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1.
In support of requirements for the U.S. Air Force Cambridge Research Laboratories, gravity anomalies have been upward continued to several elevations in different areas of the United States. One area was 340 to 400 N in latitude and 960 to 1030 W in longitude, generally called the Oklahoma area. The computations proceeded from 26, 032 point anomalies to the prediction of mean anomalies in 14, 704, 2.5′×2.5′ blocks and 9,284, 5′×5′ blocks. These anomalies were upward continued along 28 profiles at 5′ intervals for every 30′ in latitude and longitude. These anomalies at elevations were meaned in various patterns to form mean 30′×30″, 10×10, 50×50 blocks. Comparisons were then made to the corresponding ground values. The results of these comparisons lead to practical recommendations on the arrangement of flight profiles in airborne gravimetry.  相似文献   

2.
A detailed gravimetric geoid in the North Atlantic Ocean, named DGGNA-77, has been computed, based on a satellite and gravimetry derived earth potential model (consisting in spherical harmonic coefficients up to degree and order 30) and mean free air surface gravity anomalies (35180 1°×1° mean values and 245000 4′×4′ mean values). The long wavelength undulations were computed from the spherical harmonics of the reference potential model and the details were obtained by integrating the residual gravity anomalies through the Stokes formula: from 0 to 5° with the 4′×4′ data, and from 5° to 20° with the 1°×1° data. For computer time reasons the final grid was computed with half a degree spacing only. This grid extends from the Gulf of Mexico to the European and African coasts. Comparisons have been made with Geos 3 altimetry derived geoid heights and with the 5′×5′ gravimetric geoid derived byMarsh andChang [8] in the northwestern part of the Atlantic Ocean, which show a good agreement in most places apart from some tilts which porbably come from the satellite orbit recovery.  相似文献   

3.
Knudsen 《Journal of Geodesy》1987,61(2):145-160
The estimation of a local empirical covariance function from a set of observations was done in the Faeroe Islands region. Gravity and adjusted Seasat altimeter data relative to theGPM2 spherical harmonic approximation were selected holding one value in celles of1/8°×1/4° covering the area. In order to center the observations they were transformed into a locally best fitting reference system having a semimajor axis1.8 m smaller than the one ofGRS80. The variance of the data then was273 mgal 2 and0.12 m 2 respectively. In the calculations both the space domain method and the frequency domain method were used. Using the space domain method the auto-covariances for gravity anomalies and geoid heights and the cross-covariances between the quantities were estimated. Furthermore an empirical error estimate was derived. Using the frequency domain method the auto-covariances of gridded gravity anomalies was estimated. The gridding procedure was found to have a considerable smoothing effect, but a deconvolution made the results of the two methods to agree. The local covariance function model was represented by a Tscherning/Rapp degree-variance model,A/((i−1)(i−2)(i+24))(R B /R E )2i+2, and the error degree-variances related to the potential coefficient setGPM2. This covariance function was adjusted to fit the empirical values using an iterative least squares inversion procedure adjusting the factor A, the depth to the Bjerhammar sphere(R E −R B ), and a scale factor associated with the error degree-variances. Three different combinations of the empirical covariance values were used. The scale factor was not well determined from the gravity anomaly covariance values, and the depth to the Bjerhammar sphere was not well determined from geoid height covariance values only. A combination of the two types of auto-covariance values resulted in a well determined model.  相似文献   

4.
This research deals with some theoretical and numerical problems of the downward continuation of mean Helmert gravity disturbances. We prove that the downward continuation of the disturbing potential is much smoother, as well as two orders of magnitude smaller than that of the gravity anomaly, and we give the expression in spectral form for calculating the disturbing potential term. Numerical results show that for calculating truncation errors the first 180∘ of a global potential model suffice. We also discuss the theoretical convergence problem of the iterative scheme. We prove that the 5′×5′ mean iterative scheme is convergent and the convergence speed depends on the topographic height; for Canada, to achieve an accuracy of 0.01 mGal, at most 80 iterations are needed. The comparison of the “mean” and “point” schemes shows that the mean scheme should give a more reasonable and reliable solution, while the point scheme brings a large error to the solution. Received: 19 August 1996 / Accepted: 4 February 1998  相似文献   

