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## Auteur Kurt Seitz |

### Documents disponibles écrits par cet auteur (5)

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[article]inJournal of geodesy > vol 96 n° 6 (June 2022) . - n° 39

Titre : The RTM harmonic correction revisited Type de document : Article/Communication Auteurs : R. Klees, Auteur ; Kurt Seitz, Auteur ; D.C. Slobbe, Auteur Année de publication : 2022 Article en page(s) : n° 39 Note générale : bibliographie Langues : Anglais ( eng)Descripteur : [Vedettes matières IGN] Géodésie physique

[Termes IGN] analyse harmonique

[Termes IGN] anomalie de pesanteur

[Termes IGN] Auvergne

[Termes IGN] correction des altitudes

[Termes IGN] géoïde local

[Termes IGN] harmonique sphérique

[Termes IGN] hauteur ellipsoïdale

[Termes IGN] méthode des moindres carrés

[Termes IGN] modèle de géopotentiel local

[Termes IGN] modèle numérique de terrain

[Termes IGN] Norvège

[Termes IGN] quasi-géoïde

[Termes IGN] résiduRésumé : (auteur) In this paper, we derive improved expressions for the harmonic correction to gravity and, for the first time, expressions for the harmonic correction to potential and height anomaly. They need to be applied at stations buried inside the masses to transform internal values into harmonically downward continued values, which are then input to local quasi-geoid modelling using least-squares collocation or least-squares techniques in combination with the remove-compute-restore approach. Harmonic corrections to potential and height anomaly were assumed to be negligible so far resulting in yet unknown quasi-geoid model errors. The improved expressions for the harmonic correction to gravity, and the new expressions for the harmonic correction to potential and height anomaly are used to quantify the approximation errors of the commonly used harmonic correction to gravity and to quantify the magnitude of the harmonic correction to potential and height anomaly. This is done for two test areas with different topographic regimes. One comprises parts of Norway and the North Atlantic where the presence of deep, long, and narrow fjords suggest extreme values for the harmonic correction to potential and height anomaly and corresponding large errors of the commonly used approximation of the harmonic correction to gravity. The other one is located in the Auvergne test area with a moderate topography comprising both flat and hilly areas and therefore may be representative for many areas around the world. For both test areas, two RTM surfaces with different smoothness are computed simulating the use of a medium-resolution and an ultra-high-resolution reference gravity field, respectively. We show that the errors of the commonly used harmonic correction to gravity may be as large as the harmonic correction itself and attain peak values in areas of strong topographic variations of about 100 mGal. Moreover, we show that this correction may introduce long-wavelength biases in the computed quasi-geoid model. Furthermore, we show that the harmonic correction to height anomaly can attain values on the order of a decimetre at some points. Overall, however, the harmonic correction to height anomaly needs to be applied only in areas of strong topographic variations. In flat or hilly areas, it is mostly smaller than one centimetre. Finally, we show that the harmonic corrections increase with increasing smoothness of the RTM surface, which suggests to use a RTM surface with a spatial resolution comparable to the finest scales which can be resolved by the data rather than depending on the resolution of the global geopotential model used to reduce the data. Numéro de notice : A2022-414 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s00190-022-01625-w Date de publication en ligne : 30/05/2022 En ligne : https://doi.org/10.1007/s00190-022-01625-w Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=100769 [article]Comparison among three harmonic analysis techniques on the sphere and the ellipsoid / Hussein Abd-Elmotaalin Journal of applied geodesy, vol 8 n° 1 (April 2014)

[article]inJournal of applied geodesy > vol 8 n° 1 (April 2014) . - pp 1 - 19

Titre : Comparison among three harmonic analysis techniques on the sphere and the ellipsoid Type de document : Article/Communication Auteurs : Hussein Abd-Elmotaal, Auteur ; Kurt Seitz, Auteur ; Mostafa Abd-Elbaky, Auteur ; Bernhard Heck, Auteur Année de publication : 2014 Article en page(s) : pp 1 - 19 Note générale : Bibliographie Langues : Anglais ( eng)Descripteur : [Vedettes matières IGN] Géodésie physique

[Termes IGN] analyse comparative

[Termes IGN] anomalie de pesanteur

[Termes IGN] Earth Gravity Model 2008

[Termes IGN] ellipsoïde (géodésie)

