全文获取类型
收费全文 | 98044篇 |
免费 | 3068篇 |
国内免费 | 3582篇 |
专业分类
测绘学 | 2653篇 |
大气科学 | 7939篇 |
地球物理 | 18936篇 |
地质学 | 36982篇 |
海洋学 | 9752篇 |
天文学 | 20986篇 |
综合类 | 1211篇 |
自然地理 | 6235篇 |
出版年
2022年 | 1276篇 |
2021年 | 1866篇 |
2020年 | 1802篇 |
2019年 | 1960篇 |
2018年 | 3325篇 |
2017年 | 3083篇 |
2016年 | 3490篇 |
2015年 | 2040篇 |
2014年 | 3420篇 |
2013年 | 5256篇 |
2012年 | 3618篇 |
2011年 | 4536篇 |
2010年 | 4103篇 |
2009年 | 4966篇 |
2008年 | 4375篇 |
2007年 | 4455篇 |
2006年 | 4127篇 |
2005年 | 2889篇 |
2004年 | 2769篇 |
2003年 | 2657篇 |
2002年 | 2699篇 |
2001年 | 2379篇 |
2000年 | 2260篇 |
1999年 | 1886篇 |
1998年 | 1864篇 |
1997年 | 1758篇 |
1996年 | 1491篇 |
1995年 | 1484篇 |
1994年 | 1256篇 |
1993年 | 1174篇 |
1992年 | 1078篇 |
1991年 | 1064篇 |
1990年 | 1009篇 |
1989年 | 915篇 |
1988年 | 818篇 |
1987年 | 937篇 |
1986年 | 820篇 |
1985年 | 982篇 |
1984年 | 1098篇 |
1983年 | 1056篇 |
1982年 | 971篇 |
1981年 | 908篇 |
1980年 | 808篇 |
1979年 | 763篇 |
1978年 | 735篇 |
1977年 | 626篇 |
1976年 | 635篇 |
1975年 | 627篇 |
1974年 | 597篇 |
1973年 | 668篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
661.
Geomasking sensitive health data and privacy protection: an evaluation using an E911 database 总被引:2,自引:0,他引:2
Allshouse WB Fitch MK Hampton KH Gesink DC Doherty IA Leone PA Serre ML Miller WC 《国际地球制图》2010,25(6):443-452
Geomasking is used to provide privacy protection for individual address information while maintaining spatial resolution for mapping purposes. Donut geomasking and other random perturbation geomasking algorithms rely on the assumption of a homogeneously distributed population to calculate displacement distances, leading to possible under-protection of individuals when this condition is not met. Using household data from 2007, we evaluated the performance of donut geomasking in Orange County, North Carolina. We calculated the estimated k-anonymity for every household based on the assumption of uniform household distribution. We then determined the actual k-anonymity by revealing household locations contained in the county E911 database. Census block groups in mixed-use areas with high population distribution heterogeneity were the most likely to have privacy protection below selected criteria. For heterogeneous populations, we suggest tripling the minimum displacement area in the donut to protect privacy with a less than 1% error rate. 相似文献
662.
Muhammad Adnan Paul A Longley Alex D Singleton Chris Brunsdon 《Transactions in GIS》2010,14(3):283-297
Geodemographic classifications provide discrete indicators of the social, economic and demographic characteristics of people living within small geographic areas. They have hitherto been regarded as products, which are the final “best” outcome that can be achieved using available data and algorithms. However, reduction in computational cost, increased network bandwidths and increasingly accessible spatial data infrastructures have together created the potential for the creation of classifications in near real time within distributed online environments. Yet paramount to the creation of truly real time geodemographic classifications is the ability for software to process and efficiency cluster large multidimensional spatial databases within a timescale that is consistent with online user interaction. To this end, this article evaluates the computational efficiency of a number of clustering algorithms with a view to creating geodemographic classifications “on the fly” at a range of different geographic scales. 相似文献
663.
This article reports on an empirical study of the trends and patterns of research activities in Geographic Information Science (GIScience) during the years 1997–2007. The GIScience research priorities identified by the University Consortium of Geographic Information Science (UCGIS) were used as guidelines to examine the 985 research articles published in six well‐recognized academic journals. Latent Semantic Analysis (LSA) was employed to investigate the association among the different GIScience research themes. The spatial and temporal patterns of the association between the publications and the different GIScience themes were examined to show the development of GIScience research during the study period. Furthermore, correlation analyses between the publications were conducted following the LSA results to reveal GIScience research networks, including the networks of the published articles and those formed by the research places. In this article, we applied an approach that was developed within information science to depict what GIS research activities were conducted when and where and how they connect to each other through sharing common research themes. The related findings pave the way for future efforts to describe the paradigm of GIScience as well as the pattern of GIScience research. 相似文献
664.
665.
666.
