Large-scale Topographic Features Research Articles

Uncertainty in primary and secondary topographic attributes caused by digital elevation model spatial resolution

Digital elevation models (DEMs) are increasingly being used to explain different soil and site-related characteristics using a variety of modelled topographic properties and their frequencies. It is well known that the grid cell size of a raster DEM significantly affects derived primary and secondary topographic attirbutes, such as elevation, slope, topographic position index, and wind exposition index. This study investigates the impact of increasing DEM spatial resolution on spatial distortion (defined by mean absolute error (MAE)) in two contrasting sites and tests two different data-obtaining procedures (average plot vs. single point). Findings detected and quantified significant error(s) caused by coarsening DEM spatial resolution. In addition, the plot average measurement procedure was more representative for any bounded area than a value extracted from a single point measurement. These findings provide important guidance for creating and using topographic attributes across a range of applications. This study recommends using DEM resolution of <15 m to derive primary and secondary topographic attributes. However, the optimal resolution is scale dependent and related with landscape; henceforth, coarser resolution may be sufficient for representing large-scale topographic features.

Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?

Abstract Topography on a wet rocky exoplanet could raise land above its sea level. Although land elevation is the product of many complex processes, the large-scale topographic features on any geodynamically active planet are the expression of the convecting mantle beneath the surface. This so-called “dynamic topography” exists regardless of a planet’s tectonic regime or volcanism; its amplitude, with a few assumptions, can be estimated via numerical simulations of convection as a function of the mantle Rayleigh number. We develop new scaling relationships for dynamic topography on stagnant lid planets using 2D convection models with temperature-dependent viscosity. These scalings are applied to 1D thermal history models to explore how dynamic topography varies with exoplanetary observables over a wide parameter space. Dynamic topography amplitudes are converted to an ocean basin capacity, the minimum water volume required to flood the entire surface. Basin capacity increases less steeply with planet mass than does the amount of water itself, assuming a water inventory that is a constant planetary mass fraction. We find that dynamically supported topography alone could be sufficient to maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately 10−4 times their mass, in the most favorable thermal states. By considering only dynamic topography, which has ∼1 km amplitudes on Earth, these results represent a lower limit to the true ocean basin capacity. Our work indicates that deterministic geophysical modeling could inform the variability of land propensity on low-mass planets.

Multiband Photometry of a Patroclus–Menoetius Mutual Event: Constraints on Surface Heterogeneity

Abstract We present the first complete multiband observations of a binary asteroid mutual event. We obtained high-cadence, high-signal-to-noise photometry of the UT 2018 April 9 inferior shadowing event in the Jupiter Trojan binary system Patroclus–Menoetius in four Sloan bands—g′, r′, i′, and z′. We use an eclipse light-curve model to fit for a precise mideclipse time and estimate the minimum separation of the two eclipsing components during the event. Our best-fit mideclipse time of is 19 minutes later than the prediction of Grundy et al. The minimum separation between the center of Menoetius’s shadow and the center of Patroclus is 72.5 ± 0.7 km—slightly larger than the predicted 69.5 km. Using the derived light curves, we find no evidence for significant albedo variations or large-scale topographic features on the Earth-facing hemisphere and limb of Patroclus. We also apply the technique of eclipse mapping to place an upper bound of ∼0.15 mag on wide-scale surface color variability across Patroclus.