[Termes IGN] harmonique ellipsoïdale

[Termes IGN] harmonique sphérique

[Termes IGN] méthode des moindres carrés

[Termes IGN] transformation rapide de FourierRésumé : (Auteur) The paper presents a comparison among three different techniques for harmonic analysis on the sphere and the ellipsoid. The EGM2008 global geopotential model has been used up to degree and order 360 in order to create gravity anomaly fields on both the sphere and the ellipsoid as the function fields of the current investigation. Harmonic analysis has then been carried out to compute the dimensionless potential coeficients using the created function fields. Three different harmonic analysis techniques have been applied: the least-squares technique, the Fast Fourier Transform (FFT) technique and the Gauss-Legendre numerical integration technique. The computed coeficients in spherical harmonics have then been compared with EGM2008 (in the frequency domain) and the computed fields on the sphere and the ellipsoid have been compared with fields created by EGM2008 up to degree and order 360 (in the space domain) in order to estimate the accuracy of the three different harmonic analysis techniques used within the current investigation. The results proved that the least-squares technique gives the best accuracy both in frequency and space domain. The FFT technique provides quite good results in a very short cpu time. The Gauss-Legendre technique gives the worst results among the presented techniques, but still the residuals in the space domain are negligibly small. Numéro de notice : A2014-270 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1515/jag-2013-0008 En ligne : http://www.degruyter.com/view/j/jag.2014.8.issue-1/jag-2013-0008/jag-2013-0008.x [...] Format de la ressource électronique : URL Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=33173 [article]## Réservation

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Code-barres Cote Support Localisation Section Disponibilité 158-2014011x SL Revue Centre de documentation Revues en salle DisponibleOptimized formulas for the gravitational field of a tesseroid / Thomas Grombeinin Journal of geodesy, vol 87 n° 7 (July 2013)

[article]inJournal of geodesy > vol 87 n° 7 (July 2013) . - pp 645 - 660

Titre : Optimized formulas for the gravitational field of a tesseroid Type de document : Article/Communication Auteurs : Thomas Grombein, Auteur ; Kurt Seitz, Auteur ; Bernard Heck, Auteur Année de publication : 2013 Article en page(s) : pp 645 - 660 Note générale : Bibliographie Langues : Anglais ( eng)Descripteur : [Vedettes matières IGN] Géodésie physique

[Termes IGN] champ de pesanteur terrestre

[Termes IGN] coordonnées cartésiennes géocentriques

[Termes IGN] coordonnées sphériques

[Termes IGN] intégrale de Newton

[Termes IGN] tesseroidRésumé : (Auteur) Various tasks in geodesy, geophysics, and related geosciences require precise information on the impact of mass distributions on gravity field-related quantities, such as the gravitational potential and its partial derivatives. Using forward modeling based on Newton’s integral, mass distributions are generally decomposed into regular elementary bodies. In classical approaches, prisms or point mass approximations are mostly utilized. Considering the effect of the sphericity of the Earth, alternative mass modeling methods based on tesseroid bodies (spherical prisms) should be taken into account, particularly in regional and global applications. Expressions for the gravitational field of a point mass are relatively simple when formulated in Cartesian coordinates. In the case of integrating over a tesseroid volume bounded by geocentric spherical coordinates, it will be shown that it is also beneficial to represent the integral kernel in terms of Cartesian coordinates. This considerably simplifies the determination of the tesseroid’s potential derivatives in comparison with previously published methodologies that make use of integral kernels expressed in spherical coordinates. Based on this idea, optimized formulas for the gravitational potential of a homogeneous tesseroid and its derivatives up to second-order are elaborated in this paper. These new formulas do not suffer from the polar singularity of the spherical coordinate system and can, therefore, be evaluated for any position on the globe. Since integrals over tesseroid volumes cannot be solved analytically, the numerical evaluation is achieved by means of expanding the integral kernel in a Taylor series with fourth-order error in the spatial coordinates of the integration point. As the structure of the Cartesian integral kernel is substantially simplified, Taylor coefficients can be represented in a compact and computationally attractive form. Thus, the use of the optimized tesseroid formulas particularly benefits from a significant decrease in computation time by about 45 % compared to previously used algorithms. In order to show the computational efficiency and to validate the mathematical derivations, the new tesseroid formulas are applied to two realistic numerical experiments and are compared to previously published tesseroid methods and the conventional prism approach. Numéro de notice : A2013-399 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1007/s00190-013-0636-1 Date de publication en ligne : 18/05/2013 En ligne : https://doi.org/10.1007/s00190-013-0636-1 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=32537 [article]## Réservation

