Surface Elevation Maps Research Articles

Comparative Analysis of Tomographic Indicators Forecasting Decompensation in Fuchs Endothelial Corneal Dystrophy.

To compare the performance of 3 commercially available tomographers (the Pentacam Scheimpflug camera, the swept-source optical coherence tomography Casia, and the blue light slit-scanning tomographer Precisio) in the identification of patterns associated with Fuchs endothelial corneal dystrophy (FECD) decompensation. This was a clinic-based cross-sectional imaging study. Pachymetry maps and posterior surface elevation maps were acquired with the 3 devices from 61 eyes affected by FECD. The maps were graded according to the evidence of tomographic patterns predictive of FECD decompensation (loss of parallel isopachs, displacement of the thinnest point, and focal posterior depression) by 2 blind cornea specialists. The loss of parallel isopachs was significantly less frequently evident in Pentacam pachymetry maps [8%, 95% confidence interval (CI) (3%, 18%)] compared with both the Casia [31%, 95% CI (20%, 44%), P = 0.01] and Precisio devices [24%, 95% CI (15%, 37%), P = 0.05]. The displacement of the thinnest point was graded as most evident in a significantly higher proportion of Precisio pachymetry maps [43%, 95% CI (31%, 55%)] compared with both the Pentacam [13%, 95% CI (6%, 24%), P = 0.001] and Casia devices [21%, 95% CI (12%, 33%), P = 0.03]. There were no significant differences in the identification of focal posterior depression on posterior elevation maps across the 3 devices. Identification of patterns predictive of FECD prognosis on pachymetry and posterior elevation maps is possible with different devices. However, their evidence varies across tomographers, and the results from different devices are not interchangeable.

Sea surface reconstruction from marine radar images using deep convolutional neural networks

The sea surface reconstructed from radar images provides valuable information for marine operations and maritime transport. The standard reconstruction method relies on the three-dimensional fast Fourier transform (3D-FFT), which introduces empirical parameters and modulation transfer function (MTF) to correct the modulation effects that may cause errors. In light of the convolutional neural networks’ (CNN) success in computer vision tasks, this paper proposes a novel sea surface reconstruction method from marine radar images based on an end-to-end CNN model with the U-Net architecture. Synthetic radar images and sea surface elevation maps were used for training and testing. Compared to the standard reconstruction method, the CNN-based model achieved higher accuracy on the same data set, with an improved correlation coefficient between reconstructed and actual wave fields of up to 0.96-0.97, and a decreased non-dimensional root mean square error (NDRMSE) of around 0.06. The influence of training data on the deep learning model was also studied. Additionally, the impact of the significant wave height and peak period on the CNN model’s accuracy was investigated. It has been demonstrated that the accuracy will fluctuate as the wave steepness increases, but the correlation coefficient remains above 0.90, and the NDRMSE remains less than 0.11.

Topographic Data Analysis of Observed Wind Data Recorded at 15 Different Stations in Thailand

This study focuses on the collection and observation of mean wind speed and power density of 15 stations in Thailand merged with the topographic map of the stations. The wind data was collected by installing anemometers at 10m, 15m, 20m,25m and 30m height at 15 selected stations around Thailand. Wind Analysis and Application Program (WAsP) is used to generate mean wind speed and power density. Meanwhile, roughness and surface elevation map are produced and merged with the data in WAsP. The results showed the highest wind speed in Songkhla station which was 3.16 to 12.15m/s and on the other hand data from Narathiwat showed the lowest mean wind ranging from 1.13 to 1.72m/s. Finally, Songkhla station power density ranges from 24-1372W/m2 and in Narathiwat station ranging from 2-5W/m2 in terms of power density. In Thailand, the landscape is diverse such as plateau, plains, coastal plains, land, mountains, mountain ranges and hills. Generally, the wind speeds and directions change due to landscape. For this season, to study wind resource, investigation on topography is vital.