Shape and spin determination of Barbarian asteroids

Context. The so-called Barbarian asteroids share peculiar, but common polarimetric properties, probably related to both their shape and composition. They are named after (234) Barbara, the first on which such properties were identified. As has been suggested, large scale topographic features could play a role in the polarimetric response, if the shapes of Barbarians are particularly irregular and present a variety of scattering/incidence angles. This idea is supported by the shape of (234) Barbara, that appears to be deeply excavated by wide concave areas revealed by photometry and stellar occultations. Aims. With these motivations, we started an observation campaign to characterise the shape and rotation properties of Small Main- Belt Asteroid Spectroscopic Survey (SMASS) type L and Ld asteroids. As many of them show long rotation periods, we activated a worldwide network of observers to obtain a dense temporal coverage. Methods. We used light-curve inversion technique in order to determine the sidereal rotation periods of 15 asteroids and the con- vergence to a stable shape and pole coordinates for 8 of them. By using available data from occultations, we are able to scale some shapes to an absolute size. We also study the rotation periods of our sample looking for confirmation of the suspected abundance of asteroids with long rotation periods. Results. Our results show that the shape models of our sample do not seem to have peculiar properties with respect to asteroids with similar size, while an excess of slow rotators is most probably confirmed.

Constraints on cometary surface evolution derived from a statistical analysis of 67P’s topography

We present a statistical analysis of the distribution of large scale topographic features on comet 67P/Churyumov-Gerasimenko. We observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 +- 0.02. When this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. For instance, the northern hemisphere topography is dominated by structures on the 100~m scale while the southern hemisphere topography, illuminated at perihelion, is dominated by 10~m scale terrain features. Our study suggest that the current size of a cliff is controlled not only by material cohesion but by the dominant erosional process in each region. This observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. In effect, our model shows that a measure of the topography provides a quantitative assessment of a comet's erosional history, i.e. its evolutionary age.

Environmental characteristics drive variation in Amazonian understorey bird assemblages.

Tropical bird assemblages display patterns of high alpha and beta diversity and, as tropical birds exhibit strong habitat specificity, their spatial distributions are generally assumed to be driven primarily by environmental heterogeneity and interspecific interactions. However, spatial distributions of some Amazonian forest birds are also often restricted by large rivers and other large-scale topographic features, suggesting that dispersal limitation may also play a role in driving species’ turnover. In this study, we evaluated the effects of environmental characteristics, topographic and spatial variables on variation in local assemblage structure and diversity of birds in an old-growth forest in central Amazonia. Birds were mist-netted in 72 plots distributed systematically across a 10,000 ha reserve in each of three years. Alpha diversity remained stable through time, but species composition changed. Spatial variation in bird-assemblage structure was significantly related to environmental and topographic variables but not strongly related to spatial variables. At a broad scale, we found bird assemblages to be significantly distinct between two watersheds that are divided by a central ridgeline. We did not detect an effect of the ridgeline per se in driving these patterns, indicating that most birds are able to fly across it, and that differences in assemblage structure between watersheds may be due to unmeasured environmental variables or unique combinations of measured variables. Our study indicates that complex geography and landscape features can act together with environmental variables to drive changes in the diversity and composition of tropical bird assemblages at local scales, but highlights that we still know very little about what makes different parts of tropical forest suitable for different species.

The effects of seascape pattern on algal patch structure, sea urchin barrens, and ecological processes

Seascapes are the marine analog of terrestrial landscapes, and their importance for ecological processes in marine ecosystems is generally poorly known. This is especially true for giant kelp (Macrocystis pyrifera) forests for which the acquisition of high resolution acoustic terrain data is problematic due to canopy cover. The goal of this study was to determine the effects of geologically forced seascape structure on algal patch structure, and to address the potential for seascape pattern to affect higher order ecological processes within giant kelp forests. The difficulty of terrain data acquisition within giant kelp forests was overcome using a narrow single beam sonar system and the development of processing algorithms that resulted in high resolution (10m) models of bathymetry and algal patch structure. Algal patch structure was determined for the canopy kelps Macrocystis pyrifera and Pelagophycus porra, and pooled understory algal species. The analysis also enabled estimation of the spatial distribution of a large (>2ha) persistent sea urchin barrens. The results highlight the importance of bottom topography and bottom depth distributions on algal distributions, algal patch structure, and the dynamics of the sea urchin barren. Large scale topographic features (ca. 500m), depth, and their interaction are significantly related to canopy and understory distributions among the forests and known patterns of algal resiliency and persistence. Canopy is preferentially distributed toward large scale topographic highs, whereas understory is preferentially distributed towards topographic lows. Pelagophycus porra, however, does not exhibit a distributional preference among hard bottom topographic features. Large scale topography clearly controls algal patch structure. Greater algal patch density and complexity are associated with topographic highs and lows, whereas patches in planar areas exhibit large core areas with less edge habitat and less interspersion with different canopy guilds. Comparisons of the joint probability distributions of bottom and algal depth distributions among forests indicates that density dependent positive feedback processes potentially reinforce the predominance of their differential canopy guild composition. Seascape structure (1) controls and interacts with algal patch structure to define edge habitat structure and potentially foraging and migration pathways, and (2) defines the margins of the large persistent sea urchin barrens and likely the dynamics of sea urchin feeding and movement. The results reinforce the need for adequate sampling design and replication for probability based benthic assessments when high resolution habitat structure is not available, or the utilization of model based purposive sampling strategies when habitats are known.