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Code-barres Cote Support Localisation Section Disponibilité 266-2013071 SL Revue Centre de documentation Revues en salle DisponibleEllipsoidische und topographische Effekte im geodätischen Randwertproblem / Kurt Seitz (1997)

Titre : Ellipsoidische und topographische Effekte im geodätischen Randwertproblem Titre original : [Les effets ellipsoïdiques et topographiques dans le problème géodésique de valeurs limites] Type de document : Thèse/HDR Auteurs : Kurt Seitz, Auteur Editeur : Munich : Bayerische Akademie der Wissenschaften Année de publication : 1997 Collection : DGK - C Sous-collection : Dissertationen num. 483 Importance : 139 p. Format : 21 x 30 cm ISBN/ISSN/EAN : 978-3-7696-9523-6 Note générale : Bibliographie Langues : Allemand ( ger)Descripteur : [Vedettes matières IGN] Géodésie physique

[Termes IGN] analyse harmonique

[Termes IGN] approximation

[Termes IGN] champ de pesanteur terrestre

[Termes IGN] linéarisation

[Termes IGN] potentiel de pesanteur terrestre

[Termes IGN] problème des valeurs limitesRésumé : (Editeur) To determine both the external gravity potential W and the geometry of the earth's surface 5, various boundary value problems (bvp) can be formulated. They depend on the utilised observaWes L and whether the boundary is supposed to be known or unknown. If the geometry of 5 is already determined by the classical terrestrial techniques or by methods of satellite geodesy, then the fixed boundary value problem is under consideration. Otherwise the resulting bvp is of free type. The relation between the unknowns W, S and the observables L is given by boundary conditions. Generally, they are of non-linear structure.

This thesis focuses on the scalar free bvp and the fixed gravimetric bvp.

Analytical and numerical aspects require the linearisation of the boundary conditions. Therefore, suitable approximations have to be introduced for the unknowns. For the gravity potential, the normal gravity potential w is introduced. In the case of the scalar free boundary value problem, the boundary surface is approximated by the telluroid s, resulting from some telluroid mapping. In this context, the gravity potential W is substituted by the disturbing potential 6w := W w.

As shown in previous studies (Heck and Seitz, 1993, 1995; Seitz et al., 1994), the non-linear terms in the boundary condition can amount to maximum values of ±200 10~8ma~2 in case of the scalar free bvp, and ±40 10~8ms~2 for the fixed gravimetric problem. As a consequence of omitting the non-linear terms, the equipotential surfaces of the solution for the disturbing potential are shifted in vertical direction. Utilising a normal gravity field of Somigiiana-Pizzetti-type (e.g. GRS80), this shift takes values up to 4mm. If the gravitational portion of the normal gravity field is represented by a truncated series of spherical harmonics (e.g. Nv = 20), this effect can be reduced to 2mm. In the vicinity of the earth's physical surface, the accuracy of present day global solutions of the earth's gravity potential varies in the scope of dm-m. Therefore, the non-linear effects in the boundary condition can be neglected for the purpose of this thesis.

f

The linear boundary condition is now based on a linear operator D acting on the disturbing potential 6w. Applying the evaluation operator E,, the resulting linear functional is restricted to the telluroid. In case of the fixed bvp, the evaluation operator es is applied to D{Sw} and restricts the boundary condition to the physical earth's surface. The coefficients of the differential operator D are functional of the normal gravity potential w. Introducing an approximation wa of the normal gravity potential tu, the coefficients of D can be represented in terms of a Taylor series expansion.

For the scalar free boundary value problem as well as for the fixed gravimetric bvp the second order approximation of the differential operator D is derived. Thereby, the approximation wa of the normal gravity potential is represented by the potential of a Helmert's normal spheroid. This potential is symmetrical with respect to the mean earth's equatorial plane and independent of the geocentric longitude. In addition to the centrifugal part, this normal potential contains the isotropic term fj?/r and, furthermore, two gravitational terms, which are proportional to the zonal harmonic coefficients «/2 and «/4.

The analytical representation of the differential operator is carried out on the basis of wa. The extension of wa by the coefficient J± leads to the second order approximation of D. Further simplifications of wa result in the first order approximation and the isotropic approximation of D{6w} (fundamental equation of physical geodesy). This evaluation is done for a general normal potential and a Somigliana-Pizzetti reference field respectively.