A Low-Cost Stereo Video System for Measuring Directional Wind Waves

Typical oceanographic instruments are expensive, complex to build, and hard to deploy and require constant and specialized maintenance. In this paper, we present a cheap and simple technique to estimate a three-dimensional surface elevation map, η(x,y,t), the directional spectrum, and the main sea state parameters using inexpensive smartphones. The proposed methodology uses Time Lagged Cross Correlation (TLCC) between the audio signals from two independent video records to perform the frame synchronization. This makes the system much easier to deploy, where the main requirement is a fixed or moving platform close to the sea. The time records are mostly limited by the equipment storage space and battery life, although it can be easily replaced or recharged. Here, we pose the basis for an inexpensive yet powerful stereo reconstruction device and discuss its capabilities and limitations. The smartphone system capabilities were illustrated here by near shore experiment, at Leme beach in the Southeast of Brazil, and the results were compared against a pressure sensor. For this particular setup, the root mean square error in terms of significant wave height is of the order of 11% with perfect estimation of the peak period. The results are promising and demonstrate the validity and applicability of the technique.

Airborne observations of arctic-boreal water surface elevations from AirSWOT Ka-Band InSAR and LVIS LiDAR

AirSWOT is an experimental airborne Ka-band radar interferometer developed by NASA-JPL as a validation instrument for the forthcoming NASA Surface Water and Ocean Topography (SWOT) satellite mission. In 2017, AirSWOT was deployed as part of the NASA Arctic Boreal Vulnerability Experiment (ABoVE) to map surface water elevations across Alaska and western Canada. The result is the most extensive known collection of near-nadir airborne Ka-band interferometric synthetic aperture radar (InSAR) data and derivative high-resolution (3.6 m pixel) digital elevation models to produce water surface elevation (WSE) maps. This research provides a synoptic assessment of the 2017 AirSWOT ABoVE dataset to quantify regional WSE errors relative to coincident in situ field surveys and LiDAR data acquired from the NASA Land, Vegetation, and Ice Sensor (LVIS) airborne platform. Results show that AirSWOT WSE data can penetrate cloud cover and have nearly twice the swath-width of LVIS as flown for ABoVE (3.2 km vs. 1.8 km nominal swath-width). Despite noise and biases, spatially averaged AirSWOT WSEs can be used to estimate sub-seasonal hydrologic variability, as confirmed with field GPS surveys and in situ pressure transducers. This analysis informs AirSWOT ABoVE data users of known sources of measurement error in the WSEs as influenced by radar parameters including incidence angle, magnitude, coherence, and elevation uncertainty. The analysis also provides recommended best practices for extracting information from the dataset by using filters for these four parameters. Improvements to data handing would significantly increase the accuracy and spatial coverage of future AirSWOT WSE data collections, aiding scientific surface water studies, and improving the platform’s capability as an airborne validation instrument for SWOT.

Probabilistic methods for the estimation of seismic Fa and Fv maps—application to Montreal

In most national building codes, site classification is based on the Vs30 parameter, the average shear wave velocity for the first 30 m of soil below the surface, and defines amplification factors (e.g. Fa and Fv) to be applied to ground motions for a reference condition. Seismic microzonation mapping is usually achieved by combining information from various sources, each with varying degrees of uncertainties. A preliminary microzonation can be derived from surface geology or surface elevation maps, while a more detailed and accurate map is usually based on extensive seismic surveys. A procedure is proposed that progressively allows the integration of information from various sources and to estimate the degree of uncertainty on the microzonation. This allows planners to determine where microzonation maps require further investigations given current or future urban development plans. The proposed procedure uses conditional second moment estimation and provides the best linear unbiased estimates of Vs30 and its uncertainty. Next, these estimates are used to derive soil classification probability maps and to compute the expected values and variance of soil amplification factors Fa ad Fv to be used in probabilistic seismic risk analyses. The proposed procedure is demonstrated for the seismic microzonation of the island of Montreal.

