| 1. |
- Eshagh, Mehdi, et al.
(författare)
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A Strategy for Optimum Designing of the Geodetic Networks from the Cost, Reliability and Precision Views
- 2007
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Ingår i: Acta Geodaetica et Geophysica Hungarica. - 1217-8977 .- 1587-1037. ; 42:3, s. 297-308
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Tidskriftsartikel (refereegranskat)abstract
- There are different criteria for designing a geodetic network in an optimal way. An optimum network can be regarded as a network having high precision, reliability and low cost. Accordingly, corresponding to these criteria different single-objective models can be defined. Each one can be subjected to two other criteria as constraints. Sometimes the constraints can be contradictory so that some of the constraints are violated. In this contribution, these models are mathematically reviewed. It is numerically shown how to prepare these mathematical models for optimization process through a simulated network. We found that the reliability model yields small position changes between those obtained using precision respectively. Elimination of some observations may happen using precision and cost model while the reliability model tries to save number of observations. In our numerical studies, no contradictions can be seen in reliability model and this model seems to be more suitable for designing of the geodetic and deformation networks.
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| 2. |
- Eshagh, Mehdi, 1977-, et al.
(författare)
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Evaluation of robust techniques in suppressing the impact of outliers in a deformation monitoring network – A case study on the Tehran Milad tower network
- 2007
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Ingår i: Acta Geodaetica et Geophysica Hungarica. - : Springer Netherlands. - 1217-8977 .- 1587-1037. ; 42:4, s. 449-463
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Tidskriftsartikel (refereegranskat)abstract
- The problem of handling outliers in a deformation monitoring network is of special importance, because the existence of outliers may lead to false deformation parameters. One of the approaches to detect the outliers is to use robust estimators. In this case the network points are computed by such a robust method, implying that the adjustment result is resisting systematic observation errors, and, in particular, it is insensitive to gross errors and even blunders. Since there are different approaches to robust estimation, the resulting estimated networks may differ. In this article, different robust estimation methods, such as the M-estimation of Huber, the “Danish”, and the L 1-norm estimation methods, are reviewed and compared with the standard least squares method to view their potentials to detect outliers in the Tehran Milad tower deformation network. The numerical studies show that the L 1-norm is able to detect and down-weight the outliers best, so it is selected as the favourable approach, but there is a lack of uniqueness. For comparison, Baarda’s method “data snooping” can achieve similar results when the outlier magnitude of an outlier is large enough to be detected; but robust methods are faster than the sequential data snooping process.
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| 3. |
- Eshagh, Mehdi
(författare)
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Impact of vectorization on global synthesis and analysis in gradiometry
- 2009
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Ingår i: Acta Geodaetica et Geophysica Hungarica. - 1217-8977. ; 44:3, s. 323-342
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Tidskriftsartikel (refereegranskat)abstract
- Computational time is an important matter in numerical aspects and it depends on the algorithm and computer that is used. An inappropriate algorithm can increase computation time and cost. The main goal of this paper is to present a vectorization algorithm to speed up the global gradiometric synthesis and analysis. The paper discusses details of this technique and its very high capabilities. Numerical computations show that the global gradiometric synthesis with 0.5 degrees x 0.5 degrees resolution can be done in a few minutes (6 minutes) by vectorization, which is considerable less compared to several hours (9 hours) by an inappropriate algorithm. The global gradiometric analysis of representation by spherical harmonics up to degree and order of 360, can be performed within one hour using vectorization, but if an inconvenient algorithm is used it can be delayed more than 1 day. Here we present the vectorization technique to gradiometric synthesis and analysis, but it can also be used in many other computational aspects and disciplines.
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| 4. |
- Eshagh, Mehdi, et al.
