The effect of EGM2008-based normal, normal-orthometric and Helmert orthometric height systems on the Australian levelling network

Abstract

This paper investigates the normal-orthometric correction used in the definition of the Australian Height Datum (AHD), and also computes and evaluates normal and Helmert orthometric corrections for the Australian National Levelling Network (ANLN). Testing these corrections in Australia is important to establish which height system is most appropriate for any new Australian vertical datum. An approximate approach to assigning gravity values to ANLN benchmarks (BMs) is used, where the EGM2008-modelled gravity field is used to 're-construct' observed gravity at the BMs. Network loop closures (for first-and second-order levelling) indicate reduced misclosures for all height corrections considered here, particularly in the mountainous regions of south eastern Australia. Differences between Helmert orthometric and normal-orthometric heights reach 44 cm in the Australian Alps, and differences between Helmert orthometric and normal heights are about 26 cm in the same region. Normal-orthometric heights differ from normal heights by up to 18 cm in mountainous regions > 2000 m. This indicates that the quasigeoid is not compatible with normalorthometric heights.

The first Australian gravimetric quasigeoid model with location-specific uncertainty estimates

“…Table 1 (column 9) also shows that the NOC is a reasonable approximation of the NC (cf. Filmer et al 2010) in relation to the expected precision of the levelling data (Table 2). Another consideration is the inherent precision of the GNSS-levelling data and thus its ability to discriminate among different quasigeoid model solutions.…”

“…The NC system is theoretically consistent with the quasigeoid. Gravity values at ANLN benchmarks required for the NC were derived from EGM2008, as in Filmer et al (2010).…”

Estudo das correções gravimétricas para altitudes físicas aplicadas aos desníveis da RAAP

“…A forma mais simples de determinação da altitude ortométrica utiliza a redução gravimétrica simplificada de Poincaré-Prey, sendo esta conhecida como altitude ortométrica de Helmert (FILMER et al, 2010). A redução de Poincaré-Prey aplica uma série de simplificações com respeito às massas externas ao geóide, como a negligencia do efeito do terreno e variações de densidade nas massas topográficas .…”

“…Este tipo de altitude substitui completamente o uso do potencial da Terra real (W), pelo uso do esferopotencial (U). Dessa forma, os números do geopotencial (C) são substituídos pelos números do potencial normal (C N ) (Filmer et al 2010). Existem inúmeros tipos de correções normal-ortométrica, podendo-se citar Bomford (1980) além de Amos e Featherstone (2009).…”

Deterministic, stochastic, hybrid and band-limited modifications of Hotine’s integral

“…In Australia, for instance, the vertical datum contains a confirmed tilt with respect to the geoid (Featherstone and Filmer 2012), regional distortions [e.g., Featherstone et al (2011) and the citations therein], and uses a height system that is not compatible with the geoid (Filmer et al 2010). These will cause errors in the computed terrestrial gravity anomalies (cf .…”