Stochastic reconstruction of paleovalley bedrock morphology from sparse datasets

Stochastic groundwater models enable the characterization of geological uncertainty. Often the major source of uncertainty is not related to aquifer heterogeneity, but to the general shape of the aquifer. This is especially the case in paleovalley-type alluvial aquifers where the bedrock surface limits the extent of easily extractable groundwater. Determining the shape of a bedrock surface is not straightforward, because it is typically non-stationary and defined by few data points that are generally far apart. This paper presents a new workflow for the stochastic reconstruction of bedrock surfaces using limited datasets that are typically available for aquifer characterization. The method is based on a lateral propagation of basement cross-sections interpreted from geophysical surveys, and conditions the reconstructed surface to existing well-log data and digital elevation model. To alleviate the typical limitations of sparse data, we use an analog approach to incorporate prior geological knowledge. We test the methodology on a synthetic example and a dataset from an alluvial aquifer in Northern Chile. Results of these case studies show that the algorithm is capable of enforcing the general notion of structural continuity, with the aquifer shape being conceptualized as an elongated, continuous and connected valley-shaped body. Our method captures the large-scale topographic features of fluvial incision into bedrock and the uncertainty in the positioning of the surface. Small-scale spatial variability is incorporated using Sequential Gaussian Simulation informed by geological analogs. Being stochastic, the methodology allows characterization of the uncertainty associated with positioning of the bedrock surface, by generating an ensemble of models via a Monte-Carlo analysis. This makes it possible to quantify the uncertainty associated with estimating the aquifer volume. We also discuss how this methodology may be used to better quantify the influence of uncertainty associated with defining the aquifer geometry on water resource assessment and management.

The Statistical Predictability of Surface Winds over Western and Central Canada

Abstract Multivariate linear regression is used to downscale reanalysis-based midtropospheric predictors (wind components and speed, temperature, and geopotential height) to historical wind observations at 44 surface weather stations during the four calendar seasons. The model performance is assessed as a function of statistical feature of the wind, averaging time scale of the wind statistics, and wind regime (as defined by how variable the vector wind is relative to its mean amplitude). Despite large differences in predictability characteristics between sites, several systematic results are observed: consistent with recent studies, a strong anisotropy of predictability for vector quantities is often observed, although no obvious relation is found between large-scale topographic features and the anisotropy orientation or magnitude. The predictability of time-averaged quantities increases with decreasing averaging time scale. In general, the predictability of mean vector wind components is superior to that of mean wind speeds or subaveraging time scale vector wind variability. These results are interpreted through empirically and theoretically based analyses of the sensitivity of mean wind speed to changes in the vector wind statistics. On longer averaging time scales, the statistical features of the wind speed are found to be highly sensitive to subaveraging time-scale vector wind variability, which is poorly predicted. On shorter averaging time scales, the mean wind speed is found to be highly sensitive to the magnitude of the mean vector wind, a quantity whose predictability can be much lower than the individual mean vector wind components. These results demonstrate limitations to the statistical downscaling of wind speed and suggest that deterministic models that resolve the short-time-scale variability may be necessary for successful predictions.