Rainfall-triggered shallow landslides at catchment scale: Threshold mechanics-based modeling for abruptness and localization

[1] Rainfall-induced shallow landslides may occur abruptly without distinct precursors and could span a wide range of soil mass released during a triggering event. We present a rainfall-induced landslide-triggering model for steep catchments with surfaces represented as an assembly of hydrologically and mechanically interconnected soil columns. The abruptness of failure was captured by defining local strength thresholds for mechanical bonds linking soil and bedrock and adjacent columns, whereby a failure of a single bond may initiate a chain reaction of subsequent failures, culminating in local mass release (a landslide). The catchment-scale hydromechanical landslide-triggering model (CHLT) was applied to results from two event-based landslide inventories triggered by two rainfall events in 2002 and 2005 in two nearby catchments located in the Prealps in Switzerland. Rainfall radar data, surface elevation and vegetation maps, and a soil production model for soil depth distribution were used for hydromechanical modeling of failure patterns for the two rainfall events at spatial and temporal resolutions of 2.5 m and 0.02 h, respectively. The CHLT model enabled systematic evaluation of the effects of soil type, mechanical reinforcement (soil cohesion and lateral root strength), and initial soil water content on landslide characteristics. We compared various landslide metrics and spatial distribution of simulated landslides in subcatchments with observed inventory data. Model parameters were optimized for the short but intense rainfall event in 2002, and the calibrated model was then applied for the 2005 rainfall, yielding reasonable predictions of landslide events and volumes and statistically reproducing localized landslide patterns similar to inventory data. The model provides a means for identifying local hot spots and offers insights into the dynamics of locally resolved landslide hazards in mountainous regions.

Glacier flow measurement and radio-echo sounding at Aurora Peak, Alaska, in 2008

AbstarctAs part of a deep ice-core drilling project, GPS and ice-radar measurements were made in the vicinity of Aurora Peak, Alaska, USA. Surface and bed elevation maps were obtained over an area of 400 m×700 m, which includes the drilling site. Surface flow velocity was measured at 18 locations in the same area. The results showed that the drilling site was situated on a gentle north–south ridge with saddle-shaped surface geometry. The horizontal ice-flow velocity at the drilling site was <0.5ma–1 and the ice generally flowed eastward and westward along the surface slope. The glacier bed slopes to the east and has more complex topography than the ice surface. The drilling site was situated above a bedrock dip, with an ice thickness of 252±10 m. These data constitute important information for estimating the effect of ice flow to the ice core retrieved from the depth of up to 180.17 m.

Strategic Planning in Topography-guided Ablation of Irregular Astigmatism After Laser Refractive Surgery

To identify an optimal customized ablation strategy in the treatment of eyes with secondary irregular astigmatism. Corneal anterior surface elevation maps of 50 eyes with secondary irregular astigmatism after decentered laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) and 50 virgin eyes were used for customized ablation simulations. Two ablation simulations with targeted postoperative surfaces perpendicular to either the visual or corneal morphological axis were made for each eye. All ablations were programmed for correction of corneal irregularities, including corneal astigmatism. The manifest refractive error was not corrected. Optical diameter was 6.5 mm and total diameter was 7.5 mm. Maximum ablation depths and maximum transition zone gradients were registered and analyzed. In eyes with secondary irregular astigmatism, mean maximum ablation depth was 48.21 +/- 25.96 microm and 26.31+/- 14.08 microm, whereas mean maximum transition zone gradient was 29.07 +/- 25.15 microm and 9.88 +/- 6.41 microm in ablation simulations based on the visual and corneal morphological axes, respectively. The difference between the ablation strategies was highly statistically significant for both parameters (P < .001). In virgin eyes, only a minor difference was noted between the visual and corneal morphological axis ablation simulations (P = .15 for maximum ablation depths and P=.19 for maximum transition zone gradient). In secondary irregular astigmatism, ablation based on the corneal morphological axis appears to minimize corneal tissue consumption and allows a smoother transition zone.

