2,682 research outputs found
Terrain analysis using radar shape-from-shading
This paper develops a maximum a posteriori (MAP) probability estimation framework for shape-from-shading (SFS) from synthetic aperture radar (SAR) images. The aim is to use this method to reconstruct surface topography from a single radar image of relatively complex terrain. Our MAP framework makes explicit how the recovery of local surface orientation depends on the whereabouts of terrain edge features and the available radar reflectance information. To apply the resulting process to real world radar data, we require probabilistic models for the appearance of terrain features and the relationship between the orientation of surface normals and the radar reflectance. We show that the SAR data can be modeled using a Rayleigh-Bessel distribution and use this distribution to develop a maximum likelihood algorithm for detecting and labeling terrain edge features. Moreover, we show how robust statistics can be used to estimate the characteristic parameters of this distribution. We also develop an empirical model for the SAR reflectance function. Using the reflectance model, we perform Lambertian correction so that a conventional SFS algorithm can be applied to the radar data. The initial surface normal direction is constrained to point in the direction of the nearest ridge or ravine feature. Each surface normal must fall within a conical envelope whose axis is in the direction of the radar illuminant. The extent of the envelope depends on the corrected radar reflectance and the variance of the radar signal statistics. We explore various ways of smoothing the field of surface normals using robust statistics. Finally, we show how to reconstruct the terrain surface from the smoothed field of surface normal vectors. The proposed algorithm is applied to various SAR data sets containing relatively complex terrain structure
Simulations of snow distribution and hydrology in a mountain basin
We applied a version of the Regional HydroâEcologic Simulation System (RHESSys) that implements snow redistribution, elevation partitioning, and windâdriven sublimation to Loch Vale Watershed (LVWS), an alpineâsubalpine Rocky Mountain catchment where snow accumulation and ablation dominate the hydrologic cycle. We compared simulated discharge to measured discharge and the simulated snow distribution to photogrammetrically rectified aerial (remotely sensed) images. Snow redistribution was governed by a topographic similarity index. We subdivided each hillslope into elevation bands that had homogeneous climate extrapolated from observed climate. We created a distributed wind speed field that was used in conjunction with daily measured wind speeds to estimate sublimation. Modeling snow redistribution was critical to estimating the timing and magnitude of discharge. Incorporating elevation partitioning improved estimated timing of discharge but did not improve patterns of snow cover since wind was the dominant controller of areal snow patterns. Simulating windâdriven sublimation was necessary to predict moisture losses
Airborne laser scanning raster data visualization
This guide provides an insight into a range of visualization techniques for high-resolution digital elevation models (DEMs). It is provided in the context of investigation and interpretation of various types of historical and modern, cultural and natural small-scale relief features and landscape structures. It also provides concise guidance for selecting the best techniques when looking at a specific type of landscape and/or looking for particular kinds of forms.The three main sections â descriptions of visualization techniques, guidance for selection of the techniques, and visualization tools â accompany examples of visualizations, exemplar archaeological and geomorphological case studies, a glossary of terms, and a list of references and recommendations for further reading. The structure facilitates people of different academic background and level of expertise to understand different visualizations, how to read them, how to manipulate the settings in a calculation, and choose the best suited for the purpose of the intended investigation.A smaller amount of books is also available in hardcover (ISBN 978-961-05-0011-7, 24 EUR).Monografija nudi vpogled v nabor tehnik prikaza visokoloÄljivih modelov viĆĄin. Napisana je v kontekstu preuÄevanja in interpretacije razliÄnih tipov zgodovinskih in modernih, kulturnih in naravnih majhnih reliefnih oblik. Daje jedrnate napotke za izbiro najboljĆĄih tehnik prikaza doloÄenih tipov pokrajine in izrazitih oblik.Tri glavna poglavja â opis tehnik prikazovanja digitalnih modelov viĆĄin, napotki za njihovo izbiro in orodja za izraÄun prikazov â, spremljajo izbrani primeri tipiÄnih arheoloĆĄkih in geomorfoloĆĄkih ĆĄtudij, slovarÄek pojmov ter seznam literature in priporoÄenega branja. Posameznikom z razliÄnih znanstvenih podroÄij in z razliÄnim predznanjem o tematiki je struktura v pomoÄ pri razumevanju razliÄnih tehnik prikazov, kako jih brati, kako izbrati prave nastavitve pri njihovem izraÄunu in kako prepoznati najbolj primerne za namen zasnovane raziskave
Using multiple data sources to enhance photogrammetry for mapping antarctic terrain
Extensive aerial photography cover is available for parts of the British Antarctic Territory, but the fisoffinfillijl characteristics of the photography, combined with the sparsity of ground control information and rugged snow-covered terrain, make photogrammetric mapping techniques difficult to apply. This paper shows, by reference to a new 1:50,000 scale topographic map of part of the Antarctic Peninsula, how merging topographic data from various sources in a GIS environment can make photogrammetric mapping more effective. Information sources used in the map compilation include three types of aerial photography, geo-referenced satellite imagery, surveyed points in a control network and satellite image-derived control points. A shape-from-shading algorithm was used to generate contours for snowfields where absence of surface detail prevented photogrammetric contouring. A horizontal and vertical accuracy of better than ±5 m was achieved in orientation of photography covering almost all of the map area. Such errors have allowed the construction of an accurate large-scale map for an area where previous mapping had been restricted to medium and small scales