(författare)
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THE EFFECT OF GEOPOTENTIAL PERTURBATIONS OF GOCE ON ITS OBSERVATIONS - A NUMERICAL STUDY
- 2009
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Ingår i: ACTA GEODAETICA ET GEOPHYSICA HUNGARICA. - 1217-8977. ; 44:4, s. 385-398
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Tidskriftsartikel (refereegranskat)abstract
- In solution of gradiometric boundary value problem in space a regular grid of satellite gravity gradiometry data is required. This grid is considered on a sphere with radius of the mean Earth sphere and altitude of satellite. However, the gravitational gradients are measured by a gradiometer mounted on GOCE satellite and orbital perturbations of the satellite influence GOCE observations as well. In this study we present that these effects are about 2 E on GOCE data. Also numerical studies on the gravitational gradients in orbital frame show that the perturbations of co-latitude are more significant than that of inclination. The effect of perturbed inclination is less than -9 mE while the effect of perturbed co-latitude is within -173 mE in one day revolution of GOCE.
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| 5. |
- Eshagh, Mehdi
(författare)
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The Effect Of Lateral Density Variations Of Crustal And Topographic Masses On Goce Gradiometric Data - A Study In Iran And Fennoscandia
- 2009
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Ingår i: ACTA GEODAETICA ET GEOPHYSICA HUNGARICA. - 1217-8977. ; 44:4, s. 399-418
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Tidskriftsartikel (refereegranskat)abstract
- The topographic and atmospheric masses influence the satellite gravity gradiometry data, and it is necessary to remove these effects as precise as possible to make the computational space harmonic and simplify the downward continuation of such data. The topographic effects have been formulated based on constant density assumption for the topographic masses. However in this paper we formulate and study the effect of lateral density variation of crustal and topographic masses on the satellite gravity gradiometry data. Numerical studies over Fennoscandia and Iran show that the lateral density variation effect of the crust on GOCE data can reach to 1.5 E in Fennoscandia and 1 E in Iran. The maximum effect of lateral density variation of topography is 0.1 E and 0.05 E in Iran and Fennoscandia, respectively.
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| 6. |
- Kiamehr, Ramin
(författare)
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A new height datum for Iran based on combination of the Gravimetric and GPS/levelling geoid models
- 2007
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Ingår i: Acta Geodaetica et Geophysica. - 1217-8977. ; 42:1, s. 69-81
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Tidskriftsartikel (refereegranskat)abstract
- A new geoid model for Iran (IRG04) was computed based on the least squares modification of the Stokes formula. IRG04 was derived from the most recent gravity anomaly database, SRTM high resolution Digital Elevation Model (DEM) and GRACE GGM02 global geopotential model. In order to define a new height datum for Iran, we attempted to combine this high resolution gravimetric geoid model with GPS/levelling data using the corrective surface approach. The corrective surface was constructed from 224 GPS/levelling points and then evaluated with 35 independent points. Different interpolation techniques were tested for the creation of the corrective surface; among them the Kriging method was selected as it gave the smallest RMS and 'noise level' at the comparisons with GPS/levelling data. The RMS fit of the new combined geoid model versus the independent GPS/levelling data is 0.09 m, it is near four times better compared to the original gravimetric geoid model. The combined model should be more convenient and useful in definition of the new height reference surface, specifically in engineering and GPS/levelling projects.
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| 7. |
- Kiamehr, Ramin
(författare)
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Qualification and refinement of the gravity database based on cross-validation approach, A case study of Iran
- 2007
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Ingår i: Acta Geodaetica et Geophysica. - 1217-8977. ; 42:3, s. 285-295
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Tidskriftsartikel (refereegranskat)abstract
- In this article, we propose a technique for the precise cleaning of the gravity anomaly database based on the cross validation approach. The terrestrial gravity anomalies were compared versus a global geopotential model and take into account the effect of topography in this comparison. The efficiency of the cross-validation technique is illustrated in outlier detection as well as in choosing the proper gridding technique as a case study in construction of the Iranian new gravity database. In order to reduce the effect of topography and the discretisation error, a special interpolation scheme is used for gridding of the free-air gravity anomalies. The final grid file was created based on the Kriging method with 80" x 90" block resolution. The overall accuracy for the new Iranian gravity database is estimated in the order of 10 mGal.
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