The Mercury Laser Altimeter Instrument for the MESSENGER Mission

The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which launched on August 3, 2004. The altimeter will measure the round-trip time of flight of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position and pointing data, gives a high-precision measurement of surface topography referenced to Mercury’s center of mass. MLA will sample the planet’s surface to within a 1-m range error when the line-of-sight range to Mercury is less than 1,200 km under spacecraft nadir pointing or the slant range is less than 800 km. The altimeter measurements will be used to determine the planet’s forced physical librations by tracking the motion of large-scale topographic features as a function of time. MLA’s laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity at 1,064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing.

Dispersal history of a spider (Stegodyphus lineatus) across contiguous deserts: vicariance and range expansion

Israel marks a crossroads between three continents encompassing several phytogeographical and zoogeographical zones. In this complex area, the flow of species from different biogeographical regions creates opportunities to study how geographical division and colonization routes affect current distribution and structure of resident populations of organisms associated with desert and arid environments, habitats that may have persisted throughout Pleistocene glacial periods. The present paper analyses the population history of the spider Stegodyphus lineatus in the contiguous Negev and Judean deserts in Israel using allozyme and mtDNA variation. The distinct patterns of variation indicate that Judean and Negev populations are vicariant lineages. The residence time was longer in Judea, where populations were more polymorphic for mtDNA, showed isolation by distance and were less structured than in the Negev. The Negev population, possibly linked to other Mediterranean populations of S. lineatus, consisted of two subdivisions derived from a recent eastward expansion across the central Negev watershed. Despite differences in age and level of structure, all lineages show similar dispersal processes dominated by restricted gene flow. The distribution patterns of allozyme and mtDNA markers are unrelated to geographical patterns of precipitation and vegetation. Rather, they follow large-scale topographic features, namely the water divide between Mediterranean and Afro-Syrian rift drainages and between eastern and western Negev drainages. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 739–754.

Bed-flow instability of the longshore current

An initially uniform longshore current on a plane erodible beach is considered and a linear stability analysis of the bed-flow system is performed in order to investigate the growth of alongshore periodic topographic features such as transverse or oblique bars. γ, numerical model based on the shallow water equations and a simple sediment transport formula is used. For a wide range of parameters instability is found, leading to the growth of large-scale topographic features (lengthscale of the order of the current width) downflow progressing. The growth rates and the dominant unstable mode depend mainly on R = c d/β parameter, where c d is the bottom friction coefficient and β is the beach slope. For a small R, say less than 0.1, instability is very weak, probably negligible. For R between 0.1 and 0.7 instability increases with R, leading typically to a quite simple transverse bars pattern. A further increase in R produces a far more complicated behaviour where complex patterns with downcurrent oriented oblique bars, bumps and holes can be dominant. In this region growth rates may either decrease or increase with R depending on the beach slope and the maximum Froude number of the basic flow, F. Usually, the most complex behaviour is found for gently sloping beaches. The physical mechanism of the instability is found to lie on the disturbances of potential vorticity caused by topographically induced differences in bottom friction. In this sense it is similar to the alternate bars growth in a river rather than the dunes or antidunes occurrence for 1D channel flow. The predictions of the model compare well with the available experimental data. The alongshore wavelength, γ, typically of the order of one to four times the width of the current, is close to four times for the most common values of R. The typical growth time is proportional to γ 2 and for a wavelength of 100 m can be of the order of one day, depending on the sediment transport rate. The migrational speed is inversely proportional to γ, in accordance to earlier field data reported by Sonu (1969) Collective movement of sediment in littoral environment.

