National Vertical Datum Research Articles

Investigating the Congruence between Gravimetric Geoid Models over India

A major motivation of precise geoid computation is to adopt it as a national vertical datum or as an alternative vertical reference surface to aid surveyors in calculating physical heights using the Global Navigation Satellite System (GNSS). There exist several methods of geoid computation, and only one particular method is generally used to calculate the national geoid model irrespective of the size and topographical landforms of the country. The validation of the developed geoid models is done with the complete GNSS-leveling data set. In such a case, it is hard to claim the consistency in the precision of the developed geoid model throughout the country. This study aims to identify the consistency of the geoid models over India computed using the three approaches primarily followed in Curtin University of Technology (CUT), the University of New Brunswick (UNB), and the Royal Institute of Technology (KTH). Three analyses have been done on the calculated geoid models: (1) clusterwise validation with the GNSS-leveling data, (2) intermodel comparison for the whole study area, and (3) intermodel comparison for Indian states and Union Territories (UT) only. The GNSS-leveling validation results show that the standard deviations of differences for all the methods are within a range of ±0.01 m with the exception of Uttar Pradesh West with the UNB method. However, inter-model comparison shows that the mean (meters) and standard deviation (meters) of the differences between the pairs (CUT-UNB), (CUT-KTH), and (KTH-UNB) are 0.241±0.854, −0.133±0.498, and 0.374±1.239, respectively, with maximum difference sometimes exceeding 5 m. There is only one UT and four states for which the mean value is within (−0.20 m, 0.20 m) and standard deviation ≤±0.05 m for all the three pairs. Therefore, the analysis shows that it is difficult to calculate a precise national geoid model using any one method alone and a strategy is required to merge various regional precise geoid models or methods to develop a consistently precise national geoid model.

A Geoid Slope Validation Survey (2017) in the rugged terrain of Colorado, USA

In the summer of 2017, the National Geodetic Survey (NGS) conducted its third and final Geoid Slope Validation Survey in the rugged terrain of southern Colorado, USA. As in previous surveys, the intent is to acquire the most accurate and precise field observations to determine geoid slopes. In turn, these data can be used to quantify the accuracy of various geoid models as NGS looks ahead to creating a highly accurate gravimetric geoid model for use as a national vertical datum. Long period GPS sessions, spirit leveling, absolute gravity, and deflection of the vertical (DoV) observations were acquired along a 360 km line, ranging from 1900 to 3300 m in elevation, with a station spacing of approximately 1.6 km. Our absolute gravity and DoV datasets are unique in that they were collected at 222 field stations in highly mountainous terrain at an unprecedented observational accuracy of 10 µGal and 0.04″, respectively. Further, by employing tailored refraction corrections to the spirit leveling data, we improved the agreement between heights derived from the DoV and spirit leveling from ± 1.9 to ± 1.3 cm RMS, or by more than 30%, across the line. At all length scales, from 1.6 to 360 km, the agreement is better than 2 cm. Finally, as a description of the validation process, we compare the observations with recent NGS experimental geoid models. We find that typical agreement is at about 3–5 cm, with no single model being best at all length scales. The data from this project are freely available to the community and should serve as test beds for not only geoid modeling comparisons, but also the refinement of numerous field techniques.

OFFSET EVALUATION OF THE ECUADORIAN VERTICAL DATUM RELATED TO THE IHRS

Abstract Considering the definition of the International Height Reference System (IHRS) in the geopotential space (Resolution 1/2015, International Association of Geodesy - IAG), among the present main objectives of the international geodetic community is the materialization of IHRS around the world. One fundamental task for this is the offset determination of each national vertical datum related to the IHRS. In this manuscript we establish the relationship between the Ecuadorian Vertical Datum (EVD) and the IHRS in the geopotential space following the foundations of the Resolution 1/2015 IAG. Gravity data, heights from the Ecuadorian Fundamental Vertical Network, Global Geopotential Models and Digital Elevation Models were used in the computations. Based on the Least Squares Collocation method, empirical covariance functions and spectral decomposition techniques, we realized the modelling of the geopotential in the study region (4° x 4° centered in the La Libertad tide gauge, Ecuador). Based on the referred approaches, we solved the free Geodetic Boundary Value Problem for determining the discrepancy of the EVD related to the IHRS. An offset of approximately 29 cm ± 3 cm was estimated for the W 0 - W 0 i relation when the GO_CONS_GCF_2_DIR_R5 model was used in the modeling of the medium and long wavelengths of the terrestrial gravity field, and approximately 43 cm ± 3 cm when the EIGEN6C4 model was used.

