GF-1垂直干旱指数的土壤湿度空间格局分析——以秭归县为例

针对中低分辨率的遥感图像在表征空间异质性较大地区的土壤湿度空间格局存在较大误差问题,该文探讨了基于高分辨率影像在小尺度上分析土壤湿度空间分布及变异规律的可行性。首先利用高分一号WFV数据构建垂直干旱指数来反映秭归县土壤湿度干湿状况,然后进一步分析了该县土壤湿度在水平及垂直方向上的空间格局和分异规律。实验结果表明,野外同步土壤表层水含量测试数据同垂直干旱指数两者表现出较好的相关程度。     

Sedimentation on the Ionian active margin (Hellenic arc) — Provenance of sediments and mechanisms of deposition

Lithological, sedimentological, mineralogical and faunal analyses of twenty cores from the western Peloponnesus margin and adjacent Matapan Trench permit the origin of the sediments and the processes of sedimentation to be defined in some detail.Except for the ash layers derived from the Ischian Province, the major part of the sediment is local in origin (Peloponnesus). Analyses of heavy, light and clay minerals enable three main provinces to be distinguished. Each of these provinces includes one or more slope basins and a portion of trench, without lateral connection between them and mixing of inputs.The diversity and complexity of sedimentary structures, the granulometric characters, the mixed fauna (molluscs and ostracods) and the variability of rates of sedimentation all testify to the prevalence of reworking phenomena and gravitative processes.These data confirm and elaborate on the model of sedimentation deduced from seismic studies in which sediments are successively trapped, released by the slope basins and transferred via channels and canyons towards the underlying trench basins. This “cascade feeding” appears characteristics of this subductive sedimentary environment.     

Characterization and quantification of data voids in the shuttle Radar topography mission data

The goal of this study was to characterize and quantify the occurrence of data voids in data from the Shuttle Radar Topography Mission (SRTM) for the conterminous United States. For this purpose, SRTM data and corresponding data from the national elevation data were downloaded in 21 samples spatially organized to cover the main topography of the U.S. Void locations in SRTM data were compared to terrain attributes and subsequently the area of individual data voids to the same attributes. It was found that data voids amounted to 0.3% of the total dataset. Data voids were found in all topographic settings but more often in slopes steeper than approximately 20/spl deg/ that face south (170/spl deg/), and also in flat areas such as lakes and rivers. It was also found that more than 50% of all data voids were composed of connected pixels in groups less than six pixels. The largest data voids could be attributed to water bodies, while the rest could be explained by terrain-radar interaction characteristics.     

Wet tropospheric effects on precise relative GPS height determination

Summary Considerable interest has been generated recently in the use of the Global Positioning System (GPS) for precise height determination. A major error source in these measurements is the propagation delay due to atmospheric water vapour. In order to achieve the high precisions required for such applications as absolute sea-level monitoring improvement of wet delay modelling is necessary. Results from a GPS campaign show a significant correlation (0.91) between the variability of the wet delay measured using a water vapour radiometer (WVR) at the Onsala site and the absolute value of the residual error in the height determination of a 134 km baseline from Onsala to Jönköping. This correlation indicates that the atmosphericvariability as inferred from the WVR data includes information on the quality of the GPS height estimate. During periods of high atmospheric activity, e.g., during the passage of a weather front, the use of a six-parameter gradient model reduces the spread for the vertical coordinate from 40 mm to 20 mm (with standard deviations of 17 mm and 9 mm respectively) over the 134 km baseline (less than 1 × 10^–7) using 8 hour data spans on 11 different days over a six month period.     

Forest cover mapping of East district,Sikkim using IRS-1A LISS II satellite data

Remote sensing techniques have been applied to classify tour density classes within each of the forest type along with other major landuse/landcover classes in the East district, Sikkim using IRS-1A LISS II satellite data pertaining to the period of November, 1988. The shadow problem in rugged terrain and difficulty in acquiring cloud free data for different seasons pose problems to achieve considerable mapping accuracy. In the present study, the forests of the district were delineated through supervised classification techniques using maximum likelihood algorithm into five forest types as sal forests, subtropical broad-leaved forests, Himalayan wet temperate forests, Rhododendron forests and alpine forests. The alpine forests were further stratified into two categories as moist alpine scrub and dry alpine scrub. The statistical data obtained from the present study shows that 55.47 percent of the total geographical area of the East district was under forest cover. An overall accuracy of more than 85 percent in correctly delineating forest classes was achieved.     

Topographic and atmospheric corrections of gravimetric geoid determination with special emphasis on the effects of harmonics of degrees zero and one

 The topographic and atmospheric effects of gravimetric geoid determination by the modified Stokes formula, which combines terrestrial gravity and a global geopotential model, are presented. Special emphasis is given to the zero- and first-degree effects. The normal potential is defined in the traditional way, such that the disturbing potential in the exterior of the masses contains no zero- and first-degree harmonics. In contrast, it is shown that, as a result of the topographic masses, the gravimetric geoid includes such harmonics of the order of several centimetres. In addition, the atmosphere contributes with a zero-degree harmonic of magnitude within 1 cm. Received: 5 November 1999 / Accepted: 22 January 2001     

Compatibility of NMEA GGA with GPS Receivers Implementation

The NMEA GGA standard for global positioning system (GPS) and GLONASS receiver interfaces provides smooth data transfer from receiver to computer for postprocessing. Specifically, the NMEA GGA specifies that the orthometric height and the undulation be listed in addition to other quantities. The Ashtech manual (Ashtech, 1997; p. 104), however, specifies that the respective field in the NMEA GGA message contains the ellipsoidal height when outputting from Ashtech's GG24 receiver. Because such an inconsistency between the official NMEA GGA specifications and the implementation by a manufacturer can potentially confuse the user, we carried out a numerical test to confirm Ashtech's implentation. The result indicates that Ashtech indeed gives the ellipsoidal height in the fielt that should actually contain the orthometric height according to the NMEA GGA specifications. Firmware version after and including GF02 have corrected this situation. ? 2000 John Wiley & Sons, Inc.