全文获取类型
收费全文 | 9147篇 |
免费 | 315篇 |
国内免费 | 100篇 |
专业分类
测绘学 | 228篇 |
大气科学 | 611篇 |
地球物理 | 2154篇 |
地质学 | 3192篇 |
海洋学 | 876篇 |
天文学 | 1442篇 |
综合类 | 22篇 |
自然地理 | 1037篇 |
出版年
2021年 | 96篇 |
2020年 | 123篇 |
2019年 | 119篇 |
2018年 | 181篇 |
2017年 | 179篇 |
2016年 | 246篇 |
2015年 | 206篇 |
2014年 | 216篇 |
2013年 | 512篇 |
2012年 | 268篇 |
2011年 | 337篇 |
2010年 | 316篇 |
2009年 | 382篇 |
2008年 | 354篇 |
2007年 | 317篇 |
2006年 | 350篇 |
2005年 | 265篇 |
2004年 | 321篇 |
2003年 | 293篇 |
2002年 | 292篇 |
2001年 | 199篇 |
2000年 | 196篇 |
1999年 | 163篇 |
1998年 | 167篇 |
1997年 | 116篇 |
1996年 | 123篇 |
1995年 | 126篇 |
1994年 | 139篇 |
1993年 | 122篇 |
1992年 | 130篇 |
1991年 | 119篇 |
1990年 | 106篇 |
1989年 | 91篇 |
1988年 | 92篇 |
1987年 | 135篇 |
1986年 | 107篇 |
1985年 | 184篇 |
1984年 | 217篇 |
1983年 | 158篇 |
1982年 | 140篇 |
1981年 | 153篇 |
1980年 | 121篇 |
1979年 | 134篇 |
1978年 | 130篇 |
1977年 | 111篇 |
1976年 | 108篇 |
1975年 | 102篇 |
1974年 | 73篇 |
1973年 | 94篇 |
1972年 | 51篇 |
排序方式: 共有9562条查询结果,搜索用时 10 毫秒
51.
Reliable land cover land use (LCLU) information, and change over time, is important for Green House Gas (GHG) reporting for climate change documentation. Four different organizations have independently created LCLU maps from 2010 satellite imagery for Malawi for GHG reporting. This analysis compares the procedures and results for those four activities. Four different classification methods were employed; traditional visual interpretation, segmentation and visual labelling, digital clustering with visual identification and supervised signature extraction with application of a decision rule followed by analyst editing. One effort did not report classification accuracy and the other three had very similar and excellent overall thematic accuracies ranging from 85 to 89%. However, despite these high thematic accuracies there were very significant differences in results. National percentages for forest ranged from 18.2 to 28.7% and cropland from 40.5 to 53.7%. These significant differences are concerns for both remote-sensing scientists and decision-makers in Malawi. 相似文献
52.
Journal of Geographical Systems - This paper raises a fundamental question about Sub-Saharan Africa: has urbanization there been accompanied by improvements in personal wellbeing? It then proceeds... 相似文献
53.
Jane Bemigisha John Carranza rew K. Skidmore Mike McCall Chiara Polce Herbert H.T. Prins 《Transactions in GIS》2009,13(3):273-293
In a project to classify livestock grazing intensity using participatory geographic information systems (PGIS), we encountered the problem of how to synthesize PGIS-based maps of livestock grazing intensity that were prepared separately by local experts. We investigated the utility of evidential belief functions (EBFs) and Dempster's rule of combination to represent classification uncertainty and integrate the PGIS-based grazing intensity maps. These maps were used as individual sets of evidence in the application of EBFs to evaluate the proposition that " This area or pixel belongs to the high, medium, or low grazing intensity class because the local expert(s) says (say) so ". The class-area-weighted averages of EBFs based on each of the PGIS-based maps show that the lowest degree of classification uncertainty is associated with maps in which "vegetation species" was used as the mapping criterion. This criterion, together with local landscape attributes of livestock use may be considered as an appropriate standard measure for grazing intensity. The maps of integrated EBFs of grazing intensity show that classification uncertainty is high when the local experts apply at least two mapping criteria together. This study demonstrates the usefulness of EBFs to represent classification uncertainty and the possibility to use the EBF values in identifying and using criteria for PGIS-based mapping of livestock grazing intensity. 相似文献
54.
There is an emerging requirement for processing global navigation satellite system (GNSS) signals indoor where the signal
is very weak and subjected to spatial fading. Typically, longer coherent integration intervals provide the additional processing
gain required for the detection and processing of such weak signals. However, the arbitrary physical motion of the handset
imputed by the user limits the effectiveness of longer coherent integration intervals due to the spatial decorrelation of
the multipath-faded GNSS signal. In this paper, limits of coherent integration due to spatial decorrelation are derived and
corroborated with experimental verification. A general result is that the processing gain resulting from direct coherent integration
saturates after the antenna has moved through a certain distance, which for typical indoor propagation, is about half a carrier
wavelength. However, a refined Doppler search coupled with a prolonged coherent integration interval extends this limit, which
is effectively a manifestation of selective diversity. 相似文献
55.
