Topographic Variations (topographic + variation)

Distribution by Scientific Domains


Selected Abstracts


Topographic variation of the choroidal watershed zone and its relationship to neovascularization in patients with age-related macular degeneration

ACTA OPHTHALMOLOGICA, Issue 3 2009
Efstratios Mendrinos
Abstract. Purpose:, To evaluate the patterns of choroidal watershed zones (WZs) in exudative age-related macular degeneration (AMD) and to describe their relationship with choroidal neovascularization (CNV). Methods:, We retrospectively evaluated 50 digital indocyanine green video-angiograms of 50 patients with exudative AMD demonstrating one or more WZs. In addition, the relationship between the site of CNV and the WZ was analysed. Results:, A stellate WZ was observed in 30 of 50 (60%) patients. Choroidal neovascularization occurred within the centre of the WZ in all cases. The WZ was vertically oriented in 18 of 50 (36%) patients. When the WZ coursed through or extended into the fovea, CNV occurred within the WZ, but it occurred at its margin when the WZ did not involve the fovea. An angled WZ coursing through the fovea with CNV occurring within it was observed in two of 50 (4%) patients. Conclusions:, In exudative AMD, the WZ most commonly conformed to the stellate pattern, followed by the vertical and angled patterns. Choroidal neovascularization occurred within the WZ in 44 of 50 (88%) patients. When the WZ did not involve the fovea (12%), CNV occurred at its margin. The relationship between the site of CNV and macular WZs suggests that macular WZs may be areas which are vulnerable to AMD and which are predisposed to CNV by the resulting hypoxia,ischaemia. [source]


Spatial prediction of river channel topography by kriging

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2008
Carl J. Legleiter
Abstract Topographic information is fundamental to geomorphic inquiry, and spatial prediction of bed elevation from irregular survey data is an important component of many reach-scale studies. Kriging is a geostatistical technique for obtaining these predictions along with measures of their reliability, and this paper outlines a specialized framework intended for application to river channels. Our modular approach includes an algorithm for transforming the coordinates of data and prediction locations to a channel-centered coordinate system, several different methods of representing the trend component of topographic variation and search strategies that incorporate geomorphic information to determine which survey data are used to make a prediction at a specific location. For example, a relationship between curvature and the lateral position of maximum depth can be used to include cross-sectional asymmetry in a two-dimensional trend surface model, and topographic breaklines can be used to restrict which data are retained in a local neighborhood around each prediction location. Using survey data from a restored gravel-bed river, we demonstrate how transformation to the channel-centered coordinate system facilitates interpretation of the variogram, a statistical model of reach-scale spatial structure used in kriging, and how the choice of a trend model affects the variogram of the residuals from that trend. Similarly, we show how decomposing kriging predictions into their trend and residual components can yield useful information on channel morphology. Cross-validation analyses involving different data configurations and kriging variants indicate that kriging is quite robust and that survey density is the primary control on the accuracy of bed elevation predictions. The root mean-square error of these predictions is directly proportional to the spacing between surveyed cross-sections, even in a reconfigured channel with a relatively simple morphology; sophisticated methods of spatial prediction are no substitute for field data. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Technical note: Morphometric maps of long bone shafts and dental roots for imaging topographic thickness variation

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 2 2010
Luca Bondioli
Abstract Qualitative and quantitative characterization through functional imaging of mineralized tissues is of potential value in the study of the odontoskeletal remains. This technique, widely developed in the medical field, allows the bi-dimensional, planar representation of some local morphometric properties, i.e., topographic thickness variation, of a three-dimensional object, such as a long bone shaft. Nonetheless, the use of morphometric maps is still limited in (paleo)anthropology, and their feasibility has not been adequately tested on fossil specimens. Using high-resolution microtomographic images, here we apply bi-dimensional virtual "unrolling" and synthetic thickness mapping techniques to compare cortical bone topographic variation across the shaft in a modern and a fossil human adult femur (the Magdalenian from Chancelade). We also test, for the first time, the possibility to virtually unroll and assess for dentine thickness variation in modern and fossil (the Neanderthal child from Roc de Marsal) human deciduous tooth roots. The analyses demonstrate the feasibility of using two-dimensional morphometric maps for the synthetic functional imaging and comparative biomechanical interpretation of cortical bone thickness variation in extant and fossil specimens and show the interest of using this technique also for the subtle characterization of root architecture and dentine topography. More specifically, our preliminary results support the use of virtual cartography as a tool for assessing to what extent internal root morphology is capable of responding to loading and directional stresses and strains in a predictable way. Am J Phys Anthropol, 2010. © 2010 Wiley-Liss, Inc. [source]


