GLOBAL GEOID ADJUSTMENT ON LOCAL AREA FOR GIS APPLICATIONS USING GNSS PERMANENT STATION COORDINATES

Ugo Falchi,Claudio Parente,Giuseppina Prezioso

doi:10.3846/gac.2018.4356

Abstract

Orthometric heights, useful for many engineering and geoscience applications, can be obtained by GPS (Global Positioning System) surveys only when an accurate geoid undulation model (that supplies the vertical separation between the geoid and WGS84 ellipsoid) is available for the considered topic area. Global geoid height models (i.e., EGM2008), deriving from satellite gravity measurements suitably integrated with other data are free available on web, but their accuracy is often not sufficient for the user’s purposes. More accurate local models can nevertheless be acquired, but often only for a fee. GPS/levelling surveys are suitable for determining a local, accurate geoid model, but may be too expensive. This paper aims to demonstrate that GNSS (Global Navigation Satellite System) Permanent Station documents (monographs), freely available on the web and supplying orthometric and ellipsoidal heights, permit to calculate precise geoidal undulations useful to perform global geoid modelling on a local area. In fact, in this study 25 GNSS Permanent Stations (GNSS PS), located in North-Western Italy are considered: the differences between GNSS PS geoidal heights and the corresponding EGM2008 1′ × 1′ ones are used as a starting dataset for Ordinary Kriging applications. The resulting model is summed to the EGM2008 1′ × 1′, obtaining a better-performed model of the interest area. The accuracy tests demonstrate that the resulting model is better than EGM2008 grids to produce contours from a GPS dataset for large-scale mapping.

Highlights

The geoid, namely the shape that the surface of the oceans would take under the influence of Earths gravity and rotation alone, in the absence of other influences such as tides and winds (Pugh, 1987), is the reference surface to which levelled heights refer
The aim of this paper is to assess the accuracy of the geoid model resulting by fitting EGM2008 in a local area using a limited number of control points uniformly distributed on the territory, such as GNSS Permanent Stations (GNSS PS)
This study confirms that global geoid models presenting in local areas accuracy values that are not suitable for applications at large scale may be performed using accurate undulations values, e.g., derived by GPS/levelling

Summary

Introduction

The geoid, namely the shape that the surface of the oceans would take under the influence of Earths gravity and rotation alone, in the absence of other influences such as tides and winds (Pugh, 1987), is the reference surface to which levelled heights refer. The OSU91A (Rapp & Wang, 1993), EGM96 (Lemoine et al, 1998) and EGM2008 (Pavlis, Holmes, Kenyon, & Factor, 2008), are some of the models used to determine the orthometric height from GPS measurements. They are global geopotential models from which geoid height files are extracted, e.g., WGS84 EGM96 15-Minute Geoid Height File is a 15-minute grid of point values in the tide-free system, using the EGM96 Geopotential Model to degree and order 360 (NGA/NASA, n.d.)

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Conclusion