Rigorous equations in compact symbolic matrix notation are introduced to transform coordinates and velocities between ITRF
frames and modern GPS-based geocentric geodetic datums. The theory is general but, after neglecting higher than second-order
terms, it is shown that the equations revert to the formulation currently applied in most major continental datums. We discuss
several examples: the North American Datum of 1983 (NAD83), the European Terrestrial Reference System of 1989 (ETRS89), the Geodetic Datum of Australia of 1994 (GDA94), and the South American Geocentric Reference System (SIRGAS).
Electronic Publication 相似文献
56.
This paper describes a framework for an image processing procedure for operational agricultural crop area estimation. This operational framework has been conceived within the development of an Advanced Agricultural Information System (AAIS) for the “Regione del Veneto “ (RdV ‐ Veneto Region) in northeastern Italy. The objective of this program is to develop the ability to generating timely and accurate area estimates and production information for four major agricultural crops: soybeans, sugar beets, corn, and small grains. AAIS uses state of the art methods in remote sensing and geographic information systems (GIS) technology and integrates a variety of data types including satellite imagery. This paper describes the methodology developed for image and ancillary data processing for the production of crop area statistics. Using a combination of standard unsupervised classification and GIS operations that incorporate knowledge about the agricultural system, a “sequential masking” classification procedure was derived. This sequential masking procedure yielded crop classification accuracies that at the study site level range between 76% and 99% depending on the crop under study. We believe that classification accuracies will improve with full system implementation, along with the incorporation of new and/or improved thematic information and operational experience using AAIS‐based estimation. 相似文献
57.
Kristine M. Larson Eric E. Small Ethan Gutmann Andria Bilich Penina Axelrad John Braun 《GPS Solutions》2008,12(3):173-177
Measurements of soil moisture are important for studies of climate and weather forecasting, flood prediction, and aquifer
recharge studies. Although soil moisture measurement networks exist, most are sparsely distributed and lack standardized instrumentation.
Measurements of soil moisture from satellites have extremely large spatial footprints (40–60 km). A methodology is described
here that uses existing networks of continuously-operating GPS receivers to measure soil moisture fluctuations. In this technique,
incoming signals are reflected off and attenuated by the ground before reception by the GPS receiver. These multipath reflections
directly affect signal-to-noise ratio (SNR) data routinely collected by GPS receivers, creating amplitude variations that
are a function of ground reflectivity and therefore soil moisture content. After describing this technique, multipath reflection
amplitudes at a GPS site in Tashkent, Uzbekistan are compared to estimates of soil moisture from the Noah land surface model.
Although the GPS multipath amplitudes and the land surface model are uncalibrated, over the 70-day period studied, they both
rise sharply following each rainfall event and slowly decrease over a period of ∼10 days. 相似文献
58.
GPS tomography in the polar cap: comparison with ionosondes and in situ spacecraft data 总被引:1,自引:1,他引:1
Dimitry Pokhotelov P. T. Jayachandran Cathryn N. Mitchell John W. MacDougall Michael H. Denton 《GPS Solutions》2011,15(1):79-87
Tomographic 4D reconstructions of ionospheric anomalies appearing in the high-latitude polar cap region are compared with
plasma density measurements by digital ionosonde located near the north magnetic pole at Eureka station and with in situ plasma
measurements on-board DMSP spacecraft. The moderate magnetic storm of 14–17 October 2002 is taken as an example of a geomagnetic
disturbance which generates large-scale ionospheric plasma anomalies at mid-latitudes and in the polar cap region. Comparison
of the GPS tomographic reconstructions over Eureka station with the ionosonde measurements of the F layer peak densities indicates
that the GPS tomography correctly predicts the time of arrival and passage of the ionospheric tongue of ionization over the
magnetic pole area, although the tomographic technique appears to under-estimate the value of F peak plasma density. Comparison
with the in situ plasma measurements by the DMSP SSIES instruments shows that the GPS tomography correctly reproduces the
large-scale spatial structure of ionospheric anomalies over a wide range of latitudes from mid-latitudes to the high-latitude
polar cap region, though the tomographic reconstructions tend to over-estimate the density of the topside ionosphere at 840 km
DMSP orbit. This study is essential for understanding the quality and limitations of the tomographic reconstruction techniques,
particularly in high-latitude regions where GPS TEC measurements and other ionospheric data sources are limited. 相似文献
59.
分析了GPS高程信号和Heave信号的频段特征,利用数字信号处理技术,提取了GPS高程和Heave中的有效频段信号,合成了一个全新的信号。该合成信号能够全面反映多波束换能器的实际垂直运动,并能够克服传统多波束测量和数据处理方法的缺陷,大大提高了多波束在垂直方向的成果精度。实验证明了该方法的正确性和可行性。 相似文献
60.