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171.
Seabirds and fronts: a brief overview 总被引:4,自引:0,他引:4
DAVID C. SCHNEIDER 《Polar research》1990,8(1):17-21
Oceanographic fronts are the sites of enhanced physical and biological activity, including locally concentrated feeding by marine birds. Two general hypotheses relating marine birds to fronts have been developed. The first is that enhanced primary production at fronts increases prey supply through increased animal growth, reproduction, or immigration. The second is that prey patches develop at fronts either through behavioural responses of prey to thermal or salinity gradients, or through interaction between prey behaviour and circulatory patterns. Several recent studies support the second hypothesis. The first hypothesis, that birds benefit from enhanced primary production at fronts, has yet to be evaluated. We need a better understanding of trophic mechanisms at fronts in order to determine (1) the proportion of daily energy requirements extracted by predators at fronts; (2) the proportion of natural mortality of prey populations at fronts; and (3) the probability of contact between seabirds and point source pollutants. 相似文献
172.
173.
The Grande Prairie region of northwestern Alberta was partially covered by glacial Lake Peace, which was dammed against the retreating Laurentide ice sheet. Two levels of glacial Lake Peace are identified in the study are by closely spaced groups of strandlines and minor deltas lying at 805 to 840 m a. s. l., and 655 to 710 m a.s.l. Sedimentation associated with the upper of the two lake levels is marked by rhythmites of silt and clay deposited by turbid underflow, interbedded with diamicton deposited by debris flow. Dropstones and dump structures indicate common ice-rafting. Thick sequences are only found on the axes of major valleys, where sediment gravity flows were concertrated. Thin sequences of ice proximal glaciolacustrine sediments reflect topographic setting and do not indicate a short-lived lake. Retreat of the ice front resulted in a decrease in ice-rafted material and diamicton in sediments. The fall in lake level to the second stage resulted in deposition of sequences of vaguely laminated silt and clay close to the modern Beaverlodge River. These sediments were deposited by suspension settling from interflow or overflow of the Beaverlodge River as it entered the lake. Lake sedimentation was dominated by inflow from unglaciated areas, rather than the ice front. 相似文献
174.
175.
DAVID J. MOSSMAN 《Meteoritics & planetary science》1972,7(1):71-74
A crater, 80 m diameter by 6 m deep, was created in 1959 on the Nyika Plateau between Zambia and Malawi. It may have been the result of a meteorite. 相似文献
176.
Development and application of global databases: considerable progress,but more collaboration needed
DAVID A. HASTINGS JOHN J. KINEMAN DAVID M. CLARK 《International journal of geographical information science》2013,27(1):137-146
Abstract This paper reviews the development of international collaboration in the availability of global data and processing abilities in the environmental field, beginning in 1957 with tie pioneering activities of the World Data Centres. Considerable progress has been made, particularly through the availability of satellite data, but there are considerable shortcomings in these achievements, arising from the isolated nature of many developments, the difficulties of integrating different sources and weaknesses in the top-down approach. There are now attempts to develop a complementary bottom-up approach, notably through the Global Change Diskette Project Developments in hardware, such as the widespread availability of compatible personal computers operating on MS-DOS and UNIX systems; and easier storage on compact disks, and in software, through access to low-cost and public-domain software such as GRASS and IDRISI, are helping but improved collaboration is needed, in which existing institutions such as the World Data Centers have a major part to play. 相似文献
177.
DAVID J. ABEL DAVID M. MARK 《International journal of geographical information science》2013,27(1):21-31
Abstract Many spatial analysis algorithms for cellular representations and data structures are based on an ordering of cells or objects to transform a two-dimensional problem to a one-dimensional one. Several orderings are available and their differing properties influence the performance of the data structures and algorithms. The relative merits of five orderings (row, row prime, Hilbert, Morton and Gray code) are assessed empirically for four paradigmatic geographical data-processing tasks in spatial analysis and data management. It is concluded that the Hilbert ordering deserves further investigation. 相似文献
178.
HELENA MITASOVA LUBOS MITAS WILLIAM M. BROWN DAVID P. GERDES IRINA KOSINOVSKY TERRY BAKER 《International journal of geographical information science》2013,27(4):433-446
Abstract The concept of GRASS (Geographic Resources Analysis Support System) as an open system has created a favourable environment for integration of process based modelling and GIS. To support this integration a new generation of tools is being developed in the following areas: (a) interpolation from multidimensional scattered point data, (b) analysis of surfaces and hypersurfaces, (c) modelling of spatial processes and, (d) 3D dynamic visualization. Examples of two applications are given-spatial and temporal modelling of erosion and deposition, and multivariate interpolation and visualization of nitrogen concentrations in the Chesapeake Bay. 相似文献
179.
STEVE CARVER IAN HEYWOOD SARAH CORNELIUS DAVID SEAR 《International journal of geographical information science》2013,27(4):475-486
ABSTRACT This paper examines the potential of using GIS (Geographical Information Systems) in the field for environmental characterization, modelling and decision support particularly in isolated areas where data collection is difficult. Observations are based on experiences gained during two Anglo-Russian expeditions to the Altai Mountains of south central Siberia aimed at evaluating proposals for a new national park in the Katunsky Ridge area of the Belukha Massif. The use of GIS together with GPS (Global Positioning Systems) for primary data collection and verification/update of existing data is described and the use of field-based systems for on-the-spot environmental modelling and decision support is evaluated. 相似文献
180.