5.
About half a million marine gravity measurements over a 30∘×30∘ area centered on Japan have been processed and adjusted to produce a new free-air gravity map from a 5′×5′ grid. This map seems to have a better resolution than those previously published as measured by its correlation with bathymetry. The grid was used together with a high-degree and -order spherical harmonics geopotential model to compute a detailed geoid with two methods: Stokes integral and collocation. Comparisons with other available geoidal surfaces derived either from gravity or from satellite altimetry were made especially to test the ability of this new geoid at showing the sea surface topography as mapped by the Topex/Poseidon satellite. Over 2 months (6 cycles) the dynamic topography at ascending passes in the region (23∘47∘N and 123∘147∘E) was mapped to study the variability of the Kuroshio current. Received: 15 July 1994 / Accepted: 17 February 1997  相似文献   

6.
 Global mean sea surface heights (SSHs) and gravity anomalies on a 2′×2′ grid were determined from Seasat, Geosat (Exact Repeat Mission and Geodetic Mission), ERS-1 (1.5-year mean of 35-day, and GM), TOPEX/POSEIDON (T/P) (5.6-year mean) and ERS-2 (2-year mean) altimeter data over the region 0∘–360∘ longitude and –80∘–80∘ latitude. To reduce ocean variabilities and data noises, SSHs from non-repeat missions were filtered by Gaussian filters of various wavelengths. A Levitus oceanic dynamic topography was subtracted from the altimeter-derived SSHs, and the resulting heights were used to compute along-track deflection of the vertical (DOV). Geoidal heights and gravity anomalies were then computed from DOV using the deflection-geoid and inverse Vening Meinesz formulae. The Levitus oceanic dynamic topography was added back to the geoidal heights to obtain a preliminary sea surface grid. The difference between the T/P mean sea surface and the preliminary sea surface was computed on a grid by a minimum curvature method and then was added to the preliminary grid. The comparison of the NCTU01 mean sea surface height (MSSH) with the T/P and the ERS-1 MSSH result in overall root-mean-square (RMS) differences of 5.0 and 3.1 cm in SSH, respectively, and 7.1 and 3.2 μrad in SSH gradient, respectively. The RMS differences between the predicted and shipborne gravity anomalies range from 3.0 to 13.4 mGal in 12 areas of the world's oceans. Received: 26 September 2001 / Accepted: 3 April 2002 Correspondence to: C. Hwang Acknowledgements. This research is partly supported by the National Science Council of ROC, under grants NSC89-2611-M-009-003-OP2 and NSC89-2211-E-009-095. This is a contribution to the IAG Special Study Group 3.186. The Geosat and ERS1/2 data are from NOAA and CERSAT/France, respectively. The T/P data were provided by AVISO. The CLS and GSFC00 MSS models were kindly provided by NASA/GSFC and CLS, respectively. Drs. Levitus, Monterey, and Boyer are thanked for providing the SST model. Dr. T. Gruber and two anonymous reviewers provided very detailed reviews that improved the quality of this paper.  相似文献   

7.
Sampling Design for Global Scale Mapping and Monitoring of Agriculture   总被引：2，自引：0，他引：2  
Gathering timely information of the global agriculture production of major and commercially important crops has become essential with globalization of the agriculture commodities. Remote sensing based crop production forecasting and monitoring is emerging as one of the most viable solutions for such large area monitoring task. A suitable sampling strategy is the basic requirement towards this. In the present study, different sampling sizes using agricultural area as the sampling frame has been used to analyse the optimum sampling size for continent level assessment. Land use/cover map of the world using 300 m resolution MERIS data was used to generate the agriculture area mask. Grid size of (i) 5° × 5° (ii) 1° × 1° (iii) 30′ × 30′ (iv) 15′ × 15′ (v) 7.5′ × 7.5′ and (vi) 5′ × 5′ were used. Percent crop area was estimated for the grids of all sizes. The grid size of 15′ × 15′ was found to be optimum for global monitoring, as not much change Ws observed in the distribution of the grids after reducing the sample size. Stratification was done using simple random and stratified random sampling method. Stratification using the ‘cumulative square-root of frequency method that resulted in five strata performed best in terms of the variance of the population.  相似文献   