Towards extracting artistic sketches and maps from digital elevation models
The main trend of computer graphics is the creation of photorealistic images however, there is increasing interest in the simulation of artistic and illustrative techniques. This thesis investigates a profile based technique for automatically extracting artistic sketches from regular grid digital elevation models. The results resemble those drawn by skilled cartographers and artists.The use of cartographic line simplification algorithms, which are usually applied to complex two-dimensional lines such as coastlines, allow a set of most important points on the terrain surface to be identified, these form the basis for sketching.This thesis also contains a wide ranging review of terrain representation techniques and suggests a new taxonomy
Earthshine as an Illumination Source at the Moon
Earthshine is the dominant source of natural illumination on the surface of
the Moon during lunar night, and at locations within permanently shadowed
regions that never receive direct sunlight. As such, earthshine may enable the
exploration of areas of the Moon that are hidden from solar illumination. The
heat flux from earthshine may also influence the transport and cold trapping of
volatiles present in the very coldest areas. In this study, Earth's spectral
radiance at the Moon is examined using a suite of Earth spectral models created
using the Virtual Planetary Laboratory (VPL) three dimensional modeling
capability. At the Moon, the broadband, hemispherical irradiance from Earth
near 0 phase is approximately 0.15 watts per square meter, with comparable
contributions from solar reflectance and thermal emission. Over the simulation
timeframe, spanning two lunations, Earth's thermal irradiance changes less than
a few mW per square meter as a result of cloud variability and the
south-to-north motion of sub-observer position. In solar band, Earth's
diurnally averaged light curve at phase angles < 60 degrees is well fit using a
Henyey Greenstein integral phase function. At wavelengths > 0.7 microns, near
the well known vegetation "red edge", Earth's reflected solar radiance shows
significant diurnal modulation as a result of the longitudinal asymmetry in
projected landmass, as well as from the distribution of clouds. A simple
formulation with adjustable coefficients is presented for estimating Earth's
hemispherical irradiance at the Moon as a function of wavelength, phase angle
and sub-observer coordinates. It is demonstrated that earthshine is
sufficiently bright to serve as a natural illumination source for optical
measurements from the lunar surface.Comment: 27 pages, 15 figures, 1 tabl
Cascading water underneath Wilkes Land, East Antarctic ice sheet, observed using altimetry and digital elevation models
We describe a major subglacial lake drainage close to the ice divide in
Wilkes Land, East Antarctica, and the subsequent cascading of water
underneath the ice sheet toward the coast. To analyse the event, we combined
altimetry data from several sources and subglacial topography. We estimated
the total volume of water that drained from Lake Cook<sub>E2</sub> by differencing
digital elevation models (DEM) derived from ASTER and SPOT5 stereo imagery
acquired in January 2006 and February 2012. At 5.2 ± 1.5 km<sup>3</sup>, this
is the largest single subglacial drainage event reported so far in
Antarctica. Elevation differences between ICESat laser altimetry spanning
2003â2009 and the SPOT5 DEM indicate that the discharge started in November
2006 and lasted approximately 2 years. A 13 m uplift of the surface,
corresponding to a refilling of about 0.6 ± 0.3 km<sup>3</sup>, was observed
between the end of the discharge in October 2008 and February 2012. Using
the 35-day temporal resolution of Envisat radar altimetry, we monitored the
subsequent filling and drainage of connected subglacial lakes located
downstream of Cook<sub>E2</sub>. The total volume of water traveling within the
theoretical 500-km-long flow paths computed with the BEDMAP2 data set is
similar to the volume that drained from Lake Cook<sub>E2</sub>, and our
observations suggest that most of the water released from Lake Cook<sub>E2</sub>
did not reach the coast but remained trapped underneath the ice sheet. Our
study illustrates how combining multiple remote sensing techniques allows
monitoring of the timing and magnitude of subglacial water flow beneath the
East Antarctic ice sheet
Constraints on effusive cryovolcanic eruptions on Europa using topography obtained from Galileo images
Images of Europa's surface taken by the Galileo Solid State Imager (SSI) show
smooth features measuring a few kilometers, potentially resulting from
eruptions of low-viscosity material such as liquid cryomagma. We estimated the
volume of four of these smooth features by producing digital elevation models
(DEMs) of four Galileo/SSI images. We used the shape-from-shading technique
with special care to estimate the uncertainties on the produced DEMs and
estimated feature volumes to be between ( m and (
m. We discussed the implications for putative sub-surface liquid
reservoir dimensions in the case of eruptions induced from freezing reservoirs.
Our previous cryovolcanic eruption model was improved by considering a cycle of
cryomagma freezing and effusion and by estimating the vaporized cryolava
fraction once cryolava spreads onto Europa's surface. Our results show that the
cryomagma reservoirs would have to be relatively large to generate these smooth
features (1 to 100 km if the flow features result from a single eruption,
and 0.4 to 60 km for the full lifetime of a reservoir generating cyclic
eruptions). The two future missions JUICE (ESA) and Europa Clipper (NASA)
should reach Europa during the late 2020s. They shall give more information on
those putative cryovolcanic regions which appear as interesting targets that
could provide a better understanding of the material exchanges between the
surface, sub-surface and ocean of Europa
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