Subtraction Of Digital Elevation Model Research Articles

Bayesian estimation of glacier surface elevation changes from DEMs

Accurate estimates of glacier surface elevation changes are paramount for various aspects of the study of the cryosphere, from glacier flow and thickness estimates to hydrological forecasts and projections of sea-level-rise. We present a novel probabilistic framework to filter outliers and estimate uncertainties in glacier surface elevation changes computed from the subtraction of digital elevation models (DEM). Our methodology frames outlier filtering as a Bayesian inference problem, thus characterizing the state of knowledge on glacier surface elevation changes through the posterior distribution as the combination of glacier volume variation observations and prior knowledge arising from previously collected data and/or modeled results. We validate this technique with experiments using Gaussian random fields to generate artificial noise in glacier surface elevation variation observations and show that the model satisfactorily culls the simulated outliers. Surface elevation change estimates are consistent with results computed from widely-used outlier filtering and uncertainty estimation techniques. The Bayesian framework allows unifying DEM error models with physical considerations on glacier surface elevation changes within a simple, statistically coherent model preventing temporal correlation and additional biases in other techniques. On the basis of these results, we discuss the implications of DEM uncertainty and offer suggestions for the glaciological community.

Measuring change at Earth’s surface: On-demand vertical and three-dimensional topographic differencing implemented in OpenTopography

AbstractTopographic differencing measures landscape change by comparing multitemporal high-resolution topography data sets. Here, we focused on two types of topographic differencing: (1) Vertical differencing is the subtraction of digital elevation models (DEMs) that span an event of interest. (2) Three-dimensional (3-D) differencing measures surface change by registering point clouds with a rigid deformation. We recently released topographic differencing in OpenTopography where users perform on-demand vertical and 3-D differencing via an online interface. OpenTopography is a U.S. National Science Foundation–funded facility that provides access to topographic data and processing tools. While topographic differencing has been applied in numerous research studies, the lack of standardization, particularly of 3-D differencing, requires the customization of processing for individual data sets and hinders the community’s ability to efficiently perform differencing on the growing archive of topography data. Our paper focuses on streamlined techniques with which to efficiently difference data sets with varying spatial resolution and sensor type (i.e., optical vs. light detection and ranging [lidar]) and over variable landscapes. To optimize on-demand differencing, we considered algorithm choice and displacement resolution. The optimal resolution is controlled by point density, landscape characteristics (e.g., leaf-on vs. leaf-off), and data set quality. We provide processing options derived from metadata that allow users to produce optimal high-quality results, while experienced users can fine tune the parameters to suit their needs. We anticipate that the differencing tool will expand access to this state-of-the-art technology, will be a valuable educational tool, and will serve as a template for differencing the growing number of multitemporal topography data sets.

Performance Assessment of TanDEM-X DEM for Mountain Glacier Elevation Change Detection

TanDEM-X digital elevation model (DEM) is a global DEM released by the German Aerospace Center (DLR) at outstanding resolution of 12 m. However, the procedure for its creation involves the combination of several DEMs from acquisitions spread between 2011 and 2014, which casts doubt on its value for precise glaciological change detection studies. In this work we present TanDEM-X DEM as a high-quality product ready for use in glaciological studies. We compare it to Aerial Laser Scanning (ALS)-based dataset from April 2013 (1 m), used as the ground-truth reference, and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) V003 DEM and SRTM v3 DEM (both 30 m), serving as representations of past glacier states. We use a method of sub-pixel coregistration of DEMs by Nuth and Kääb (2011) to determine the geometric accuracy of the products. In addition, we propose a slope-aspect heatmap-based workflow to remove the errors resulting from radar shadowing over steep terrain. Elevation difference maps obtained by subtraction of DEMs are analyzed to obtain accuracy assessments and glacier mass balance reconstructions. The vertical accuracy (± standard deviation) of TanDEM-X DEM over non-glacierized area is very good at 0.02 ± 3.48 m. Nevertheless, steep areas introduce large errors and their filtering is required for reliable results. The 30 m version of TanDEM-X DEM performs worse than the finer product, but its accuracy, −0.08 ± 7.57 m, is better than that of SRTM and ASTER. The ASTER DEM contains errors, possibly resulting from imperfect DEM creation from stereopairs over uniform ice surface. Universidad Glacier has been losing mass at a rate of −0.44 ± 0.08 m of water equivalent per year between 2000 and 2013. This value is in general agreement with previously reported mass balance estimated with the glaciological method for 2012–2014.

