航空重力测量厄特弗斯改正公式注记

针对当前国内外机构和学者在使用航空重力测量厄特弗斯改正公式中存在的不一致性问题,详细分析了不同时期、不同形式改正计算公式的来源及其相互关系,并比较了它们在数值上的差异,指出我国作业部门目前使用近似公式存在的误用问题和统一使用严密公式的必要性。     

Temperature Sensitivity of Timing Measurements Using Dorne Margolin Antennas

We have inferred the temperature sensitivity of Dorne Margolin choke ring antennas by direct estimation from differential clock estimates for two GPS sites separated by 2400 km. At each site, the cable and receiver systems are very well isolated from environmental variations. By direct comparison of the observed clock variations between these sites with local temperature measurements, empirical temperature coefficients for each system have been estimated. These thermal coefficients most likely apply to the only uncontrolled components of the systems, the Dorne Margolin choke ring antennas. Based on these results, the short-term (diurnal) stability of the antennas appears to be better than 2 ps/°C. The possibility that longer-term effects exist due to sensitivity in the daily average of the pseudorange observations has not been tested and cannot be excluded. ? 2001 John Wiley & Sons, Inc.     

Lineaments and seismicity of Kerala — a remote sensing based analysis

Seismically active lineaments of Kerala State were identified by correlating the lineaments mapped using IRS LISS-I data with the earthquake occurrences. There are 31 earthquake incidences in Kerala since 1821, out of which 22 are falling on/close to 9 major lineaments/faults (length more than 20 km) indicating the possible correlation between lineaments/faults and earthquakes. It was observed that the earthquake occurrences are mostly associated with the NNW-SSE to NW-SE trending lineaments (6 out of 9 lineaments), which are considered to be formed sympathetic to the West Coast Fault. Hence, there is a need to monitor these seismically active lineaments using advanced techniques such as GPS, SAR Interferometry etc. for better understanding of the influence of these lineaments on the seismic activities of Kerala.     

A Geographic Information System approach to evaluation of groundwater potentiality of Shamri micro-watershed in the Shimla Taluk, Himachal Pradesh

GIS. a potential tool for facilitating the generation and use of thematic information, has been applied to groundwater potentiality of the Shamri micro-watershed in Shimla Taluk. The role of various parameters, namely, drainage. lineament. lithology . slope and landuse have been emphasised for delineation of groundwater potential iones. IRS-I C IAN and LISS Ill FCC merged satellite images on 1:25000 scale and Topographic map no. 53L/4 SI together with field traverses have been used as the data source. A multi-criteria evaluation following probability weighted approach has been applied for overlay analysis that allows a linear combination of weights of each thematic map with the individual capability value. 1 he resultant map indicates a high groundwater potentiality in the flood plains, river terraces and river channels in the vicinity of the Shamri nala. Other sites of high potentiality include places showing break in slopes and criss-crossing of lineaments.     

Remote Sensing and Geographical Information System for canopy cover mapping

Availability of remote sensing data from earth observation satellites has made it convenient to map and monitor land use/land cover at regional to local scales. A land cover map is very critical for a various planning activities including watershed planning. The spectral and spatial resolutions are major constraints for mapping the crop resources at microlevel. The cropping pattern zones have been mapped using the false color composite, physiography, irrigation and toposheets. The IRS LISS-III data is classified into various categories depending on spectral reflectance from crop canopy and are overlaid on cropping zones map. The re-classified resultant map provides land use/land cover information including dominant cropping systems. The canopy cover is estimated monthly considering the crop calendar for the area.     

Chlorophyll — A Concentration Along West Coast of India Using IRS-P3 MOS-B Data

Remote sensing of ocean colour yields information on the constituents of sea water, such as the concentration of phytoplankton pigments, suspended sediments and yellow substances. It is well understood that the study of ocean colour is significantly related with the primary production and zonation of potential fishing sites in coastal and oceanic waters. The major pigment constituent is predominated by chlorophyll-a (ocean colour pigment of phytoplankton). The chlorophyll mapping on regular basis plays a major role in assessing water quality and classifying different water types. IRS P-3 MOS-B satellite data for three consecutive passes of path 94, during the period of January-February 1997 have been used to derive chlorophyll-a concentration. The present study emphasizes on the chlorophyll mapping using IRS-P3 MOS-B data for the coastal and offshore water of Maharashtra coast, India.     