Hurricane Directional Wave Spectrum Spatial Variation in the Open Ocean

The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane in open water using the NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane hunter aircraft at 1.5 km height. The SRA measures the energetic portion of the directional wave spectrum by generating a topographic map of the sea surface. At 8 Hz, the SRA sweeps a radar beam of 1 deg half-power width (two-way) across the aircraft ground track over a swath equal to 0. 8 of the aircraft height, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 positions. These slant ranges are multiplied by the cosine of the incidence angles to determine the vertical distances from the aircraft to the sea surface. Subtracting these distances from the aircraft height produces the sea surface elevation map. The sea surface topography is interpolated to a uniform grid, transformed by a two-dimensional FFT, and Doppler corrected. The data presented were acquired on 24 August 1998 when hurricane Bonnie was east of the Bahamas and moving slowly to the north. Wave heights up to 18 m were observed and the spatial variation of the wave field was dramatic. The dominant waves generally propagated at significant angles to the downwind direction and at times there were wave fields traveling at right angles to each other. The NOAA aircraft spent over five hours within 180 km of the hurricane Bonnie eye, and made five eye penetrations. A 2-minute animation of the directional wave spectrum spatial variation over this period will be shown.

Development and testing of a contamination potential mapping system for a portion of the General Separations Area, Savannah River Site, South Carolina

A methodology was developed to evaluate and map the contamination potential or aquifer sensitivity of the upper groundwater flow system of a portion of the General Separations Area (GSA) at the Department of Energy's Savannah River Site (SRS) in South Carolina. A Geographic Information System (GIS) was used to integrate diverse subsurface geologic data, soils data, and hydrology utilizing a stack-unit mapping approach to construct mapping layers. This is the first time that such an approach has been used to delineate the hydrogeology of a coastal plain environment. Unit surface elevation maps were constructed for the tops of six Tertiary units derived from over 200 boring logs. Thickness or isopach maps were created for five hydrogeologic units by differencing top and basal surface elevations. The geologic stack-unit map was created by stacking the five isopach maps and adding codes for each stack-unit polygon. Stacked-units were rated according to their hydrogeologic properties and ranked using a logarithmic approach (utility theory) to establish a contamination potential index. Colors were assigned to help display relative importance of stacked-units in preventing or promoting transport of contaminants. The sensitivity assessment included the effects of surface soils on contaminants which are particularly important for evaluating potential effects from surface spills. Hydrogeologic/hydrologic factors did not exhibit sufficient spatial variation to warrant incorporation into contamination potential assessment. Development of this contamination potential mapping system provides a useful tool for site planners, environmental scientists, and regulatory agencies.

A combined surface-and volume-scattering model for ice-sheet radar altimetry

AbstractOver the last 15 years, satellite-altimeter data have been used to produce surface-elevation maps of the Greenland and Antarctic ice sheets with a 2 m accuracy. Analysis of Seasat and Geosat cross-over points showed that satellite altimeters can measure changes in the mass balance of the ice sheets. The retracking algorithm used to extract surface elevations from Seasat and Geosat return wave forms is based upon a modified form of the Brown surface-scattering model. Recent work has shown that altimeter wave forms over higher-altitude regions of the ice sheets are affected by sub-surface volume-scattering. Here, we develop a theoretical model for altimeter return wave forms over the ice sheets that is based on a combination of surface-and volume-scattering. By approximating the altimeter’s antenna pattern and transmitted pulse shape with Gaussian functions, we derive a closed-form analytical solution for the return-power volume-scattered from beneath the ice-sheet surface. We then combine the volume-scattering model with the Brown model and apply it to average wave forms from the Greenland and Antarctic ice sheets. The results show that the combined model accurately describes variations in altimeter wave-form shapes that are produced by differing contributions of surface-and volume-scattering to the received power. The combined model is then used to simulate return wave forms from a dual-frequency altimeter. The simulation shows that a two-frequency system can provide quantitative estimates of the absorption and scattering coefficients for near-surface snow.