On steady quasi-geostrophic flow component formation over undulating bottom topography: PART II. a two-layer low-order model of the open ocean

Abstract An equilibrium quasi-linear regime consisting of forced topographic waves and a steady flow component correlated with bottom relief is analysed using the low-order spectral model of quasi-geostrophic ocean dynamics. Two functions - a correlation C between potential vorticity and the topography and a topographic stress force D - are employed to study the qualitative structure of the steady flow component. Both a negative and positive C appear in the model. This function is denned mostly by topographically induced energy fluxes among barotropic and/or among baroclinic modes when the horizontal scale of the bottom undulations is smaller than the internal Rossbi radius. Over the large-scale topographic features, the interface displacements, controlled by the energy transfer between barotropic and baroclinic components, dominate in C. The β-effect tries to develop negative homogeneous C. D is derected oppositively to the phase velocity of the incident wave for motions on an f-plane. The β-effect induc...

A functional approach to the study of Labrador/Newfoundland shelf macrofauna

A functional approach based on feeding type, microhabitat preference, motility pattern, and body size of benthic organisms is compared to a taxonomic (family) based method in the study of the trophodynamics of marine soft-bottom communities. Grab samples were collected from the continental shelf and slope along the east coast of Canada at depths ranging from 85 to 622 m ( X = 247 m). Canonical multivariate analysis was used to produce functional and taxonomic classifications of macrofaunal polychaete assemblages that were correlated with environmental variables. No distinct faunal associations were identified from either approach. Instead there appeared to be a continuum of faunal groups. Groups of samples along the continuum appeared to be loosely associated with large-scale topographic features of the Labrador shelf and slope. Nearly 50% of the variation in the biological data was explained by environmental variables such as water depth, station latitude, and sediment grain size. The functional approach provided a more direct way of interpreting the effects of environmental factors on the community structure. The way in which the biological and environmental data are collected influences the outcome of multivariate analysis and its interpretation.

Geologic analyses of Shuttle Imaging Radar (SIR-B) data of Kilauea Volcano, Hawaii

Analyses of imaging radar data of volcanic terranes on Earth and Venus have emphasized the need for a clearer understanding of how these data can be most effectively used to accomplish important volcanological goals, including the interpretation of eruptive styles and the characterization of the geologic history of volcanic centers. The second Shuttle Imaging Radar experiment (SIR-B) obtained two digital images over the summit caldera and the Southwest Rift Zone of Kilauea Volcano in 1984. Our geologic analyses of these images indicate that SIR-B data are particularly useful for delineating the distribution and surface textural variations of a9a lava flows, for mapping large-scale topographic features with radar-facing slopes, and for identifying an areally extensive pyroclastic deposit. Analyses of the SIR-B data of Kilauea, however, do not permit unambiguous identification of landforms such as pahoehoe lava flows, cinder cones, and fissures. Although separation of low-return units such as pahoehoe lava flows and adjacent pyroclastic ash is not greatly improved using standard image-enhancement techniques, the texture-analysis technique applied here did facilitate discrimination of such smooth-surfaced volcanic deposits. Although analyses of the SIR-B data permit a generally accurate interpretation of the eruptive history of Kilauea, the inability to distinguish low-return pahoehoe flows results in misinterpretation of several aspects of Kilauea volcanism, suggesting that caution should be exercised in the interpretation of SAR data of volcanic terranes.

Glacial erosion and ice sheet divides, northeastern Laurentide Ice Sheet, on the basis of the distribution of limestone erratics

Paleozoic limestone erratics are common in tills and glaciomarine sediments along the outermost coast of eastern Baffin Island, Northwest Territories. They are rare immediately east of the Paleozoic limestone-Precambrian shield contact on western Baffin Island. Therefore, we conclude that (1) the presence of abundant limestone erratics in east coast tills suggests that limestone bed rock of Paleozoic age occurs at the mouths of and within many of the major sounds and fiords of eastern Baffin Island; thus, these large-scale topographic features are primarily tectonic in origin; and (2) the rarity of limestone erratics along probable flow lines from a hypothetical ice center over Hudson Bay refutes the concept of a single-domed Laurentide Ice Sheet; an appealing alternative is a more complex ice mass with an ice divide lying above the coast of western Baffin Island.

