Propagating effects of database generalization on the viewshed   总被引：2，自引：0，他引：2  

PETER FISHER 《Transactions in GIS》1996,1(2):69-81

Few studies have systematically examined the effects of different possible generalization methods on the products of GIS operations. In this paper the effects of generalizing a Digital Elevation Model (DEM) on the area which is determined to be visible (the viewshed) is examined. Among the many different methods of generalization which are possible, a limited set of operators is examined here. First, they all fit geometrically from one resolution to another, and they can be grouped into two types: regular sampling and statistical summaries. In the latter class four different operations are used: for any cell at the target resolution, elevations are determined, from the arithmetic mean, the maximum, the minimum, and the maximum difference from the mean of the cells within the kernel at the original scale. Changes in resolution of 0.5, 0.33, 0.25, and 0.2 of the original study area are studied at 100 viewing points in each of two study areas. At the original resolution a 120 × 120 pixel area was examined, giving generalizations to 60 × 60, 40 × 40, 30 × 30, and 24 × 24 pixels. The viewsheds determined over these different resolution DEMs are compared with a number of possible viewsheds derived by generalization of the viewshed over the original DEM. Of those tested, the maximum deviation from the mean within kernel emerges as the most reliable estimate of the pattern and area of the viewshed at all resolutions. The importance of this conclusion is that different generalization operators yield more or less faithful versions of the ungeneralized product (the viewshed), and it seems indicative that a similar variety in stability of the product with generalization operator will result in many, if not all, complex products of spatial analysis.  相似文献   

AN INTEGRAL EQUATION FOR THE GEOELECTRIC RESPONSE OF THIN RESISTIVE BODIES1     

PETER FURNESS 《Geophysical Prospecting》1992,40(7):701-720

Thin sheet-like forms are common target bodies in geoelectric prospecting. Depending on their mineralogy and other factors these bodies may be relatively conductive or relatively resistive with respect to their surroundings. For suitably remote field points (relative to the thickness) these features manifest themselves geoelectrically in terms of their conductivity-thickness product for relatively conductive bodies or in terms of their resistivity-thickness product for resistive forms. While the case of a conductive sheet has received some attention in the geophysical literature, resistive sheets have been largely ignored. Accordingly an efficient technique to model the geoelectric responses of a resistive lamina is presented here. The technique involves representing the lamina in terms of a distribution of normally directed current dipole moment whose density is shown to satisfy an inhomogeneous Fredholm integral equation of the second kind. The technique is rigorously tested in a 2D environment and is shown to produce reliable and suitably accurate results. An application of the method is presented in which the apparent resistivity and chargeability responses measured with a gradient array over a dipping resistive ribbon are computed. These are compared with the responses observed over a relatively conductive ribbon in the same orientation.  相似文献   

Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands     

STEPHEN J. BURNS  PETER K. SWART 《Sedimentology》1992,39(2):285-304

The sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions. In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca²⁺, Mg²⁺ and Sr²⁺ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio-irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition. Stratigraphic analysis and δ¹³C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr^?1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high-Mg calcite dissolution and low-Mg calcite precipitation, from 0 to 35 cm; (2) Ca- or Mg-sulphate dissolution and low-Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X-ray diffraction analyses, probably because of the comparatively young age of the island.  相似文献   

DETACHMENT OF FIRST-GENERATION CARBONATE CEMENT ON ONCOLITES BY CRYSTALLIZATION OF SECONDARY ANHYDRITE     

PETER ROTHE 《Sedimentology》1969,13(3-4):311-318

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Need for an Iron-Formation Reference Sample*     

PETER W. UITTERDIJK APPEL 《Geostandards and Geoanalytical Research》1980,4(1):19-19

An international iron-formation reference sample is warranted as a basis for geochemical investigations of Precambrian iron-formations. A better knowledge of the geochemistry of iron formations will hopefully elucidate the long standing discussion on the genesis of iron-formations. The reference sample can furthermore be used in studies of deep-sea metalliferous deposits.  相似文献   

THIN-SECTION SIZE ANALYSIS: A FURTHER NOTE     

PETER H. STAUFFER 《Sedimentology》1966,7(3):261-263

Criticisms by VAN DER PLAS (1962) of thin-section point counting as a method of size analysis appear to be based on confusion between number percent and volume percent. Points are samples of volume, and point counts yield volume percents. This is as much true of size classes as of other types of constituents, though distortions are caused in the distribution by the sectioning effect. The “ribbon” method advocated by Van der Plas counts individuals rather than points and hence yields number percents. The two types of statistic are fundamentally different.  相似文献   

Review     

PETER H. THOMPSON 《Journal of Metamorphic Geology》1986,4(2):231-233

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Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla,Switzerland – a consequence of resetting at the glacier bed?     

DARREL A. SWIFT  DAVID C. W. SANDERSON  PETER W. NIENOW  ROBERT G. BINGHAM  IAN C. COCHRANE 《Boreas: An International Journal of Quaternary Research》2011,40(3):446-458

Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques.  相似文献   

Real world and real time political geography     

PETER PERRY 《New Zealand geographer》1987,43(1):33-34

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Reefs ancient and modern     

PETER ELLIS 《Geology Today》1986,2(2):41-42

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