A combined surface-and volume-scattering model for ice-sheet radar altimetry

AbstractOver the last 15 years, satellite-altimeter data have been used to produce surface-elevation maps of the Greenland and Antarctic ice sheets with a 2 m accuracy. Analysis of Seasat and Geosat cross-over points showed that satellite altimeters can measure changes in the mass balance of the ice sheets. The retracking algorithm used to extract surface elevations from Seasat and Geosat return wave forms is based upon a modified form of the Brown surface-scattering model. Recent work has shown that altimeter wave forms over higher-altitude regions of the ice sheets are affected by sub-surface volume-scattering. Here, we develop a theoretical model for altimeter return wave forms over the ice sheets that is based on a combination of surface-and volume-scattering. By approximating the altimeter’s antenna pattern and transmitted pulse shape with Gaussian functions, we derive a closed-form analytical solution for the return-power volume-scattered from beneath the ice-sheet surface. We then combine the volume-scattering model with the Brown model and apply it to average wave forms from the Greenland and Antarctic ice sheets. The results show that the combined model accurately describes variations in altimeter wave-form shapes that are produced by differing contributions of surface-and volume-scattering to the received power. The combined model is then used to simulate return wave forms from a dual-frequency altimeter. The simulation shows that a two-frequency system can provide quantitative estimates of the absorption and scattering coefficients for near-surface snow.

Satellite radar altimetry

A brief review of the historical development and principles of satellite radar altimetry is presented, with special emphasis on the unique capability of the microwave altimeter to provide valuable information for global geoscientific studies. Altimeter data over the ocean are used to monitor mean sea levels, wave height, wind speeds, and surface topographical features. Over the ice sheets, the altimeter data are used to produce surface elevation maps, while repeated measurements are used to monitor volume changes. The success of earlier altimeter mission has promoted the development of future missions that will provide more accurate data sets.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ice-Sheet Topography From Geosat Radar Altimetry (Abstract)

Ice-sheet surface topography is the principal ice parameter obtainable from satellite radar altimetry. Surface-elevation maps of the East Antarctic ice sheet north of 72°S from Seasat data, collected between July and October 1978, and preliminary maps from Geosat data, collected between March 1985 and September 1986, are described. The Geosat data, obtained from the U.S. Navy as an unclassified data set, have greatly increased the density of elevation measurements. A principal correction to the altimeter measurements is obtained by applying a computer curve-fitting procedure to each radar waveform to correct for errors in the automatic range-tracking circuitry of the altimeter. The errors are caused by slow response to range variations due to undulations of the ice surface between successive measurements made at intervals of 662 m along track. The retracking correction for Geosat data has a standard deviation of 2.4 m and a mean value of 1.1m, values which are about 20% smaller than the corresponding values for Seasat. The positive mean correction indicates a common tendency of the altimeters' automatic tracking to give an excessive range to the surface. The precision of the measurements, given by the standard deviation of the range differences at cross-over points, is about 1.6 m before adjustment for errors in the radial position of the satellite orbit. The preliminary surface-elevation maps from Geosat data are improved over those produced from Seasat, mainly due to optimal spacing of successive ground tracks. The locations of ice divides and drainage basins along the East Antarctic coast are delineated by several methods, including vector plots of surface slope.

Ice-Sheet Topography From Geosat Radar Altimetry (Abstract)

Ice-sheet surface topography is the principal ice parameter obtainable from satellite radar altimetry. Surface-elevation maps of the East Antarctic ice sheet north of 72°S from Seasat data, collected between July and October 1978, and preliminary maps from Geosat data, collected between March 1985 and September 1986, are described. The Geosat data, obtained from the U.S. Navy as an unclassified data set, have greatly increased the density of elevation measurements. A principal correction to the altimeter measurements is obtained by applying a computer curve-fitting procedure to each radar waveform to correct for errors in the automatic range-tracking circuitry of the altimeter. The errors are caused by slow response to range variations due to undulations of the ice surface between successive measurements made at intervals of 662 m along track. The retracking correction for Geosat data has a standard deviation of 2.4 m and a mean value of 1.1m, values which are about 20% smaller than the corresponding values for Seasat. The positive mean correction indicates a common tendency of the altimeters' automatic tracking to give an excessive range to the surface. The precision of the measurements, given by the standard deviation of the range differences at cross-over points, is about 1.6 m before adjustment for errors in the radial position of the satellite orbit. The preliminary surface-elevation maps from Geosat data are improved over those produced from Seasat, mainly due to optimal spacing of successive ground tracks. The locations of ice divides and drainage basins along the East Antarctic coast are delineated by several methods, including vector plots of surface slope.

