remote sensing (SRS) and geographical information technology (GIT) provide a very useful method to study the watersheds. In view of the facts, watershed/ natural resources management in Gilgit river basin, application of geospatial techniques to various elevation datasets is required in order to obtain more accurate results using these elevation datasets. To achieve this goal, the topographic feature extraction has been studied in the catchment of Gilgit river using different Digital Elevation Models (DEMs) viz., SRTM, ASTER GDEM and GTOPO30. Several small watersheds for the Phakor, Karamber, East Gammu, Bhort and Bad-e-Swat glaciers were delineated for the basin definition. The delineated watersheds have been visually analyzed against the optical Landsat 8 OLI imagery for mountainous ridge matching. The results revealed that, SRTM 30m (radar based) exhibited more accuracy among these DEMs because of its precise delineation in the Gilgit sub-basin. However, it is appropriate to say that computed area from all three DEMs generally show close agreement. This study is a good contribution towards better understanding of the watershed management and the hydrological responses in Gilgit watershed of the upper Indus catchment.

Multi-temporal UAV based repeat monitoring of rivers sensitive to flood

ABSTRACT Multi-temporal repeat monitoring of flood-vulnerable rivers is crucial due to rapid alteration of morphological properties of in-channel landforms. Besides, the characteristics of the river crossing bridges may deteriorate due to flood induced scouring around bridge piles or due to flood loads. Thus, in this study high-resolution topography of the study region was acquired during two consecutive years by unmanned aerial vehicle (UAV) based surveys using Structure-from-Motion (SfM) processing. Following the extraction of digital elevation models (DEM), repeat data that were obtained at each UAV survey were compared using Geomorphic Change Detection (GCD) to calculate volumes of deposition and erosion via DEM of difference (DoD) algorithm. Thus, detailed high-resolution maps of the river channels can be rapidly and efficiently generated by low cost UAV based measurement methods in order for continuous tracking of stream channel morphology for the rivers sensitive to floods.

Calanchi badlands reconstructions and long-term change detection analysis from historical aerial and UAS image processing

In Italy, highly dissected badlands may be found widespread (over the entire Apennine region) and are locally described as “calanchi” (singular: “calanco”). Accelerated geomorphic processes led to the rapid evolution of these erosional landunits with intense drawbacks on agriculture, infrastructure and society. Historical aerial photographs (HAP) of such badland areas exist worldwide. To reconstruct the past surface and related topographic changes offers high potential for the assessment of long-term topographic changes of many interesting landscapes. Due to technical and conceptual demands of traditional photogrammetry those photographs have been used comparable little in long-term geomorphology. Processing them with user-friendly SfM-MVS photogrammetry would provide a new perspective on the assessment of long-term erosion rates on the catchment and slope scale and the related badland evolution. Therefore, this multi-temporal approach focusses on the appliance of SfM-MVS photogrammetry in order to reconstruct the past landscape and to calculate the related long-term surface changes of a badland located in the upper Val d’Orcia (Tuscany, Italy). The analysis is based on a set of high-resolution UAV images (2016) and two sets of historical aerial images of varying quality and scale (1976 and 1994). Digital elevation models (DEMs), shaded relief raster, several morphometric parameters and DEMs of difference (DoD) (resolution: 1 m) were used to obtain overall terrain changes and to calculate sediment balances and annual surface change rates on catchment and slope scale. Change detection analysis revealed a mean surface lowering of 0.01 m*a–1 (1976–2016) on the catchment scale and 0.03 m*a–1 (1976–1994) to 0.06 m*a–1 (1994–2016) on the slope scale. Our results further show a reduced activity of fluvial erosion processes from the mid 90s’ on while mass movements dominating the topographic changes in the second half of the 40 years observation period. Erosion processes are enhanced near the tear-off edges of the lateral valleys. Deposition mainly occurs in the main valley floor which can be classified as a temporary sediment sink/store. The geomorphic development of the catchment indicates a self-stabilizing behaviour towards an equilibrium state. A reduction of the mean slope gradient in the main valley from 25° in 1976 to 14° in 2016 confirms the self-stabilizing theory. The study also revealed the potential and limitations of historical aerial images for long-term geomorphic change detection and showed that SfM-MVS algorithms operate less reliable when applied to only a few images (n=3) of a singular flightline. Nevertheless, the good agreement of our results with the long term rates obtained by other authors with different methods/techniques and different time scales proof the great potential of historical aerial photographs for 3D surface reconstructions (SfM-MVS) of semi-arid badlands when ceratin quality parameters, mainly high image overlaps and entropy, are fulfilled.

Resolving Biases in DEM Differencing for Estimation of Change in Elevation of Glacier Surfaces Using Cartosat-I Stereo Data

Understanding the impact of climatic variations on Himalayan-Karakoram glaciers has now become a necessity for planning water resources of Indus-Ganga–Brahmaputra basins. One of the most important parameters to assess this is the mass balance of glaciers. Besides ground-based studies of mass balance, differencing of Digital Elevation Models (DEMs) is now commonly employed for estimating the long-term changes for this purpose. Before applying this approach, it is necessary to evaluate the accuracy of elevation changes of glacier surfaces with respect to terrain parameters and the DEMs used. This study demonstrates the factors governing DEM differencing using an example of four glaciers of Zanskar region by comparing DEMs extracted from Cartosat-1 stereo data of ablation season of the years 2005 and 2015. A strong relation has been observed between slope of the terrain and elevation changes between the two DEMs on non-glaciated stable terrain (R2 = 0.8144). The regression equation thus developed has been used to correct elevation changes on glacier surface. Lowering of ablation zone of the four sample glaciers over a period of ten years has been of the order of 3.92, 11.17, 4.99 and 4.39 m. The results obtained by using Cartosat-I DEMs have also been validated using ASTER DEMs of similar time interval.