The numerical studies clearly show that the error in the boundary condition, caused by isotropic approximation, exceeds the accuracy of relative gravimetric measurements by the multiple. It is also larger, by a factor of 5-10, than the non-linear terms in the corresponding boundary condition. According to this result, the ellipsoidal terms must not be neglected, as it is done in the isotropic approximation. If a Somigiiana-Pizzetti-field is used as normal field w, the first order approximation of the linear boundary condition produces a maximum error of ±0.5 10~8ms~2. The analytical

approximation of the differential operator finds its qualitative end, if the second order ellipsoidal terms, which are caused by the zonal coefficient J^, are taken into account. The errors due to the resulting second order approximation of the ellipsoidal terms in the linear boundary condition can be neglected in practice. The residuals are less than ±0.01 10~8ms~2.

The properties are different, when the normal field w contains tessera! and sectorial terms. In the present thesis, the normal gravity potential is given in terms of spherical harmonics, with a maximum degree Nv = 20. The apposition of the zonal coefficient J^ in the approximated normal field wa gives no better results for the approximated ellipsoidal terms. The maximum residuals are ±7 10~8m.s~2 for both levels of analytical approximation, the first order and the second order approximation. Even if second order tesseral and sectorial terms are included in w0, the accuracy of the approximation cannot be improved. According to the magnitude of the low degree spherical harmonic coefficients, all terms (e.g. up to degree and order 4) have to be included into the analytical approximation of the ellipsoidal terms in the boundary condition. The necessary analytical and numerical expenditure cannot be justified. Because of the slowly decreasing spectrum of this normal gravity potential, it must be concluded that the maximum degree Nv of the spherical harmonic representation has to be in accordance with the order of analytical approximation.

To solve the fixed or scalar free boundary value problem by the aid of a harmonic analysis, the respec--tive boundary condition must be related to a geometrically defined surface of revolution, symmetrical to the mean earth's rotation axis. In this context, a sphere K 9 k with radius r = a and the surface E 9 e of an ellipsoid of revolution is considered. The explained algorithm used for the harmonic ana-lysis results in the spherical harmonic spectrum, whether the (reduced) boundary data are given on K 3 k or E 9 e.

The analytical continuation of the boundary condition is realised by a formal Taylor series expansion of the evaluation operator E, or es respectively. Additionally, the optimal choice of the Taylor point has to be discussed. The Taylor point can be chosen at the auxiliary surface on which the data are analyticaly continued (sphere or surface of an ellipsoid). The Taylor point can also be situated at the original boundary surface. In case of the fixed bvp it can be situated at the physical earth's surface S 9 P, and on the telluroid s 3 p in case of the scalar free boundary value problem. The order of the Taylor series expansion of the evaluation operator is set up in accordance with the error level of the second order analytical approximation of the boundary condition. By this consideration the absolute error level of 10~8ms~2, which is aspired for the analytical approximation of the differential operator, is assigned to the analytically continued boundary condition. For each boundary condition we end up with a representation, where the analytical side contains an isotropic term and so-called ellipsoidal and topographical constituents. The isotropic term refers to the surface K or E. The ellipsoidal and topographical terms in the boundary condition are caused by the anisotropy of the normal potential and by the difference between the boundary surface and the auxiliary surface (K or E), on which the boundary condition is continued. They have to be evaluated either on the original boundary surface (S 3 P - fixed bvp; s 9 p - scalar free bvp) or on the auxiliary surface (K 9 k or E B e) according to the choice of the Taylor point. The ellipsoidal and topographical terms can be applied as a reduction to the original boundary data. The result is a spherical boundary value problem on the sphere K (Neumann-problem in the case of the fixed bvp), or rather a quasi-isotropic bvp with an ellipsoidal boundary surface. If the complete ellipsoidal and topographical terms are neglected, the constant radius approximation will be obtained. The reduction terms are functionals of the disturbing potential Sw we solve for. Therefore, an iterative procedure is required.

To guarantee an absolute accuracy of 1 lQ~8ms~2 for the continuation terms (ellipsoidal and topogra--phical terms), the partial derivatives of the disturbing potential are considered up to the 8th order. The partial derivatives of

j%- are considered up to the 4th order, with respect to the geocentric distance r. The numerical studies confirmed that this high degree of evaluation is necessary in the framework of the analytical continuation to the sphere. Otherwise, the level of accuracy obtained for the ellipsoidal terms cannot be reached for the continuation terms. If the analytical continuation is performed to the surface of an ellipsoid, the Taylor series can be truncated after the 5th order terms without anyloss of accuracy. The accuracy of approximation is not influenced by the choice of the Taylor point. The extremal values of the whole ellipsoidal and topographical part of the linear boundary condition, that has to be modelled by the analytical representation, decreases from ±100 10~5ms~2 in the case of a sphere to ±20 10~5ms~2, if the boundary operator is continued to the surface of an ellipsoid. These terms are identical with the neglected terms in case of the spherical approximation on K or the isotropic approximation on E, respectively. They have the same order of magnitude as the boundary data itself. Undoubtedly, this kind of approximation, where all ellipsoidal and topographical terms are neglected, leads to no high-precision solution of the fixed or scalar free boundary value problem.