The morphology of ice streams A, B, and C, west Antarctica, and their environs

Airborne radar soundings of the ice sheet surface made in 1984–1985, together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams A, B, and C and much of the region around them. The flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking. Ice thicknesses were also measured along the flight lines but have not yet been mapped. The surfaces of active ice streams A and B exhibit a longitudinal ridgetrough topography of uncertain origin. Prominent surface valleys are associated with most, but not all, of their marginal shear zones. There is a deep subglacial trough beneath the grid northeastern side of ice stream A that connects to the subglacial trough beneath Reedy Glacier. The boundary between ice streams Bl and B2, the two tributaries of ice stream B, persists as a suture zone for 250 km downstream from their merger. Ice stream Bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream B until it disappears close to the grounding line. Between ice streams Bl and B2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice, which suggest to us that “rafts” of ice are being incorporated into the ice streams. The previously reported negative mass balance for the drainage system of ice stream B may arise from the expansion of the ice stream into the interior of the West Antarctic ice sheet. Crevasse bands at some places on ridge AB, together with high glacial velocities reported elsewhere, also suggest this expansion. Close to the Ross Ice Shelf, ice stream B widens into a nearly flat area where the mean surface slope is only 3.5 × 10−4, and the surface elevations are only a few tens of meters above hydrostatic equilibrium. Inactive ice stream C differs from active ice streams A and B in surficial and basal characteristics. No elongated ridges and troughs are observed; instead, the ice stream surface exhibits several terraces, including some maxima and minima in elevation. Radar sounding reveals areas where basal echoes are strong and steady, indicating subglacial water, alternating with areas of weaker echoes with short fading lengths. Calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick, depending on the electrical conductivity of the water. Due to reversals in the surface slope, the hydrologic potential gradient changes sign several times along ice stream C, which implies that subglacial water should collect in some locations.

West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance

Extensive airborne radar soundings, closely tied to accurately located ground stations made over a portion of the West Antarctic inland ice sheet and the Ross Ice Shelf, have enabled us to map the boundaries of the ice streams and their flow bands on the Ross Ice Shelf. The surfaces of the active ice streams, A and B, are heavily crevassed, but there are no visible crevasses on ice stream C. However, the existence of numerous crevasses at a depth of about 35 m implies that ice stream C ceased to be active about 250 years ago. There is a complex zone at the head of ice stream B that suggests that ice stream B is currently widening and advancing toward the interior of the ice sheet. The grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance, and the character of the radar echoes. The inland ice is more extensive than previously mapped, and includes a large, weakly grounded area downstream of ice streams A and B, where the surface elevations are barely above hydrostatic equilibrium. Crary Ice Rise extends 70 km upstream of the previously mapped position. To relate surface elevation data from satellite observations to sea level, three geoidal models were tested. We conclude that the GEM 10C model is the best of the three for this section of Antarctica. A new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams A and B, but not on ice stream C, whose surface contours show signs of stagnation. Ice stream C shows a strongly positive and ice stream B a significantly negative net balance rate. Ice streams A and D also show a significantly negative net flux, whereas ice streams E and F do not. The overall net balance of this part of the West Antarctic inland ice is suggestively negative (−23 ± 15 km3 yr−1). From the measured flux into the Ross Ice Shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0.12 ± 0.03 m yr−1.

Antarctic topography from balloons

ANTARCTIC ice surface elevation maps are far from being complete, due to the difficulties in applying regular surveying and mapping techniques in Antarctica. A new wealth of ice sheet surface topography data is now available, from many balloons, floating at an altitude of about 12.5 km, which traversed the Antarctic during its 1975 summer. These balloons were part of 411 constant density balloons launched in the Southern Hemisphere during the Tropical Wind, Energy conversion and Reference Level Experiment (TWERLE)1. Each balloon carries three sensors: radio altimeter2, pressure sensor3 and ambient temperature sensor. The balloon transmits data once per minute, as long as its solar panel is sufficiently illuminated4. The data are received by the Random Access Measuring System (RAMS)5 on board the NIMBUS–6 satellite, whenever the balloon is within line of sight.