Differential astrometry over 15°     

M. A. Prez-Torres  J. M. Marcaide  J. C. Guirado  E. Ros 《New Astronomy Reviews》1999,43(8-10)

We observed the pair of radio sources 1150+812 and 1803+784 in November 1993 with a VLBI array, simultaneously recording at 8.4 and 2.3 GHz. We determined the angular separation between the two sources with submilliarcsecond accuracy by using differential techniques. This result demonstrates the feasibility of high precision differential astrometry for radio sources separated in the sky by almost 15°, and opens the avenue to its application to larger samples of radio sources.  相似文献   

Is the jet of AO 0235+164 rotating?     

Y. J. Chen  F. J. Zhang  L. O. Sjouwerman   《New Astronomy Reviews》1999,43(8-10)

We present the VLBI map of a superluminal radio source AO 0235+164 at 5 GHz. This shows that the object's radio structure is dominated by a strong, nearly unresolved core with two extended asymmetric weak jet components. Based on the variation of its flux density with time at 5 GHz, we suggest that the flux density of AO 0235+164 exhibits two periodic variations: a shorter-period variation of 1.81±0.06 yr and a longer-period one of 3.64±0.08 yr. These two periodic variations could be the result of the joint action of jet outbursts and jet rotation.  相似文献   

位置不确定性与属性不确定性的场模型   总被引：23，自引：2，他引：21  

张景雄  杜道生 《测绘学报》1999,28(3):244-250

不确定性是自地理信息系统发展与应用以来一个引起关注的课题。位置不确定性与属性不确定常常不加区分地被看作是可以单个讨论的问题。本文将借助场的概念和模型使二者得以统一的描述和分析;对于明确定义的离散目标,二者虽然可分别讨论,但却在数学上有着联合的基础;对于非明确定义的地理现象,二者以连续体的形式而存在,位置不确定性可以作为属性不确定性的空间映射而提取出来。  相似文献   

Fast variability of BL Lacertae at 1 μm     

E. Massaro  R. Nesci  M. Maesano  F. Montagni  & F. D'Alessio 《Monthly notices of the Royal Astronomical Society》1998,299(1):47-50

We present the results of photometric observations of BL Lacertae at 1 μm performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by ∼ 0.5 mag in about 2 h: the variation amplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.  相似文献   

Mark 421天体多波段辐射机制研究     

康世举  黄邦蓉  康婷  梁继华  郑永刚 《天文学报》2011,52(5):357-364

基于对数抛物线型电子分布,用单区、均匀同步自康普顿(Synchrotron Self-Compton,SSC)辐射模型计算BL Lac天体S5 0716+714的多波段能谱,并与Paggi等人的结果进行了比较.模型的计算结果与Paggi等人直接用δ函数近似得到的结果不同,导致该差异的主要原因可能是由于单电子同步辐射的δ函数近似丢失了电子的部分能量而影响逆康普顿散射结果.把该模型分别应用于Mark 421天体的高、中、低3种不同状态下的多波段观测结果,理论计算结果能与不同状态下的观测结果符合得很好.分析认为,观测到的Mark 421天体的不同状态可能是由于喷流内电子分布变化引起的.  相似文献   

Public stealth and boundary objects: Coping with integrated water resource management and the post-political condition in Montana’s portion of the Yellowstone River watershed     

《Geoforum》2017

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一种机载雷达点云数据的快速分类方法   总被引：1，自引：0，他引：1  

李慧盈  李文辉  陈圣波 《吉林大学学报(地球科学版)》2010,40(5):1205-1210

机载激光雷达扫描技术可以以点云形式快速获取地形表面高精度三维信息。基于激光雷达扫描数据及建筑物本身的拓扑信息就可以对建筑物进行精确的重建,而重建中最关键的技术是对点云数据进行分类,进而进行地物识别。对大规模三维点云数据进行快速分类,提出一种采用区域分割结合基于最小二乘平差的多项式拟合方法,将大量离散的三维点云分割后进行多项式拟合,并将二维数据分类转化为一维数据分类。在分类的基础上,将建筑物几何规则作为约束条件提取了房屋边缘。实验分析表明,该方法既能去除多余噪声,又能有效保留特征点,分类的总误差率低于3%。  相似文献   

GIS and remote sensing integration for environmental applications     

J. C. Hinton 《International journal of geographical information science》2013,27(7):877-890

Abstract

The combined use of remotely-sensed images and vector GIS data has received considerable interest in recent years. This review article discusses the historical move towards closer integration of remote sensing and GIS technologies and the requirements of integrated software systems to enable remotely-sensed data to be combined with vector datasets for maximum effect. The benefits of integration to users of both GIS and remote sensing for environmental applications are reviewed and some thoughts are given on terminology and future directions in this field.  相似文献   

Directed movements in probabilistic time geography     

Stephan Winter  Zhang-Cai Yin 《International journal of geographical information science》2013,27(9):1349-1365

This article studies probabilistic time geography for space–time prisms, that is, for situations where observers know the location of an agent at one time and then again at another time. In the intervening period, the agent would have moved freely, according to its time budget. The article demonstrates that the probability of finding the agent somewhere in the space–time prism is not equally distributed, so any attempt of a quantitative time geographic analysis must consider the actual probability distribution. This article develops, implements, and demonstrates this distribution. A preceding article introduced probabilistic time geography for space–time cones. With cones and prisms, the elementary space–time volumes of time geography are provided.  相似文献   

Cardinal directions: a comparison of direction relation matrix and objects interaction matrix     

Sanjiang Li  Weiming Liu 《International journal of geographical information science》2013,27(2):194-216

How to express and reason with cardinal directions between extended objects such as lines and regions is an important problem in qualitative spatial reasoning (QSR), a common subfield of geographical information science and Artificial Intelligence (AI). The direction relation matrix (DRM) model, proposed by Goyal and Egenhofer in 1997, is one very expressive relation model for this purpose. Unlike many other relation models in QSR, the set-theoretic converse of a DRM relation is not necessarily representable in DRM. Schneider et al. regard this as a serious shortcoming and propose, in their work published in ACM TODS (2012), the objects interaction matrix (OIM) model for modelling cardinal directions between complex regions. OIM is also a tiling-based model that consists of two phases: the tiling phase and the interpretation phase. Although it was claimed that OIM is a novel concept, we show that it is not so different from DRM if we represent the cardinal direction of two regions a and b by both the DRM of a to b and that of b to a. Under this natural assumption, we give methods for computing DRMs from OIMs and vice versa, and show that OIM is almost the same as DRM in the tiling phase, and becomes less precise after interpretation. Furthermore, exploiting the similarity between the two models, we prove that the consistency of a complete basic OIM network can be decided in cubic time. This answers an open problem raised by Schneider et al. regarding efficient algorithms for reasoning with OIM.  相似文献