Localised geoid modelling using GPS and precise levelling data

Munro, Frazer (2009) Localised geoid modelling using GPS and precise levelling data. [USQ Project] (Unpublished)

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Abstract

This project evaluates the accuracy of GPS heights derived using the current New Zealand geoid model (NZGeoid05) as compared to an empirically based spirit level network over a small study area(55 km x 50 km) in the South Island of New Zealand. It analyses the current New Zealand geoid model; NZGeoid05, and the history of levelling datums in New Zealand with specific focus on the Lyttelton 1937 datum. It then uses GPS observations to derive ellipsoid heights at known points that have precise level information, a calculation of geoid – ellipsoid separation values is performed and used to build a local geoid model. This is done mainly using the software package Grid Factory. The results of this research show that while there is a measureable difference between GPS derived heights and precise levelling heights, the differences are mostly within the specified accuracy of NZGeoid05. As the two datums are independent interpretations of the equipotential surface, it is not surprising there are some differences. Further investigation shows that the reliability of the Lyttelton 1937 datum in terms of accurately representing the equipotential surface is now questionable due to poor initial definition, tectonic deformation and sea level change. This is a problem that will worsen with time. Another disadvantage of the Lyttelton 1937 datum is that it is orientated towards conventional technology and techniques. With the increasing popularity of modern GPS surveying, vertical determination methods using geoid models are becoming more common. The localised geoid model was successfully created. This proves that is it possible to build local geoid models. It was tested against NZGeoid05 and EGM08 to determine which produced height values that best correlated with the Lyttelton 1937 datum. The ProjectGeoid proved to provide the best solution. However, the evaluation of the model shows that it is not an independent interpretation of the equipotential surface but rather, it has given the Lyttelton interpretation a new expression. Because the Lyttelton interpretation of the equipotential surface is outdated and subject to continued degradation, this new interpretation will then not provide orthometic heights that truly represent actual MSL


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Item Type: USQ Project
Refereed: No
Item Status: Live Archive
Depositing User: epEditor USQ
Faculty / Department / School: Historic - Faculty of Engineering and Surveying - Department of Surveying and Land Information
Date Deposited: 27 Jul 2010 05:15
Last Modified: 29 Jul 2010 00:39
Uncontrolled Keywords: geoid modelling; GPS; NZGeoid05
Fields of Research (FOR2008): 09 Engineering > 0909 Geomatic Engineering > 090906 Surveying (incl. Hydrographic Surveying)
09 Engineering > 0909 Geomatic Engineering > 090903 Geospatial Information Systems
URI: http://eprints.usq.edu.au/id/eprint/8543

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