The spatiotemporal dynamics of a primary succession

JOURNAL OF ECOLOGY, Issue 2 2008
N. A. Cutler
Summary 1Conceptual models of ecosystem development commonly predict a phase of initial colonization characterized by the nucleation, growth and coalescence of discrete patches of pioneer plants. Spatiotemporal dynamics during subsequent development may follow one of three different models: the classical model, in which initially discrete patches of competitive dominant (secondary) colonists coalesce to form a homogeneous cover; the patch dynamics model, in which renewal mechanisms such as disturbance create a shifting mosaic of patches at different stages; and the geoecological model, in which the vegetation gradually differentiates along edaphic gradients related to the underlying physical template. 2These models of spatiotemporal dynamics were tested using vegetation and soil data from an 850-year chronosequence, comprised of seven lava flows on Mt Hekla, Iceland. The scale and intensity of spatial pattern were quantified on each flow using spatial analyses (mean-variance ratios, quadrat variance techniques and indices of autocorrelation). Changes in spatial pattern with increasing terrain age were compared with predicted trajectories, in order to identify which of the models of spatiotemporal dynamics was most consistent with the observations. 3The early stages of ecosystem development were characterized by colonization of the pioneer species, especially Racomitrium mosses, in discrete patches (,Pioneer colonization stage', < 20 years), which then grew laterally and coalesced to form a continuous, homogeneous carpet (,Pioneer expansion stage', 20,100 years). Later in the sequence, higher plants established in discrete patches within this pioneer matrix (,Higher plant colonization stage', 100,600 years). Over time, heterogeneity re-emerged at a larger spatial scale as the vegetation differentiated according to topographic variations in the underlying substrate (,Differentiation stage', > 600 years). 4Synthesis. The spatiotemporal dynamics observed in the early stages of this succession were consistent with a model of pioneer nucleation in micro-scale safe sites, followed by growth, coalescence and eventual fragmentation of pioneer patches. The spatial patterns which emerged later in development support the geoecological model, with spatial differentiation of vegetation related to meso-scale substrate topography. The findings provide insight on how vegetation patterns emerge at different stages of ecosystem development in response to differing scales of heterogeneity in the underlying physical environment. [source]


Using farmers' knowledge for defining criteria for land qualities in biophysical land evaluation

LAND DEGRADATION AND DEVELOPMENT, Issue 6 2001
I. Messing
Abstract The objective of this paper is to present a way of complementing empirical results with farmers' perceptions in defining limiting biophysical land properties in a land suitability evaluation using the FAO framework methodology. The farmers' perceptions were identified using rapid and participatory rural appraisal (RRA/PRA) tools. The study catchment, having a semiarid continental climate and located on the Loess Plateau in northern China, covered an area of 3.5 km2. Most of the land users were dependent on subsistence agriculture. There were important topographic variations in the catchment and arable cropping on steep slopes brought about degradation of land due to water erosion. The biophysical monitoring, soil survey and RRA/PRA survey, carried out one year prior to the present investigation, supplied the data needed for identification of preliminary limiting land properties and land evaluation units. The land properties that needed further investigation in the present study were slope aspect, soil workability, flooding hazard and farmers' criteria on choice of land-use type. The farmers were able to give a comprehensive picture of the spatial and temporal variation and the importance for land-use options of the land properties concerned, and thereby complement the information gained from empirical results (measurements). In order to guarantee good production for dry as well as wet years, both south- and north-facing sites were chosen for most crops, and the slope aspect did not need to be differentiated in the final land suitability evaluation for arable crops. Grassland, however, was considered to be more suitable than woodland on south-facing sites. Hard soil layers were found to be important, since they affected soil workability and erosion negatively, giving slightly reduced suitability for the land units in which they occurred. Flooding events affecting crops on alluvial soils negatively were considered to occur once every 5 to 10 years, which is considered to be a low rate, so this property was therefore not included in the final suitability evaluation. Copyright © 2001 John Wiley & Sons, Ltd. [source]


Analytical models for the mean flow inside dense canopies on gentle hilly terrain

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 634 2008
D. Poggi
Abstract Simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments. Laser Doppler anemometry (LDA) measurements are conducted in a neutrally-stratified boundary-layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy. The hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy (H/L , 1 and Lc/L , 1, where H is the hill height, L the half-length, and Lc the canopy adjustment length-scale). The LDA data suggest that the often-criticized linearizations of the advective terms, turbulent-shear-stress gradients and drag force appear reasonable except in the deep layers of the canopy. As predicted by a previous analytical model, the LDA data reveal a recirculation region within the lower canopy on the lee slope. Adjusting the outer-layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model's performance. For the velocity perturbations in the deeper layers of the canopy, a new analytical model, which retains a balance between mean horizontal advection, mean pressure gradient and mean drag force but neglects the turbulent-shear-stress gradient, is developed. The proposed model reproduces the LDA measurements better than the earlier analytical model, which neglected advection but retained the turbulent-shear-stress gradient in the lower layers of the canopy and near the hill top. This finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small. In essence, the newly-proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean-momentum equation. We briefly discuss how to integrate all these findings with recent advances in canopy lidar remote-sensing measurements of general topography and canopy height. Copyright © 2008 Royal Meteorological Society [source]