8.
On the basis of gravity field model （EIGEN_CG01C）, together with multi-altimeter data, the improved deflection of the vertical gridded in 2＇×2＇ in China marginal sea and gridded in 5＇×5＇ in the global sea was determined by using the weighted method of along-track least squares, and the accuracy is better than 1.2^# in China marginal sea. As for the quality of the deflection of the vertical, it meets the challenge for the gravity field of high resolution and accuracy, it shows that, compared with the shipboard gravimetry in the sea, the accuracy of the gravity anomalies computed with the marine deflection of the vertical by inverse Vening-Meinesz formula is 7.75 m.s ^-2.  相似文献   

9.
Since the publication of the Earth gravitational model (EGM)96 considerable improvements in the observation techniques resulted in the development of new improved models. The improvements are due to the availability of data from dedicated gravity mapping missions (CHAMP, GRACE) and to the use of 5′ × 5′ terrestrial and altimetry derived gravity anomalies. It is expected that the use of new EGMs will further contribute to the improvement of the resolution and accuracy of the gravity and geoid modeling in continental and regional scale. To prove this numerically, three representative Earth gravitational models are used for the reduction of several kinds of data related to the gravity field in different places of the Earth. The results of the reduction are discussed regarding the corresponding covariance functions which might be used for modeling using the least squares collocation method. The contribution of the EIGEN-GL04C model in most cases is comparable to that of EGM96. However, the big difference is shown in the case of EGM2008, due not only to its quality but obviously to its high degree of expansion. Almost in all cases the variance and the correlation length of the covariance functions of data reduced to this model up to its maximum degree are only a few percentages of corresponding quantities of the same data reduced up to degree 360. Furthermore, the mean value and the standard deviation of the reduced gravity anomalies in extended areas of the Earth such as Australia, Arctic region, Scandinavia or the Canadian plains, vary between −1 and +1 and between 5 and 10 × 10−5 ms−2, respectively, reflecting the homogenization of the gravity field on a regional scale. This is very important in using least squares collocation for regional applications. However, the distance to the first zero-value was in several cases much longer than warranted by the high degree of the expansion. This is attributed to errors of medium wavelengths stemming from the lack of, e.g., high-quality data in some area.  相似文献   

10.
A 2×2 arc-minute resolution geoid model, CARIB97, has been computed covering the Caribbean Sea. The geoid undulations refer to the GRS-80 ellipsoid, centered at the ITRF94 (1996.0) origin. The geoid level is defined by adopting the gravity potential on the geoid as W 0=62 636 856.88 m2/s2 and a gravity-mass constant of GM=3.986 004 418×1014 m3/s2. The geoid model was computed by applying high-frequency corrections to the Earth Gravity Model 1996 global geopotential model in a remove-compute-restore procedure. The permanent tide system of CARIB97 is non-tidal. Comparison of CARIB97 geoid heights to 31 GPS/tidal (ITRF94/local) benchmarks shows an average offset (h–H–N) of 51 cm, with an Root Mean Square (RMS) of 62 cm about the average. This represents an improvement over the use of a global geoid model for the region. However, because the measured orthometric heights (H) refer to many differing tidal datums, these comparisons are biased by localized permanent ocean dynamic topography (PODT). Therefore, we interpret the 51 cm as partially an estimate of the average PODT in the vicinity of the 31 island benchmarks. On an island-by-island basis, CARIB97 now offers the ability to analyze local datum problems which were previously unrecognized due to a lack of high-resolution geoid information in the area. Received: 2 January 1998 / Accepted: 18 August 1998  相似文献   

11.
An intrresting variation on the familiar method of determining the earth's equatorial radius ae, from a knowledge of the earth's equatorial gravity is suggested. The value of equatorial radius thus found is 6378,142±5 meters. The associated parameters are GM=3.986005±.000004 × 1020 cm3 sec-−2 which excludes the relative mass of atmosphere ≅10−6 ξ GM, the equatorial gravity γe 978,030.9 milligals (constrained in this solution by the Potsdam Correction of 13.67 milligals as the Potsdam Correction is more directly, orless indirectly, measurable than the equatorial gravity) and an ellipsoidal flattening of f=1/298.255.  相似文献   