First of all, the error of approximation depends on the order of the Taylor series, which are setup for the analytical continuation of the boundary condition. Therefore, the surface of an ellipsoid should be favoured above the sphere. The modelling of the normal field has a minor effect. With regard to the analytical handling, a normal field of SomigJiana-Pizzetti-type should be preferred to a truncated sphe--rical harmonic axpansion. The error in the developed analytical representation of the ellipsoidal and topographical terms reaches maximum values of ±1 10~8ms~2, if the GRS80 (Somigliana-Pizzetti-field) is used as a normal field. If the normal gravity potential is modelled by a spherical harmonic expansion, (OSU91alf) truncated at degree Nv = 20, the approximation errors slightly increase up to ±7 10-8ms-2.

When no ellipsoidal and topographical reductions are applied to the scalar gravity anomalies and when this data is analysed on a sphere, the resulting effect in the solution of the disturbing potential increases to ±40m2s~2. This corresponds to a vertical shift up to ±4m of the equipotential surfaces in the vicinity of the earth's surface. If the unreduced scalar gravity anomalies are analysed on the surface of an ellipsoid, the corresponding effect will be ±2m.

The proposed iterative procedure for solving the linear boundary value problem is investigated with respect to its convergence behaviour. When the analytical continuation to the surface of an ellipsoid is applied, a strong convergence can be observed. Already after 6 iteration steps, the spectrum of the solved disturbing potential does not change any more. The spectrum of the residuals represents the error of the solution in the frequency domain. It is the difference between the solved and the given spectrum, which was used to calculate the boundary data. The residuals illustrate the error in the space domain. The resulting vertical shift of the equipotential surfaces in the vicinity of the earth's physical surface reaches a maximum of ±4mm.

In case of the analytical continuation to a sphere, the iterative approach diverges. At first, the residual spectrum is decreasing within the scope of low and medium frequencies. Simultaneously, the error in the high frequency coefficients is increasing. The spectrum deteriorates in the whole frequency domain after about 5 iteration steps.

Only if the reduction terms (ellipsoidal and topographical terms) are generated in the first iteration step by the use of the given (exact) disturbing potential, convergence will be obtained. This was done in order to check the algorithms. For that reason it is supposed that the inaccurate initial solution is responsible for the divergent behaviour of the iterative process. It can be presumed that the analytical continuation to a sphere reacts more sensibly upon errors in the initial solution, than the analytical continuation to the surface of an ellipsoid. In this context, the problem of downward continuation is crucial, requiring further investigations.

(Editeur) To determine both the external gravity potential W and the geometry of the earth's surface 5, various boundary value problems (bvp) can be formulated. They depend on the utilised observaWes L and whether the boundary is supposed to be known or unknown. If the geometry of 5 is already determined by the classical terrestrial techniques or by methods of satellite geodesy, then the fixed boundary value problem is under consideration. Otherwise the resulting bvp is of free type. The relation between the unknowns W, S and the observables L is given by boundary conditions. Generally, they are of non-linear structure.

This thesis focuses on the scalar free bvp and the fixed gravimetric bvp.

Analytical and numerical aspects require the linearisation of the boundary conditions. Therefore, suitable approximations have to be introduced for the unknowns. For the gravity potential, the normal gravity potential w is introduced. In the case of the scalar free boundary value problem, the boundary surface is approximated by the telluroid s, resulting from some telluroid mapping. In this context, the gravity potential W is substituted by the disturbing potential 6w := W ? w.

