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Dépouillements
Ajouter le résultat dans votre panierA numerical test of the topographic bias / Lars E. Sjöberg in Journal of geodetic science, vol 8 n° 1 (January 2018)
Titre : A numerical test of the topographic bias Type de document : Article/Communication Auteurs : Lars E. Sjöberg, Auteur ; Mehdi S. Shafiei Joud, Auteur Année de publication : 2018 Article en page(s) : pp 14 - 17 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique
[Termes IGN] analyse numérique
[Termes IGN] anomalie de pesanteur
[Termes IGN] erreur systématiqueRésumé : (Auteur) In 1962 A. Bjerhammar introduced the method of analytical continuation in physical geodesy, implying that surface gravity anomalies are downward continued into the topographic masses down to an internal sphere (the Bjerhammar sphere). The method also includes analytical upward continuation of the potential to the surface of the Earth to obtain the quasigeoid. One can show that also the common remove-compute-restore technique for geoid determination includes an analytical continuation as long as the complete density distribution of the topography is not known. The analytical continuation implies that the downward continued gravity anomaly and/or potential are/is in error by the so-called topographic bias, which was postulated by a simple formula of L E Sjöberg in 2007. Here we will numerically test the postulated formula by comparing it with the bias obtained by analytical downward continuation of the external potential of a homogeneous ellipsoid to an inner sphere. The result shows that the postulated formula holds: At the equator of the ellipsoid, where the external potential is downward continued 21 km, the computed and postulated topographic biases agree to less than a millimetre (when the potential is scaled to the unit of metre). Numéro de notice : A2018-612 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0002 Date de publication en ligne : 07/02/2018 En ligne : https://doi.org/10.1515/jogs-2018-0002 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92645
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 14 - 17[article]
Titre : Bayesian statistics and Monte Carlo methods Type de document : Article/Communication Auteurs : Karl Rudolf Koch, Auteur Année de publication : 2018 Article en page(s) : pp 18 - 29 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Statistiques
[Termes IGN] matrice de covariance
[Termes IGN] méthode de Monte-Carlo
[Termes IGN] propagation d'erreur
[Termes IGN] théorème de Bayes
[Termes IGN] variable aléatoire
[Termes IGN] vecteur aléatoire multidimensionnelRésumé : (Auteur) The Bayesian approach allows an intuitive way to derive the methods of statistics. Probability is defined as a measure of the plausibility of statements or propositions. Three rules are sufficient to obtain the laws of probability. If the statements refer to the numerical values of variables, the so-called random variables, univariate and multivariate distributions follow. They lead to the point estimation by which unknown quantities, i.e. unknown parameters, are computed from measurements. The unknown parameters are random variables, they are fixed quantities in traditional statistics which is not founded on Bayes’ theorem. Bayesian statistics therefore recommends itself for Monte Carlo methods, which generate random variates from given distributions. Monte Carlo methods, of course, can also be applied in traditional statistics. The unknown parameters, are introduced as functions of the measurements, and the Monte Carlo methods give the covariance matrix and the expectation of these functions. A confidence region is derived where the unknown parameters are situated with a given probability. Following a method of traditional statistics, hypotheses are tested by determining whether a value for an unknown parameter lies inside or outside the confidence region. The error propagation of a random vector by the Monte Carlo methods is presented as an application. If the random vector results from a nonlinearly transformed vector, its covariance matrix and its expectation follow from the Monte Carlo estimate. This saves a considerable amount of derivatives to be computed, and errors of the linearization are avoided. The Monte Carlo method is therefore efficient. If the functions of the measurements are given by a sum of two or more random vectors with different multivariate distributions, the resulting distribution is generally not known. The Monte Carlo methods are then needed to obtain the covariance matrix and the expectation of the sum. Numéro de notice : A2018-613 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0003 Date de publication en ligne : 02/03/2018 En ligne : https://doi.org/10.1515/jogs-2018-0003 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92646
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 18 - 29[article]
Titre : On the topographic bias and density distribution in modelling the geoid and orthometric heights Type de document : Article/Communication Auteurs : Lars E. Sjöberg, Auteur Année de publication : 2018 Article en page(s) : pp 30 - 33 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique
[Termes IGN] altitude orthométrique
[Termes IGN] analyse numérique
[Termes IGN] erreur systématique
[Termes IGN] géoïde
[Termes IGN] incertitude géométrique
[Termes IGN] montagneRésumé : (Auteur) It is well known that the success in precise determinations of the gravimetric geoid height (N) and the orthometric height (H) rely on the knowledge of the topographic mass distribution. We show that the residual topographic bias due to an imprecise information on the topographic density is practically the same for N and H, but with opposite signs. This result is demonstrated both for the Helmert orthometric height and for a more precise orthometric height derived by analytical continuation of the external geopotential to the geoid. This result leads to the conclusion that precise gravimetric geoid heights cannot be validated by GNSS-levelling geoid heights in mountainous regions for the errors caused by the incorrect modelling of the topographic mass distribution, because this uncertainty is hidden in the difference between the two geoid estimators. Numéro de notice : A2018-614 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0004 Date de publication en ligne : 02/03/2018 En ligne : https://doi.org/10.1515/jogs-2018-0004 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92662
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 30 - 33[article]
Titre : The effect of regional sea level atmospheric pressure on sea level variations at globally distributed tide gauge stations with long records Type de document : Article/Communication Auteurs : Huseyin Baki Iz, Auteur Année de publication : 2018 Article en page(s) : pp 55 - 71 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique
[Termes IGN] changement climatique
[Termes IGN] données marégraphiques
[Termes IGN] marégraphe
[Termes IGN] montée du niveau de la mer
[Termes IGN] niveau de la mer
[Termes IGN] pression atmosphérique
[Termes IGN] série temporelleRésumé : (Auteur) This study provides additional information about the impact of atmospheric pressure on sea level variations. The observed regularity in sea level atmospheric pressure depends mainly on the latitude and verified to be dominantly random closer to the equator. It was demonstrated that almost all the annual and semiannual sea level variations at 27 globally distributed tide gauge stations can be attributed to the regional/local atmospheric forcing as an inverted barometric effect. Statistically significant non-linearities were detected in the regional atmospheric pressure series, which in turn impacted other sea level variations as compounders in tandem with the lunar nodal forcing, generating lunar sub-harmonics with multidecadal periods. It was shown that random component of regional atmospheric pressure tends to cluster at monthly intervals. The clusters are likely to be caused by the intraannual seasonal atmospheric temperature changes,which may also act as random beats in generating sub-harmonics observed in sea level changes as another mechanism. This study also affirmed that there are no statistically significant secular trends in the progression of regional atmospheric pressures, hence there was no contribution to the sea level trends during the 20th century by the atmospheric pressure.Meanwhile, the estimated nonuniform scale factors of the inverted barometer effects suggest that the sea level atmospheric pressure will bias the sea level trends inferred from satellite altimetry measurements if their impact is accounted for as corrections without proper scaling. Numéro de notice : A2018-615 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0007 Date de publication en ligne : 30/05/2018 En ligne : https://doi.org/10.1515/jogs-2018-0007 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92663
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 55 - 71[article]
Titre : On the geoid and orthometric height vs. quasigeoid and normal height Type de document : Article/Communication Auteurs : Lars E. Sjöberg, Auteur Année de publication : 2018 Article en page(s) : pp 115 - 120 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique
[Termes IGN] altitude normale
[Termes IGN] altitude orthométrique
[Termes IGN] géoïde
[Termes IGN] quasi-géoïdeRésumé : (Auteur) The geoid, but not the quasigeoid, is an equipotential surface in the Earth’s gravity field that can serve both as a geodetic datum and a reference surface in geophysics. It is also a natural zero-level surface, as it agrees with the undisturbed mean sea level. Orthometric heights are physical heights above the geoid,while normal heights are geometric heights (of the telluroid) above the reference ellipsoid. Normal heights and the quasigeoid can be determined without any information on the Earth’s topographic density distribution, which is not the case for orthometric heights and geoid. We show from various derivations that the difference between the geoid and the quasigeoid heights, being of the order of 5 m, can be expressed by the simple Bouguer gravity anomaly as the only term that includes the topographic density distribution. This implies that recent formulas, including the refined Bouguer anomaly and a difference between topographic gravity potentials, do not necessarily improve the result. Intuitively one may assume that the quasigeoid, closely related with the Earth’s surface, is rougher than the geoid. For numerical studies the topography is usually divided into blocks of mean elevations, excluding the problem with a non-star shaped Earth. In this case the smoothness of both types of geoid models are affected by the slope of the terrain,which shows that even at high resolutions with ultra-small blocks the geoid model is likely as rough as the quasigeoid model. In case of the real Earth there are areas where the quasigeoid, but not the geoid, is ambiguous, and this problem increases with the numerical resolution of the requested solution. These ambiguities affect also normal and orthometric heights. However, this problem can be solved by using the mean quasigeoid model defined by using average topographic heights at any requested resolution. An exact solution of the ambiguity for the normal height/quasigeoid can be provided by GNSS-levelling. Numéro de notice : A2018-115 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0011 Date de publication en ligne : 31/12/2018 En ligne : https://doi.org/10.1515/jogs-2018-0011 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92664
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 115 - 120[article]
Titre : Assessment of sparse GNSS network for network RTK Type de document : Article/Communication Auteurs : Hannu Koivula, Auteur ; Jaakko Heinonen, Auteur ; Simo Marila, Auteur ; Sonja Lahtinen, Auteur ; Tuukka Matila, Auteur Année de publication : 2018 Article en page(s) : pp 136 - 144 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Systèmes de référence et réseaux
[Termes IGN] FinnRef
[Termes IGN] positionnement cinématique en temps réel
[Termes IGN] réseau géodésique permanent
[Termes IGN] test de performanceRésumé : (Auteur) We tested the accuracy and usability of a sparse GNSS reference station network for network RTK (NRTK) using the Finnish permanent GNSS network FinnRef. We modified the configuration of the FinnRef network stations used in NRTK computation. This allowed us to perform the test both inside and outside of the network area using different NRTK methods and two different RTK receivers. In the test area the average distance between the FinnRef stationswas 160 km. As a comparison,we tested also with the commercial Trimnet and HxGN SmartNet positioning services operated by Geotrim Oy and Leica Geosystems Finland, respectively. Tests showed that the horizontal and vertical rms of Trimnet servicewas 16mmand 40 mm, and of HxGN SmartNet service 23mmand 48 mm. The best rms for the sparse NLS (National Land Survey of Finland) Service was 22 mm and 56 mm. These results indicate that a good NRTK solution can be achieved with a sparser network than typically used. This study also indicates, that the methods for NRTK processing can also affect the quality of the solution. Numéro de notice : A2018-616 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1515/jogs-2018-0014 Date de publication en ligne : 31/12/2018 En ligne : https://doi.org/10.1515/jogs-2018-0014 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=92665
in Journal of geodetic science > vol 8 n° 1 (January 2018) . - pp 136 - 144[article]