12.
Based on the gravity field models EGM96 and EIGEN-GL04C, the Earth＇s time-dependent principal moments of inertia A, B, C are obtained, and the variable rotation of the Earth is determined. Numerical results show that A, B, and C have increasing tendencies; the tilt of the rotation axis increases 2.1×10^ 8 mas/yr; the third component of the rotational angular velocity, ω3 , has a decrease of 1.0×10^ 22 rad/s^2, which is around 23% of the present observed value. Studies show in detail that both 0 and ω3 experience complex fluctuations at various time scales due to the variations of A, B and C.  相似文献   

13.
When planning a satellite gravity gradiometer (SGG) mission, it is important to know the quality of the quantities to be recovered at ground level as a function of e.g. satellite altitude, data type and sampling rate, and signal variance and noise. This kind of knowledge may be provided either using the formal error estimates of wanted quantities using least-squares collocation (LSC) or by comparing simulated data at ground level with results computed by methods like LSC or Fast Fourier Transform (FFT). Results of a regional gravity field recovery in a 10o×20o area surrounding the Alps using LSC and FFT are reported. Data used as observations in satellite altitude (202 or161 km) and for comparison at ground level were generated using theOSU86F coefficient set, complete to degree 360. These observations are referred to points across simulated orbits. The simulated quantities were computed for a 45 days mission period and 4 s sampling. A covariance function which also included terms above degree 360 was used for prediction and error estimation. This had the effect that the formal error standard deviation for gravity anomalies were considerably larger than the standard deviations of predicted minus simulated quantities. This shows the importance of using data with frequency content above degree 360 in simulation studies. Using data at202 km altitude the standard deviation of the predicted minus simulated data was equal to8.3 mgal for gravity and0.33 m for geoid heights.  相似文献   

14.
The Global Positioning System (GPS) is considered in conjunction with a strapdown Inertial Measurement Unit (IMU) for measuring the gravity vector. A comparison of this system in space and on an airborne platform shows the relative importance of each system element in these two different acceleration environments. With currently available instrumentation, the acceleration measurement accuracy is the deciding factor in space, while on an Earth-bound (including airborne) platform, the attitude error of the IMU is most critical. A simulation shows that GPS-derived accelerations in space can be accurate to better than 0.1mgal for a 30s integration time, leading to estimates of 1° mean gravity anomalies on the Earth's surface with an accuracy of 4–5 mgal. On an airborne platform, the horizontal gravity estimation error is tightly coupled to the attitude error of the platform, which can only be bounded by external attitude updates. Horizontal gravity errors of 5mgal are achievable if the attitude is maintained to an accuracy of 1arcsec.  相似文献   

15.
Modern models of the Earth's gravity field are developed in the IERS (International Earth Rotation Service) terrestrial reference frame. In this frame the mean values for gravity coefficients of the second degree and first order, C 21(IERS) and S 21(IERS), by the current IERS Conventions are recommended to be calculated by using the observed polar motion parameters. Here, it is proved that the formulae presently employed by the IERS Conventions to obtain these coefficients are insufficient to ensure their values as given by the same source. The relevant error of the normalized mean values for C 21(IERS) and S 21(IERS) is 3×10−12, far above the adopted cutoff (10−13) for variations of these coefficients. Such an error in C 21 and S 21 can produce non-modeled perturbations in motion prediction of certain artificial Earth satellites of a magnitude comparable to the accuracy of current tracking measurements. Received: 14 September 1998 / Accepted: 20 May 1999  相似文献   

16.
A computer-efficient global data file, which contains digitized information that enables identification of a given latitude/longitude defined point as over land or over water, was generated from a data base which defines the world's shoreline. The method used in the generation of this land-sea boundary data map and its data structure are discussed. The data file was originally generated on a Control Data Corporation(CDC) computer, but it has been transported to other computer systems, includingIBM, DEC/VAX, UNIVAC and Cray computers. The land-sea boundary map also includes information on islands and inland lakes. The resolution of this map is 5′×5′ or an equivalent of9 km square surface blocks at the equator. The software to access this data base is structured to be easily transportable to different computers. This data base was used in the generation of the Seasat Geophysical Data Record(GDR) to identify whether a spaceborne radar altimeter measurement was over-land or over-ocean.  相似文献   