As shown in previous studies (Heck and Seitz, 1993, 1995; Seitz et al., 1994), the non-linear terms in the boundary condition can amount to maximum values of ±200 10~8ma~2 in case of the scalar free bvp, and ±40 10~8ms~2 for the fixed gravimetric problem. As a consequence of omitting the non-linear terms, the equipotential surfaces of the solution for the disturbing potential are shifted in vertical direction. Utilising a normal gravity field of Somigiiana-Pizzetti-type (e.g. GRS80), this shift takes values up to 4mm. If the gravitational portion of the normal gravity field is represented by a truncated series of spherical harmonics (e.g. Nv = 20), this effect can be reduced to 2mm. In the vicinity of the earth's physical surface, the accuracy of present day global solutions of the earth's gravity potential varies in the scope of dm-m. Therefore, the non-linear effects in the boundary condition can be neglected for the purpose of this thesis.

f

The linear boundary condition is now based on a linear operator D acting on the disturbing potential 6w. Applying the evaluation operator E,, the resulting linear functional is restricted to the telluroid. In case of the fixed bvp, the evaluation operator es is applied to D{Sw} and restricts the boundary condition to the physical earth's surface. The coefficients of the differential operator D are functional of the normal gravity potential w. Introducing an approximation wa of the normal gravity potential tu, the coefficients of D can be represented in terms of a Taylor series expansion.

For the scalar free boundary value problem as well as for the fixed gravimetric bvp the second order approximation of the differential operator D is derived. Thereby, the approximation wa of the normal gravity potential is represented by the potential of a Helmert's normal spheroid. This potential is symmetrical with respect to the mean earth's equatorial plane and independent of the geocentric longitude. In addition to the centrifugal part, this normal potential contains the isotropic term fj?/r and, furthermore, two gravitational terms, which are proportional to the zonal harmonic coefficients «/2 and «/4.

The analytical representation of the differential operator is carried out on the basis of wa. The extension of wa by the coefficient J± leads to the second order approximation of D. Further simplifications of wa result in the first order approximation and the isotropic approximation of D{6w} (fundamental equation of physical geodesy). This evaluation is done for a general normal potential and a Somigliana-Pizzetti reference field respectively.

The numerical studies clearly show that the error in the boundary condition, caused by isotropic approximation, exceeds the accuracy of relative gravimetric measurements by the multiple. It is also larger, by a factor of 5-10, than the non-linear terms in the corresponding boundary condition. According to this result, the ellipsoidal terms must not be neglected, as it is done in the isotropic approximation. If a Somigiiana-Pizzetti-field is used as normal field w, the first order approximation of the linear boundary condition produces a maximum error of ±0.5 10~8ms~2. The analytical

approximation of the differential operator finds its qualitative end, if the second order ellipsoidal terms, which are caused by the zonal coefficient J^, are taken into account. The errors due to the resulting second order approximation of the ellipsoidal terms in the linear boundary condition can be neglected in practice. The residuals are less than ±0.01 10~8ms~2.

The properties are different, when the normal field w contains tessera! and sectorial terms. In the present thesis, the normal gravity potential is given in terms of spherical harmonics, with a maximum degree Nv = 20. The apposition of the zonal coefficient J^ in the approximated normal field wa gives no better results for the approximated ellipsoidal terms. The maximum residuals are ±7 10~8m.s~2 for both levels of analytical approximation, the first order and the second order approximation. Even if second order tesseral and sectorial terms are included in w0, the accuracy of the approximation cannot be improved. According to the magnitude of the low degree spherical harmonic coefficients, all terms (e.g. up to degree and order 4) have to be included into the analytical approximation of the ellipsoidal terms in the boundary condition. The necessary analytical and numerical expenditure cannot be justified. Because of the slowly decreasing spectrum of this normal gravity potential, it must be concluded that the maximum degree Nv of the spherical harmonic representation has to be in accordance with the order of analytical approximation.

To solve the fixed or scalar free boundary value problem by the aid of a harmonic analysis, the respec--tive boundary condition must be related to a geometrically defined surface of revolution, symmetrical to the mean earth's rotation axis. In this context, a sphere K 9 k with radius r = a and the surface E 9 e of an ellipsoid of revolution is considered. The explained algorithm used for the harmonic ana-lysis results in the spherical harmonic spectrum, whether the (reduced) boundary data are given on K 3 k or E 9 e.