AUSGeoid2020 combined gravimetric\u2013geometric model: location-specific uncertainties and baseline-length-dependent error decorrelation

AUSGeoid2020 is a combined gravimetric–geometric model (sometimes called a “hybrid quasigeoid model”) that provides the separation between the Geocentric Datum of Australia 2020 (GDA2020) ellipsoid and Australia’s national vertical datum, the Australian Height Datum (AHD). This model is also provided with a location-specific uncertainty propagated from a combination of the levelling, GPS ellipsoidal height and gravimetric quasigeoid data errors via least squares prediction. We present a method for computing the relative uncertainty (i.e. uncertainty of the height between any two points) between AUSGeoid2020-derived AHD heights based on the principle of correlated errors cancelling when used over baselines. Results demonstrate AUSGeoid2020 is more accurate than traditional third-order levelling in Australia at distances beyond 3 km, which is 12 mm of allowable misclosure per square root km of levelling. As part of the above work, we identified an error in the gravimetric quasigeoid in Port Phillip Bay (near Melbourne in SE Australia) coming from altimeter-derived gravity anomalies. This error was patched using alternative altimetry data.

ON THE DEFINITION OF HEIGHT REFERENCE SURFACES OVER AN ARBITRARY SELECTED AREA BY MEANS OF DFHRS APPROACH

The aim of this study is to analyze the best fitting geoid model for an arbitrary selected area of investigation. The Digital Finite Element Height Reference Surface (DFHRS) method, developed by Hochschule Karlsruhe has been chosen to define the height reference surface for the territory of Kosovo, which should be defined as national vertical datum. This approach allows the conversion of ellipsoidal heights determined by GPS into the standard heights, which refer to the height reference surface (HRS) of an orthometric, or normal surface system. The DFHRS is defined as continues HRS in arbitrary large areas by bivariate polynomials over an irregular grid (Jager, Schneid 2001). The DFHRS approach uses wide range of the input data (Geometric and Physical) and in our case there were 30 GPS/leveling height data as well as physical derivatives from different global geopotential models. The proposed approach has been successfully applied and results are compared to actual normal heights and in selected profiles of digital elevation reference surface calculated from the national control network. Special attention has been given to the choice of the geopotential model and the selection of the pass points in source and target surface.

Relative performance of AUSGeoid09 in mountainous terrain

Abstract The Australian Height Datum (AHD) is the national vertical datum for Australia, and AUSGeoid09 is the latest quasigeoid model used to compute (normalorthometric) AHD heights from Global Navigation Satellite System (GNSS) derived ellipsoidal heights. While previous studies have evaluated the AUSGeoid09 model across Australia, such studies have generally not focused on mountainous terrain. This paper investigates the performance of AUSGeoid09 in a relative sense in the MidHunter and Snowy Mountains regions of New South Wales, from a user’s perspective. Relative (i.e. height difference between two points) comparisons were undertaken between AUSGeoid09-derived heights and officialAHDheights. The performance of AUSGeoid09 was compared to its predecessor AUSGeoid98. In both study areas, an overall improvement was evident when applying AUSGeoid09 to compute AHD height differences. AUSGeoid09 generally provided AHD height differences at the ±0.05mto ±0.09m level (1 sigma) and substantially increased the percentage of GNSS-derived height differences meeting 3rd order differential levelling specifications. This is a very encouraging result, considering the difficulties of spirit levelling in mountainous terrain and the increasing popularity of GNSS-based height transfer in practice.

Absolute Performance of AUSGeoid09 in Mountainous Regions

AbstractThe Australian Height Datum (AHD) is the current national vertical datum for Australia, and AUSGeoid09 is the latest quasigeoid model used to compute (normal-orthometric)AHDheights fromGlobalNavigation Satellite System (GNSS) derived ellipsoidal heights. While previous studies have evaluated the AUSGeoid09 model across Australia, such studies have not focused on mountainous regions in particular. This paper investigates the performance of AUSGeoid09 in an absolute sense in the Mid Hunter and Snowy Mountains regions of New South Wales. Absolute (i.e. single point) comparisons were undertaken between AUSGeoid09-derived heights and published AHD heights. The performance of AUSGeoid09 was evaluated relative to its predecessor AUSGeoid98. In both study areas, an overall improvement is evident when applying AUSGeoid09 to compute AHD heights in an absolute sense. In the MidHunter, AUSGeoid09 provided a substantial improvement over its predecessor, clearly demonstrating the benefits of its new geometric component on GNSS-derived AHD height determination. In the Snowy Mountains, moderate improvement over AUSGeoid98 was evident. However, a slope was detected for AUSGeoid09 residuals, and it appears that the geometric component may have overcompensated for sea surface topography in this area. While this appraisal of AUSGeoid09 performance in mountainous regions is encouraging, it has been shown that some discrepancies still remain between AUSGeoid09-derived heights and AHD. Eventually, a new vertical datum will be necessary to ensure homogeneity across Australia.