17.
Crustal data of surface elevations and depth of Moho (and densities) can be utilized to form model-earth anomalies. These model-anomalies can closely approximate the free-air anomaly field of the earth, and could thus be used to predict the latter. A review of several such models is presented, with some elaboration on model developments, procedures, data analysis and accuracies. One of the models approaches a prediction accuracy of ±10 mgal for5°×5° mean free-air anomalies, whose r.m.s. value was about30% higher.  相似文献   

18.
BOOK REVIEWS     
《The Photogrammetric Record》2000,16(96):1037-1037
Book review in this Article GEOGRAPHICAL INFORMATION SYSTEMS AND THE LAW: MAPPING THE LEGAL FRONTIERS. By G. Cho . John Wiley & Sons Ltd., Chichester, 1998. ISBN 0 471 94857 8. 158 × 236 mm. xix + 337 pages. 19 figures and 7 tables. Price £70 hardback. UNDERSTANDING SYNTHETIC APERTURE RADAR IMAGES. By C. J. Oliver and S. Quegan . Artech House, Inc., Norwood, USA, 1998. ISBN 0 89006 850 X. 160 × 235 mm. xxvii + 479 pages. 125 diagrams and illustrations. Price £79 hardback. REMOTE SENSING OF TROPICAL REGIONS. By E. A. Sharkov . John Wiley & Sons Ltd., in association with Praxis Publishing Ltd., Chichester, 1998. ISBN 0 471 97171 5. 172 × 247 mm. xvii + 310 pages, plus 8 pages of colour plates. 78 figures and 24 tables. Price £65 hardback. ELEMENTS OF PHOTOGRAMMETRY, WITH APPLICATIONS IN GIS. By P. R. Wolf and B. A. Dewitt . McGraw Hill, New York, 2000. ISBN 0 07 292454 3. 160 × 235 mm. xiii + 608 pages. Illustrated. Price US$60 hardback. THE WESTERN FRONT FROM THE AIR. By N. Watkis . Sutton Publishing, Stroud, 1999. ISBN 0 7509 1338. 200 × 270 mm. ix + 130 pages. Illustrated. Price £20 hardback. ADVANCES IN REMOTE SENSING AND GIS ANALYSIS. Edited by P. M. Atkinson and N. J. Tate . John Wiley & Sons Ltd., Chichester, 1999. ISBN 0 471 98577 5. 174 × 251 mm. xiv + 273 pages, plus 4 pages of colour plates. Illustrated. Price £55 hardback. MANUAL OF REMOTE SENSING, THIRD EDITION, VOLUME 3. REMOTE SENSING FOR THE EARTH SCIENCES. Edited by A. N. Rencz . John Wiley & Sons Inc., New York, USA, in co‐operation with the American Society for Photogrammetry and Remote Sensing, 1999. ISBN 0471 29405 5. 184 × 260mm. xvi + 707 pages. 280 illustrations and tables and 53 colour plates. Price £128 hardback. MANAGING NATURAL RESOURCES WITH GIS. By L. Lang . SERVING MAPS ON THE INTERNET. By C. Harder . Environmental Systems Research Institute Inc., Redlands, California, USA, 1998. ISBN 1 879102 53 6 and ISBN 1 879102 52 8, respectively. Both 229 × 189 mm. ix + 117 pages and ix + 130 pages, respectively, both plus CD‐ROM. Illustrated. Price US$19.95 each, paperback. PASSIVE MICROWAVE REMOTE SENSING OF OCEANS. By I. V. Cherny and V. Yu Raizer . John Wiley & Sons Ltd., in association with Praxis Publishing Ltd., Chichester, 1998. ISBN 0 471 97170 7. 172 × 247mm. viii + 195 pages. 127 illustrations, including 8 pages of colour plates. Price £55 hardback. VIDEO CAMERA TECHNOLOGY. By A. C. Luther . Artech House Inc., Norwood, USA, 1998. ISBN 0 89006 556 X. 160 × 235 mm. xvi + 312 pages. Black and white illustrations. Price £59 hardback. DIGITAL PHOTOGRAMMETRY, VOLUME I. By T. Schenk . TerraScience, Laurelville, U.S.A, 1999. ISBN 0 9677653 0 7 perfect bound; ISBN 0 9677653 1 5 case bound. 235 × 162mm. xx + 421 pages. 179 figures and 15 tables. Price US$49 perfect bound, US$69 case bound, plus package and postage. INFINITE PERSPECTIVES: TWO THOUSAND YEARS OF THREE‐DIMENSIONAL MAP‐MAKING. By B. M. Ambroziak and J. R. Ambroziak , with an introduction by R. Bradbury . Princeton Architectural Press, New York, 1999. ISBN 1 56898 195 3. 382 × 325mm. 111 pages. 100 colour illustrations. Price £55 hardback. EVEREST—THE MAN AND THE MOUNTAIN. By J. R. Smith . Whittles Publishing, Caithness, 1999. ISBN 1 870325 72 9. 170 × 240mm. xiv + 306 pages. 45 figures and 5 appendices. Price £37.50 paperback. TEXT BOOKS ON REMOTE SENSING AND GIS. Remote Sensing Notes edited by Japan Association of Remote Sensing . GIS Notes by S. Murai . Asian Center for Research on Remote Sensing, Thailand, 1999. 140 × 130mm. Price unknown, CD‐ROM. REMOTE SENSING CHANGE DETECTION. Edited by R. S. Lunetta and C. D. Elvidge . Ann Arbor Press, Michigan, USA, 1998. ISBN 1 57504 037 9. 183 × 260mm. xviii + 318 pages. 20 colour plates, plus numerous illustrations. Price US$69.95 hardback. NORFOLK FROM THE AIR. VOLUME 2. Edited by P. Wade ‐Martins . Photography by D. Edwards . Norfolk Museums Service, Norwich, 1999. ISBN 0 903101 68 8. 212 × 284mm. 148 pages. 135 half page aerial photographs, 89 in black and white. Price £15.95 paperback.  相似文献   