The analytical continuation of the boundary condition is realised by a formal Taylor series expansion of the evaluation operator E, or es respectively. Additionally, the optimal choice of the Taylor point has to be discussed. The Taylor point can be chosen at the auxiliary surface on which the data are analyticaly continued (sphere or surface of an ellipsoid). The Taylor point can also be situated at the original boundary surface. In case of the fixed bvp it can be situated at the physical earth's surface S 9 P, and on the telluroid s 3 p in case of the scalar free boundary value problem. The order of the Taylor series expansion of the evaluation operator is set up in accordance with the error level of the second order analytical approximation of the boundary condition. By this consideration the absolute error level of 10~8ms~2, which is aspired for the analytical approximation of the differential operator, is assigned to the analytically continued boundary condition. For each boundary condition we end up with a representation, where the analytical side contains an isotropic term and so-called ellipsoidal and topographical constituents. The isotropic term refers to the surface K or E. The ellipsoidal and topographical terms in the boundary condition are caused by the anisotropy of the normal potential and by the difference between the boundary surface and the auxiliary surface (K or E), on which the boundary condition is continued. They have to be evaluated either on the original boundary surface (S 3 P - fixed bvp; s 9 p - scalar free bvp) or on the auxiliary surface (K 9 k or E B e) according to the choice of the Taylor point. The ellipsoidal and topographical terms can be applied as a reduction to the original boundary data. The result is a spherical boundary value problem on the sphere K (Neumann-problem in the case of the fixed bvp), or rather a quasi-isotropic bvp with an ellipsoidal boundary surface. If the complete ellipsoidal and topographical terms are neglected, the constant radius approximation will be obtained. The reduction terms are functionals of the disturbing potential Sw we solve for. Therefore, an iterative procedure is required.

To guarantee an absolute accuracy of 1 lQ~8ms~2 for the continuation terms (ellipsoidal and topogra--phical terms), the partial derivatives of the disturbing potential are considered up to the 8th order. The partial derivatives of

j%- are considered up to the 4th order, with respect to the geocentric distance r. The numerical studies confirmed that this high degree of evaluation is necessary in the framework of the analytical continuation to the sphere. Otherwise, the level of accuracy obtained for the ellipsoidal terms cannot be reached for the continuation terms. If the analytical continuation is performed to the surface of an ellipsoid, the Taylor series can be truncated after the 5th order terms without anyloss of accuracy. The accuracy of approximation is not influenced by the choice of the Taylor point. The extremal values of the whole ellipsoidal and topographical part of the linear boundary condition, that has to be modelled by the analytical representation, decreases from ±100 10~5ms~2 in the case of a sphere to ±20 10~5ms~2, if the boundary operator is continued to the surface of an ellipsoid. These terms are identical with the neglected terms in case of the spherical approximation on K or the isotropic approximation on E, respectively. They have the same order of magnitude as the boundary data itself. Undoubtedly, this kind of approximation, where all ellipsoidal and topographical terms are neglected, leads to no high-precision solution of the fixed or scalar free boundary value problem.

First of all, the error of approximation depends on the order of the Taylor series, which are setup for the analytical continuation of the boundary condition. Therefore, the surface of an ellipsoid should be favoured above the sphere. The modelling of the normal field has a minor effect. With regard to the analytical handling, a normal field of SomigJiana-Pizzetti-type should be preferred to a truncated sphe--rical harmonic axpansion. The error in the developed analytical representation of the ellipsoidal and topographical terms reaches maximum values of ±1 10~8ms~2, if the GRS80 (Somigliana-Pizzetti-field) is used as a normal field. If the normal gravity potential is modelled by a spherical harmonic expansion, (OSU91alf) truncated at degree Nv = 20, the approximation errors slightly increase up to ±7 10-8ms-2.

When no ellipsoidal and topographical reductions are applied to the scalar gravity anomalies and when this data is analysed on a sphere, the resulting effect in the solution of the disturbing potential increases to ±40m2s~2. This corresponds to a vertical shift up to ±4m of the equipotential surfaces in the vicinity of the earth's surface. If the unreduced scalar gravity anomalies are analysed on the surface of an ellipsoid, the corresponding effect will be ±2m.

The proposed iterative procedure for solving the linear boundary value problem is investigated with respect to its convergence behaviour. When the analytical continuation to the surface of an ellipsoid is applied, a strong convergence can be observed. Already after 6 iteration steps, the spectrum of the solved disturbing potential does not change any more. The spectrum of the residuals represents the error of the solution in the frequency domain. It is the difference between the solved and the given spectrum, which was used to calculate the boundary data. The residuals illustrate the error in the space domain. The resulting vertical shift of the equipotential surfaces in the vicinity of the earth's physical surface reaches a maximum of ±4mm.

In case of the analytical continuation to a sphere, the iterative approach diverges. At first, the residual spectrum is decreasing within the scope of low and medium frequencies. Simultaneously, the error in the high frequency coefficients is increasing. The spectrum deteriorates in the whole frequency domain after about 5 iteration steps.