Elizabeth J. Petrie Matt A. King Philip Moore David A. Lavallée 《Journal of Geodesy》2010,84(8):491-499
This study provides a first attempt at quantifying potential signal bending effects on the GPS reference frame, coordinates
and zenith tropospheric delays (ZTDs). To do this, we homogeneously reanalysed data from a global network of GPS sites spanning
14 years (1995.0–2009.0). Satellite, Earth orientation, tropospheric and ground station coordinate parameters were all estimated.
We tested the effect of geometric bending and dTEC bending corrections, which were modelled at the observation level based,
in part, on parameters from the International Reference Ionosphere 2007 model. Combined, the two bending corrections appear
to have a minimal effect on site coordinates and ZTDs except for low latitude sites. Considering five days (DOY 301–305, 28
October–1 November 2001) near ionospheric maximum in detail, they affect mean ZTDs by up to ~1.7 mm at low latitudes, reducing
to negligible levels at high latitudes. Examining the effect on coordinates in terms of power-spectra revealed the difference
to be almost entirely white noise, with noise amplitude ranging from 0.3 mm (high latitudes) to 2.4 mm (low latitudes). The
limited effect on station coordinates is probably due to the similarity in the elevation dependence of the bending term with
that of tropospheric mapping functions. The smoothed z-translation from the GPS reference frame to ITRF2005 changes by less than 2 mm, though the effect combines positively with
that from the second order ionospheric refractive index term. We conclude that, at the present time, and for most practical
purposes, the geometric and dTEC bending corrections are probably negligible at current GPS/reference frame precisions. 相似文献
667.
In the past researchers have suggested hard classification approaches for pure pixel remote sensing data and to handle mixed
pixels soft classification approaches have been studied for land cover mapping. In this research work, while selecting fuzzy
c-means (FCM) as a base soft classifier entropy parameter has been added. For this research work Resourcesat-1 (IRS-P6) datasets
from AWIFS, LISSIII and LISS-IV sensors of same date have been used. AWIFS and LISS-III datasets have been used for classification
and LISS-III and LISS-IV data were used for reference data generation, respectively. Soft classified outputs from entropy
based FCM classifiers for AWIFS and LISS-III datasets have been evaluated using sub-pixel confusion uncertainty matrix (SCM).
It has been observed that output from FCM classifier has higher classification accuracy with higher uncertainty but entropy-based
classifier with optimum value of regularizing parameter generates classified output with minimum uncertainty. 相似文献
668.
Kinematic precise point positioning at remote marine platforms 总被引:6,自引:2,他引:6
Precise kinematic differential positioning using the global positioning system (GPS) at a marine platform usually requires
a relatively short distance (e.g. <500 km) to a land-based reference station. As an alternative, precise point positioning
(PPP) is normally considered free from this limiting requirement. However, due to the prerequisite of network-based satellite
products, PPP at a remote marine platform may still be affected by its distance to the reference network. Hence, this paper
investigates this scenario by configuring rings of reference stations with different radii centered on a to-be-positioned
marine platform. Particularly, we applied ambiguity resolution at a single station to PPP by estimating uncalibrated phase
delays (UPDs). We used three rings of reference stations centered on a vessel, with radii of roughly 900, 2,000 and 3,600 km,
to determine satellite clocks and UPDs independently. For comparison, we also performed differential positioning based on
a single reference station with baseline lengths of about 400, 1,700 and 2,800 km. We demonstrate that, despite the increasing
ring-network radius to a few 1,000 km, the overall change in accuracy of the satellite clocks that are used at the vessel
is smaller than 0.02 ns, and the RMS values of differences between the three sets of narrow-lane UPD estimates are around
0.05 cycles only. Moreover, the kinematic positioning accuracy of PPP is affected by the increasing ring-network radius, but
can still achieve several centimeters after ambiguity resolution when the vessel is over a few 1,000 km away from the ring
network, showing better performance than that of differential positioning. Therefore, we propose that ambiguity-fixed PPP
can be used at remote marine platforms that support precise oceanographic and geophysical applications in open oceans. 相似文献
669.
670.
Pavagada taluk of Tumkur district in Karnataka is one of the most backward taluks receiving less than 500 mm annual rainfall.
The maximum area of the taluk is under monocropping, reasons for the same were not documented well. The present study was
carried out using remote sensing data along with field survey and laboratory analysis for assessing the potentials and limitations
of soil. Using the basic information on soil, climate and topography based on the matching exercise between the growth and
production requirements of the crop, suitability of soils for groundnut, paddy and finger millet was assessed as per FAO land
evaluation. The soil suitability maps were prepared using Arc GIS software. About 48 per cent of the total area was moderate
to marginally suitable and 13 per cent of the area was not suitable for both groundnut and finger millet. Lowland areas covering
12 per cent of the area was highly suitable, 15 per cent was moderate to marginally suitable and 20 per cent was not suitable
for paddy cultivation. 相似文献