The surface gravity data collected via traditional techniques such as ground-based, shipboard and airborne gravimetry describe precisely the local gravity field, but they are often biased by systematic errors. On the other hand, the spherical harmonic gravity models determined from satellite missions, in particular, recent models from CHAMP and GRACE, homogenously and accurately describe the low-degree components of the Earth's gravity field. However, they are subject to large omission errors. The surface and satellite gravity data are therefore complementary in terms of spectral composition.
The need for a new vertical datum in Canada dates back to 1976 when a study group at the Geodetic Survey Division (GSD) of Natural Resources Canada investigated problems related to the existing ver...
En 2022, les États-Unis, dans le cadre de la modernisation de leur système de référence, remplaceront le Système de référence géodésique nord-américain de 1983 (NAD83) par un nouveau cadre de référence terrestre nord-américain (NATRF2022), ce qui entraînera des différences de coordonnées horizontales de 1,3 à 1,5 mètre à la frontière canado-américaine par rapport au NAD83 (SCRS) canadien. Jamais auparavant des différences aussi importantes n’avaient existé entre les cadres de référence de nos deux pays. Le présent document examine les raisons pour lesquelles les États-Unis apportent ce changement et examine ensuite la situation du Canada en ce qui concerne les cadres de référence. Il y a des raisons impérieuses pour que le Canada emboîte le pas et passe au NATRF2022 d’ici une décennie, mais cela représente aussi des défis majeurs. Que le Canada suive ou non la même voie, il y a beaucoup de travail à accomplir pour préparer le Canada à l’adoption du NATRF2022 par les États-Unis. Le présent document se veut une première étape pour informer la communauté géospatiale canadienne de la décision des États-Unis d’adopter le NATRF2022 et de ce que cela signifie pour le Canada.
Abstract In this study, we evaluate the suitability of recent Earth Gravitational Models (EGMs) for the realization of the International Height Reference System (IHRS) in Canada. Topographic gravity field models have been used to augment EGMs to spatial resolution reaching 2 ′ (about 4 km), which is comparable to regional geoid models. The advantages of using an EGM over a regional approach for the IHRS are its uniform representation of the Earth’s gravity field and its conformance to international standards and conventions. The main challenge is access to, and best use of knowledge of the regional gravity and topographic data. On the one hand, we determine that two recent hybrid models (EIGEN-6C4 and XGM2019) augmented by topographic signals give geopotential values ( W p ) with accuracy of ~0.3 m 2 s −2 , which is close to those estimated by the Canadian regional geoid models at the 11 International Height Reference Frame sites in Canada. On the other hand, two recent augmented satellite-only models (DIR-R6 and GOCO06s) give W p with accuracies between 1.5 and 1.7 m 2 s −2 in Canada.
[1] A new mean sea surface topography (MSST) is used to estimate the surface circulation of the subpolar gyre of the northwest Atlantic. The MSST is produced using a new geoid model derived from a blend of gravity data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, satellite altimeters, and terrestrial measurements. The MSST is compared with a topography produced by an ocean model which is spectrally nudged to a new Argo period temperature and salinity climatology. The mean surface circulation associated with the geodetic MSST is compared with estimates of the circulation from surface drifters, moorings, and other in situ measurements. The geodetic MSST and circulation estimate are found to be in good agreement with the other estimates, both qualitatively and quantitatively. The topography is found to be an improvement over an earlier geodetic estimate with better resolution of the coastal currents. Deficiencies are identified in the ocean model's estimate of flow over shelf regions.
Abstract The geoid-quasigeoid separation (GQS) traditionally uses the Bouguer anomalies to approximate the difference between the mean gravity and normal gravity along the plumb line. This approximation is adequate in flat and low elevation areas, but not in high and rugged mountains. To increase the accuracy, higher order terms of the corrections (potential and gravity gradient) to the approximation were computed in Colorado where the 1 cm geoid computation experiment was conducted. Over an area of 730 km by 560 km where the elevation ranges between 932 and 4,385 m, the potential correction (Pot. Corr.) reaches −0.190 m and its root mean square (RMS) is 0.019 m. The gravity gradient correction is small but has high variation: the RMS of the correction is merely 0.003 m but varies from −0.025 to 0.020 m. In addition, the difference between the Bouguer gravity anomaly and gravity disturbance causes about a 0.01 m bias and a maximum correction of 0.02 m. The total corrections range from −0.135 to 0.180 m, with an RMS value of 0.019 m for the region. The magnitude of the corrections is large enough and is not negligible considering today’s cm-geoid requirement. After the test in Colorado, the complete GQS term is computed in 1′ × 1′ grids for the experimental geoid 2020 (xGEOID20), which covers a region bordered by latitude 0–85° north, longitude 180–350° east. Over the land areas, the RMS of the GQS is 0.119 m and the maximum reaches 1.3 m. The RMS of the GQS increases with respect to the height until 4,000 m, then decreases unexpectedly. At the highest peaks (5,500–6,000 m) of Denali and Mount Logan, the RMS of the GQS ranges between 0.08 and 0.189 m. The small GQS at these high peaks are caused by steep slopes around the peaks that produce large Pot. Corr. caused by the topography. In addition, the higher order correction terms reach half of a meter in those peaks.
The objective of this study is to assess the recent GRACE and GOCE release 5 (R5) global geopotential models (GGM) in Canada. For the evaluation of the GGM against GPS-Levelling data and deflections of the vertical, the satellite models need to be combined with terrestrial gravity data to determine highresolution geoid models. This process allows a significant reduction of the omission error in the satellite gravity models. The geoid computation is founded on the remove-compute-restore Stokes-Helmert scheme, in which the Stokes kernel function modification is used to constrain the regional geoid models to the spectral bands of the satellite model to be assessed. The resulting geoid models are directly comparable to the GPS-Levelling data, and these same geoid models can be used to derive deflections of the vertical which are compared to astronomic deflections. The data analysis indicates that the GOCE R5 models provide better precision than the GOCE release 4 (R4) models beyond degree and order 180. The accuracy of the GOCE R5 models is estimated to be better than 4-5 cm up to spherical harmonic degree ~200. The astronomic deflections appear not accurate enough to measure improvements in the GOCE R5 models with respect to the GOCE R4 models. For the validation of GGM against terrestrial gravity data over land, EIGEN-6C4, which includes a GOCE R5 model, is assessed in contrast to EGM2008. Our analysis infers that the GOCE contribution in EIGEN-6C4 is more accurate than the corresponding wavelength components in EGM2008, which includes the Canadian terrestrial gravity data.