19.
The paper describes results obtained from the processing of 53 Geos-3 arcs of altimeter data obtained during the first weeks after the launch of the satellite in April, 1975. The measurement from the satellite to the ocean surface was used to obtain an approximate geoid undulation which was contaminated by long wavelength errors caused primarily by altimeter bias and orbit error. This long wavelength error was reduced by fitting with a low degree polynomial the raw undulation data to the undulations implied by the GEM 7 potential coefficients, in an adjustment process that included conditions on tracks that cross. The root mean square crossover discrepancy before this adjustment was ±12.4 meters while after the adjustment it was ±0.9 m. These adjusted undulations were used to construct a geoid map in the Geos-3 calibration area using a least squares filter to remove remaining noise in the undulations. Comparing these undulations to ones computed from potential coefficients and terrestrial gravity data indicates a mean difference of 0.25 m and a root mean square difference of ±1.92 m. The adjusted undulations were also used to estimate several 5o, 2o, and 1o anomalies using the method of least squares collocation. The resulting predictions agreed well with known values although the 1o x 1o anomalies could not be considered as reliably determined.  相似文献   

20.
The short wavelength geoid undulations, caused by topography, amount to several decimeters in mountainous areas. Up to now these effects are computed by means of digital terrain models in a grid of 100–500m. However, for many countries these data are not yet available or their collection is too expensive. This problem can be overcome by considering the special behaviour of the gravity potential along mountain slopes. It is shown that 90 per cent of the topographic effects are represented by a simple summation formula, based on the average height differences and distances between valleys and ridges along the geoid profiles, δN=[30.H.D.+16.(H−H′).D] in mm/km, (error<10%), whereH, H′, D are estimated in a map to the nearest 0.2km. The formula is valid for asymmetric sides of valleys (H, H′) and can easily be corrected for special shapes. It can be used for topographic refinement of low resolution geoids and for astrogeodetic projects. The “slope method” was tested in two alpine areas (heights up to 3800m, astrogeodetic deflection points every 170km 2) and resulted in a geoid accuracy of ±3cm. In first order triangulation networks (astro points every 1000km 2) or for gravimetric deflections the accuracy is about 10cm per 30km. Since a map scale of 1∶500.000 is sufficient, the method is suitable for developing countries, too.  相似文献   

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