Only if the reduction terms (ellipsoidal and topographical terms) are generated in the first iteration step by the use of the given (exact) disturbing potential, convergence will be obtained. This was done in order to check the algorithms. For that reason it is supposed that the inaccurate initial solution is responsible for the divergent behaviour of the iterative process. It can be presumed that the analytical continuation to a sphere reacts more sensibly upon errors in the initial solution, than the analytical continuation to the surface of an ellipsoid. In this context, the problem of downward continuation is crucial, requiring further investigations.Numéro de notice : 28009 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Thèse étrangère Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=63356 ## Réservation

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Code-barres Cote Support Localisation Section Disponibilité 28009-01 30.40 Livre Centre de documentation Géodésie Disponible28009-02 30.40 Livre Centre de documentation Géodésie DisponibleEffects of Non-Linearity in the Geodetic Boundary Value Problems / B. Heck (1993)

Titre : Effects of Non-Linearity in the Geodetic Boundary Value Problems Type de document : Monographie Auteurs : B. Heck, Auteur ; Kurt Seitz, Auteur Editeur : Munich : Bayerische Akademie der Wissenschaften Année de publication : 1993 Collection : DGK - A Sous-collection : Theoretische Geodäsie num. 109 Importance : 74 p. Format : 21 x 30 cm ISBN/ISSN/EAN : 978-3-7696-8191-8 Note générale : Bibliographie Langues : Anglais ( eng)Descripteur : [Vedettes matières IGN] Géodésie physique

[Termes IGN] analyse harmonique

[Termes IGN] champ de pesanteur terrestre

[Termes IGN] problème des valeurs limitesRésumé : (Auteur) The geodetic boundary value problem (GBVP) aims at the determination of the (external) gravity po-tential of the earth from (continuous) observations given on the earth's surface which acts as boundary surface. Essentially, three versions of the GBVP have to be distinguished: the fixed problem involving the assumption of a boundary surface with completely known geometry, and the vectorial and scalar formulations of the free GBVP where either the spatial or the vertical components of the position vector of the boundary surface occur as additional unknown functions. The number of prescribed boundary data types depends on the version of the GBVP; in order to obtain unique solutions, one observable only, e.g. the modulus of the gravity vector, is sufficient in the case of the free GBVP while two (resp. four) observables are necessary for the scalar (resp. vectorial) free problems.

Practical solutions of the GBVP are achieved by linearizing the boundary conditions, neglecting second and higher order terms. Due to this procedure the solutions of the GBVP are biased. In the present report the impact of the linearization errors on the (reduced) boundary conditions, the resolved po-tential function and - for the free problems - the geometry of the boundary surface is evaluated in the space and frequency domain. Numerical results have been obtained following two alternative concepts, a harmonic analysis and a synthesis approach. Both procedures rely on a representation of the gravi-tational potential of the earth by a truncated spherical harmonic expansion. The synthesis approach proved to be not feasible for degrees of expansion above N = 36 and had to be given up in favour of the harmonic analysis concept which was able to handle series representations up to degree N = 180.

The numerical investigations have revealed significant differences between the three formulations of the GBVP with respect to the impact of second-order non-linear effects. The smallest orders of magnitude show up in the fixed GBVP, amounting up to 15.10-8ms-2 in the boundary condition and 0.04 m2s-2 in the potential function. In contrast, the second-order effects in the vectorial free GBVP have magnitudes up to 3.10-5 ms-2 in the (reduced) boundary condition and 2 m2s-2 in the solution of the potential; the effects in the vertical component of the position correction vector amount up to 0.4 m, while the effects on the horizontal components have magnitudes up to 3 metres. The scalar free GBVP takes a mediate position between the fixed and the vectorial free GBVP, the non-linear terms in the (reduced) boundary condition amounting up to 1.10-6ms-2,0.1 m2s-2 in the potential, and 3 mm in

the vertical position correction.

From a practical point of view the bias in the solution is negligible in the case of the fixed GBVP, mostly tolerable for the scalar free GBVP, but inadmissibly large for the vectorial free GBVP, compared with the present-day observational accuracies. Due to its practical significance the results concerning the scalar free GBVP deserve most attention.Numéro de notice : 28245 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Monographie Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=63591 ## Réservation

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Code-barres Cote Support Localisation Section Disponibilité 28245-01 30.40 Livre Centre de documentation Géodésie Disponible