Corridor analysis in Rajaji-Corbett elephant reserve — A Remote sensing and GIS approach

This paper deals with corridor analysis in Rajaji-Corbett Elephant Reserve in the Shivaliks of Uttaranchal state. Efforts were made to detect changes in the state of forest covervis- à -vis the status of corridors during the three periods i.e. 1967. 1986 and 1998 using remote sensing and GIS. TheERDAS Imagine digital image processing andArcView GIS software packages were used for this purpose. Temporal satellite imagery and ground observations in the Rajaji-Corbett Elephant Reserve revealed forest loss, degradation and disturbances in the corridor areas, hindering elephant movement and restricting them to forests islands. Motichur-Chilla corridor, despite being a highly favoured habitat for the elephants was found to be highly threatened followed by Kotdwar and Ramnagar corridors. Construction of wide bridges across Kunao-Chilla Canal, recreation of corridors through reforestation, reduction of all kinds of pressures in the corridor areas and providing higher protection to corridors are recommended.     

Evaluation of RADARSAT ScanSAR synthetic aperture radar data for rice crop inventory and monitoring

Radarsat ScanSAR Narrow (SN2) data acquired on July 24 and August 17, 1997 were used to analyse the signature of rice crop in West Bengal, India. The analysis showed that the lowland practice of cultivation gives a distinct signature to rice due to the initial water background. The relatively stable backscatter from water bodies in temporal data enhanced the separability of rice fields from water using two date data. Around 94 per cent classification accuracy was achieved for rice crop using two date data. It was feasible to discriminate rice sub-classes based on their planting period like early and late crop. The analysis indicates the suitability of ScanSAR data for large area rice crop monitoring as it has a wide swath of 300 km.     

Local geoid determination combining gravity disturbances and GPS/levelling: a case study in the Lake Nasser area, Aswan, Egypt

 The use of GPS for height control in an area with existing levelling data requires the determination of a local geoid and the bias between the local levelling datum and the one implicitly defined when computing the local geoid. If only scarse gravity data are available, the heights of new data may be collected rapidly by determining the ellipsoidal height by GPS and not using orthometric heights. Hence the geoid determination has to be based on gravity disturbances contingently combined with gravity anomalies. Furthermore, existing GPS/levelling data may also be used in the geoid determination if a suitable general gravity field modelling method (such as least-squares collocation, LSC) is applied. A comparison has been made in the Aswan Dam area between geoids determined using fast Fourier transform (FFT) with gravity disturbances exclusively and LSC using only the gravity disturbances and the disturbances combined with GPS/levelling data. The EGM96 spherical harmonic model was in all cases used in a remove–restore mode. A total of 198 gravity disturbances spaced approximately 3 km apart were used, as well as 35 GPS/levelling points in the vicinity and on the Aswan Dam. No data on the Nasser Lake were available. This gave difficulties when using FFT, which requires the use of gridded data. When using exclusively the gravity disturbances, the agreement between the GPS/levelling data were 0.71 ± 0.17 m for FFT and 0.63 ± 0.15 for LSC. When combining gravity disturbances and GPS/levelling, the LSC error estimate was ±0.10 m. In the latter case two bias parameters had to be introduced to account for a possible levelling datum difference between the levelling on the dam and that on the adjacent roads. Received: 14 August 2000 / Accepted: 28 February 2001     

New analytical and numerical approaches for geopotential modeling

 The standard analytical approach which is applied for constructing geopotential models OSU86 and earlier ones, is based on reducing the boundary value equation to a sphere enveloping the Earth and then solving it directly with respect to the potential coefficients _n,m . In an alternative procedure, developed by Jekeli and used for constructing the models OSU91 and EGM96, at first an ellipsoidal harmonic series is developed for the geopotential and then its coefficients _n,m ^e are transformed to the unknown _n,m . The second solution is more exact, but much more complicated. The standard procedure is modified and a new simple integral formula is derived for evaluating the potential coefficients. The efficiency of the standard and new procedures is studied numerically. In these solutions the same input data are used as for constructing high-degree parts of the EGM96 models. From two sets of _n,m (n≤360,|m|≤n), derived by the standard and new approaches, different spectral characteristics of the gravity anomaly and the geoid undulation are estimated and then compared with similar characteristics evaluated by Jekeli's approach (`etalon' solution). The new solution appears to be very close to Jekeli's, as opposed to the standard solution. The discrepancies between all the characteristics of the new and `etalon' solutions are smaller than the corresponding discrepancies between two versions of the final geopotential model EGM96, one of them (HDM190) constructed by the block-diagonal least squares (LS) adjustment and the other one (V068) by using Jekeli's approach. On the basis of the derived analytical solution a new simple mathematical model is developed to apply the LS technique for evaluating geopotential coefficients. Received: 12 December 2000 / Accepted: 21 June 2001