The offset of the South Korean vertical datum from a global geoid

Accurate orthometric heights in a national vertical datum can be determined economically using GPS heights and gravimetrically computed geoid undulations. If this national vertical datum should be tied to a global geoid, then both the datum offset as well as the difference between the GPS ellipsoid and the globally best-fitting ellipsoid should be known. This paper elaborates on the theory and procedures to obtain the datum offset for South Korea’s national vertical datum on the basis of the global model, EGM08. Among several GPS/leveling data sets slightly different estimates are obtained, with the estimate, 26.5 +/− 1.3 cm (above the global zero-tide geoid) obtained using over 800 data points distributed in South Korea. The uncertainty is a level of precision, not absolute accuracy, and the discrepancies between estimates using different subsets of the data indicate the presence of small unsolvable biases among them. An alternative procedure to determine orthometric heights using the modern principles is also proposed that makes the vertical datum completely independent of the global geoid in the traditional way and obviates the estimation of these offsets.

Taking the “Boulder” Step From Static to Dynamic Geoid: 2009 Workshop on Monitoring North American Geoid Change; Boulder, Colorado, 21–23 October 2009

As coastal communities are increasingly affected by sea level rise and flooding from extreme weather events, and as less evident (yet significant) tectonic shifts reshape the North American continent, the need for elevation data that are accurate, consistent, updated, and easily accessible has become critical. Currently, however, the vertical datums in North America are defined by tide gauge and leveling observations, which inevitably become outdated and are costly to replace or repeat. Therefore, two North American governments (Canada and United States) have resolved that the next generation of their national vertical datums will be geoid‐ based and accessible through Global Navigation Satellite System (GNSS) technology.To adequately serve as the reference surface for a future vertical datum, the geoid must be modeled accurately and its changes over time must be monitored. But what mix of tools and techniques could fulfill this requirement? To address this question and to plan for a campaign to monitor North American geoid change, experts from North America (including United States, Canada, and Mexico) and Europe specializing in satellite and terrestrial gravimetry as well as satellite positioning convened in Colorado.

A NEW HEIGHT REFERENCE NETWORK IN TAIWAN

The island of Taiwan is located in an active tectonic zone on the boundary of the Eurasian and Philippine Sea plates. Due to the continuous mountain building process in association with the collision of the two plates, it is very important for Taiwan to periodically re-survey the island so as to maintain the intrinsic accuracy requirement of its vertical reference network. The existing height network was created in 1979 by means of the spirit levelling technique with theoretical surface gravity quantities derived from the Geodetic Reference System 1967 (GRS67) normal gravity field. Because of the lack of use of actual gravity measurements in its creation, the existing network has been found to suffer from severe systematic errors in the central mountainous areas.In this paper, we present the design and implementation of a new national vertical datum in Taiwan, Taiwan Vertical Datum 2001 (TWVD2001). The new datum was developed by optimally combining geodetic levelling and surface gravity observations collected at 1010 newly established benchmarks within a network of 2200-km first-order levelling lines. The data analyses and adjustment computation were based on geopotential numbers. Both orthometric and normal heights were computed for the new vertical reference network.

Do we need a gravimetric geoid or a model of the Australian height datum to transform GPS heights in Australia?

A proposal is made to use a model of the Australian Height Datum (AHD) instead of the classical geoid to provide a more direct transformation of Global Positioning System (GPS) ellipsoidal heights to the AHD. This approach avoids post-survey adjustment of the GPS-AUSGEOID-derived heights in order to align them with existing AHD control. Alternatively, of course, the AHD could be redefined and readjusted such that it is more coincident with the classical geoid, thus allowing the use of a pure gravimetric geoid model in the height transformation. However, the cost and inconvenience associated with implementing a new national vertical datum in the near future render the proposed approach a more practical option in the interim.

State‐of‐the‐art sea level and meteorological monitoring systems in the intra‐Americas sea

State‐of‐the‐art technology is presently being used for the acquisition of water level and meteorological data in the Intra‐Americas Sea to support the Global Sea Level Observing System (GLOSS) network of sea‐level monitoring stations. GLOSS stations provide data for the investigation of regional relative sea level change in areas of complex tectonic motion, national geodetic vertical datums, near real‐time data for input to climatic diagnostic numerical models, calibration of satellite altimeter and scatterometer data, and the evaluation of the feasibility of producing synoptic mean sea level charts for the prediction of climatic trends, long‐range weather forecasts, and ocean processes.