Using Models of the Ocean's Mean Dynamic Topography to Identify Errors in Coastal Geodetic Levelling

Abstract

Identifying errors (blunders and systematic errors) in coastal geodetic levelling networks has often been problematic, primarily for two reasons. First, mean sea level (MSL) at tide gauges cannot be directly compared to height differences from levelling because the geoid/quasigeoid and MSL are not parallel, being separated by the ocean's mean dynamic topography (MDT). Second, there is a the lack of redundancy at the edge of the levelling network. This article sets out a methodology to independently identify blunders and/or systematic errors (over long distances) in geodetic levelling using MDT models to account for the separation between the geoid/quasigeoid and MSL at tide gauges. This method is then tested in a case study using an oceanographic MDT model, MSL observations, GNSS data, and a quasigeoid model. The results are significant because the errors found could not be detected by standard levelling misclosure checks alone, with supplementary data from an MDT model, with cross-validation from GNSS-quasigeoid allowing their detection. In addition, it appears that an oceanographic-only MDT is as effective as GNSS and a quasigeoid model for detecting levelling errors, which could be particularly useful for countries with coastal levelling errors in their levelling networks that cannot be identified by conventional levelling closure checks.

Keywords:

• Australia
• coastal mean dynamic ocean topography
• geodetic levelling
• geoid
• mean sea level

Acknowledgements

I would like to thank Professor Will Featherstone for his initial suggestion for this study and his comments on earlier versions of the manuscript. I would also like to thank the following organizations for supplying data and models: Geoscience Australia (ANLN and GNSS data), CSIRO Marine Laboratories for CARS2009, the Permanent Service for Mean Sea Level (PSMSL) for tide gauge records and the Australian Hydrographic Office (AHO) for tide gauge benchmark information. Generic Mapping Tools (Wessel and Smith 1998) has been used to plot Figures 4, 5 and 6. I appreciate the constructive comments from two anonymous reviewers on the original version of the manuscript.