Mesoscale Flow over Complex Terrain during the Eastern Tennessee Trajectory Experiment (ETTEX)

Abstract Time-averaged, mesoscale flow fields over complex terrain measured during the Eastern Tennessee Trajectory Experiment (ETTEX) are analyzed in terms of the horizontal means, standard deviations from the horizontal means, and the kinetic energies of the uniform and nonuniform flow components. The vertical profiles of horizontally averaged wind speed and direction shear over flat and complex terrain during different stability conditions are compared. It is shown that the horizontally averaged flow over the ETTEX region was similar to that over a rough but otherwise flat urban area, and that a surface layer of a few hundred meters thickness existed in which the influence of the large-scale topographic features was not felt. Horizontal variability of the wind field is measured by forming 1) the standard deviations of the wind speed σs and direction σ0 and 2) the ratios of horizontal eddy to mean kinetic energy, EKE/MKE. It is shown that while both methods represent the variability well, the analysis u...

The relative age of the escarpments in the Martian polar laminated terrain based on morphology

Three theories have been proposed to explain the morphology of large-scale topographic features seen in the Martian polar laminated terrain. These features, which are subcircular in planimetric form, appear as dark bands in wide-angle Mariner 9 frames of the polar cap. Two of the theories (by Sagan and Murray) suggest that the features are constructional, built up through deformation of thick CO 2 layers or built through localized deposition of ice and dust. Another theory (Cutts) suggests the features are channels and escarpments cut into a pre-existing, featureless, layered plateau. The constructional theories suggest that the features were formed sequentially while the erosive theory suggests formation was more or less simultaneous. An experiment was performed to test whether it was possible to discern the relative age of the features. It was assumed that if the features are constructional, continued exposure to erosional processes would change their planimetric form with time. Murray has postulated which features are oldest and which are youngest. It was found, however, that the supposed oldest features were statistically neither significantly more nor less irregular than the supposed youngest escarpments. This evidence has led to the conclusion that the features were probably carved nearly simultaneously into a previously deposited, featureless, stratified plateau rather than being constructed sequentially.

Interpretation of Global Gravity Anomalies

THE absence of any significant correlation between large scale topographic features of the Earth's surface and the undulations of the geoid indicates near perfection in the isostatic balance of continent-sized features, whereas local geological features do cause distinct gravity anomalies. The difference is explicable in terms of a lithosphere of order 100 km thick which can support gravitationally unbalanced features up to 100 km or so in lateral extent, but is underlain by a yielding asthenosphere, so that larger scale features are isostatically balanced. It is therefore natural to seek a deep-seated source for the broad-scale features of the geoid. Depths greater than about 900 km become implausible, however, because the implied stresses increase very sharply with the fineness of the detail of the gravitational field which density variations are required to match, that is, with increasing order of the geoid harmonics. We can, of course, suppose that mass anomalies are distributed at all depths in the mantle, but if it can be shown that the geoid is explicable in terms of an appropriate mass distribution at a single depth, such that the stresses are approximately equal for all harmonic orders in the spherical harmonic representation of the surface, then this is strong evidence that we have found the depth of the most significant anomalies. We have reported such an analysis1 in which the plausible range of depths allowed by the uncertainty in the data was 250 km to 900 km. The analysis has since been repeated using the improved geoid coefficients of Gaposchkin and Lambeck2, which give a preferred depth of 460 km for the mass anomalies.

When did the Seychelles leave India?

INTENSIVE study of the ocean floor in the past 10 years has enabled the concept of sea-floor spreading to be examined critically, particularly by means of analysis of the magnetic field arising from the intrusion of basic rocks on ridges in the presence of regular reversals of the Earth's magnetic field1–3. Recently the work of Wilson4, McKenzie and Parker5 and Morgan6 has put on a firmer basis the geometrical aspects of rifting and faulting of plates (perhaps 100 km thick) on a sphere, and Le Pichon7 has attempted to reconstruct the Earth's surface at periods in the geological past from magnetic anomaly studies and large scale topographic features.