Significant decadal channel change 58–67 years post-dam accounting for uncertainty in topographic change detection between contour maps and point cloud models

Construction of digital elevation models (DEMs) and the subtraction of DEMs between different points in time as a method to determine temporal patterns of scour and fill is a highly valuable procedure emerging in geomorphology. These DEMs of Differences (DoDs) must be assessed for error in order to distinguish actual topographic change from uncertainty and surface error. Current methods include: (1) uniformly excluding all values that fall below a minimum threshold; (2) using a spatially variable approach such as the construction of minimum Level of Detection (LoD) grids; or (3) the creation of a fuzzy inference system. Although spatially variable methods for determining error have been more accurate in excluding noise without discarding large amounts of meaningful data, a challenge remains in performing DoDs against preexisting contour-based maps for which no original point data are available. The goals of this study were to (1) develop a method that overcomes the unknown point density of contour (and other historical) data sets and allows for some assessment of DoD uncertainty on the basis of information on topographic variability, (2) perform comprehensive uncertainty analysis testing to understand the opportunities and constraints associated with this new method, and (3) report and interpret the overall pattern and volume of decadal topographic change for a regulated river 67years post-dam in light of alternate conjectured mechanisms of post-dam longitudinal profile adjustment. The key feature of the new approach is the introduction of a high-density artificial point grid that samples the topographic variability evident in the available historical data set. The testbed used to develop and assess this new DoD method was the ~37.5-km lower Yuba River, California. Historical data consisted of 0.6-m contours from a 1999 survey, while a more detailed point cloud was available for the most recent survey in 2006–2008. To evaluate uncertainty in the method, this study applied seven different uncertainty metrics of varying strictness (t=1, t=1.96, uniform 0.3-m exclusion, t=1 plus uniform 0.3m exclusion, t=1 with LoD minimum at 0.3, t=1.96 plus uniform 0.3-m exclusion, and t=1.96 with LoD minimum at 0.3) and two different DoD adjustment methods (exclusion and subtraction). A stringent approach involving joint use of the contour half-interval and the spatially distributed LoD grid at a 95% confidence limit excluded 44.3% of the study area from spatial assessment of channel change and volumetric change computation. This preferred approach yielded an estimated 2.518millionm3 of total scour and 2.455millionm3 of total fill in 7–9years. After considering different mapping epochs, the net annual average export was 17,000m3 (~32,500tons) to the Feather River and a river-valley sediment yield of 2205tons/km2/year. This amount is 36% of the post-dam annual yield of gravel and cobble to the upstream reservoir that blocks sediment conveyance into the study domain, suggesting that the lowland system is still highly dynamic 67years after the dam was built. The scientific significance of this is that the response of rivers to dams can be far more long-lasting and complex, depending on the suite of cumulative societal impacts to rivers. The ability to account for spatially explicit DoD uncertainty (i.e., data retention when uncertainty is low and data removal when uncertainty is high) when comparing historic contour-based and modern point cloud-based DEMs will allow for more detailed and reliable DoDs and sediment budgets in these cases.

Effects of land‐use changes in vegetation cover and sidewall erosion in a gully head of the Penedès region (northeast Spain)

AbstractThe present research addresses the effects of land‐use changes in the evolution of vegetation cover on gully erosion in the Penedès region of northeast Spain. The contribution of vegetation cover to gully wall stabilization is also discussed. The catchment area of the gully is 60·2 ha and is mainly composed of vineyards. In this area, large gullies have developed on Tertiary marls and unconsolidated sandstones. Detailed aerial photographs (1 : 5000 and 1 : 7000 scale) of three dates (1975, 1995 and 2002) were ortho‐rectified to derive land‐use information in the catchment, gully vegetation cover and height information to construct detailed digital elevation models. Land‐use and gully vegetation cover mapping of the most recent period was assisted by means of fieldwork and a photo‐interpretation key constructed to help map the older dates. Soil loss in the catchment was estimated for the 1975 and 2002 scenarios to determine the relations between land‐use changes to vegetation cover changes and gully sidewall erosion. The Revised Universal Soil Loss Equation was applied for that purpose. Sidewall mass movements were determined by subtraction of digital elevation models (1 m resolution) of each period. The results show important land‐use changes in the catchment due to field reorganization for the plantation of mechanized vineyards. It increased soil loss from agricultural fields by 26·5%, which influenced vegetation cover dynamics on gully walls, with an important development of scrubland to forest, and gully erosion, with sidewall retreat at rates of 0·025 to 0·50 ha y−1 and mass movements at rates of 16 to 63·2 Mg ha−1 y−1. These results indicate that vegetation cover on gully walls, although suggesting an improvement with respect to wall protection against splash, sheet and rill erosion, did not affect sidewall retreat and mass movements. Copyright © 2009 John Wiley & Sons, Ltd.