Abstract
Abstract. The knowledge of the geoid undulation, the height of the geoid relative to a given ellipsoid of reference, is fundamental to transform the ellipsoidal heights into orthometric heights. Global geoid undulation models developed from satellite gravity measurements appropriately integrated with other data, are free accessible in internet, but their accuracy may be inadequate for specific applications. Earth Gravitational Model 2008 (EGM2008) is one of those: usually available in grid form 2.5’ × 2.5’ (a geotif is developed by Agisoft with resolution 1’ × 1’), it defines the difference between the WGS84 ellipsoid height and the mean sea level, but in some areas the discrepancies between these geoid undulations and local correspondent measured values are on the order of various decimetres. For consequence, more accurate models are necessary. This article aims to determine a geoid undulation model suitable for Campania Region (Italy), starting from the global model EGM2008 (1’ × 1’) that is locally adjusted by using geodetic network points (GNPs) and GIS interpolation functions. Three different datasets are considered including respectively 20, 40 and 60 GNPs and three deterministic interpolators are applied in global way to generate geoid undulation grids: Inverse Distance Weight (IDW), Global Polynomial 1st order (GP1), Global Polynomial 2nd order (GP2). The resultant 9 models are tested on 20 additional GNPs. The experiments demonstrate that local geoid can be produced on a little area adapting global geoid by means of GNPs: the model obtained using GP2 and 60 GNPs, the most accurate one, fits the data with ±3.2 cm root mean square error (RMSE).
Highlights
A geoid undulation model (GUM) describes the vertical separation between the WGS84 ellipsoid and a hypothetical surface corresponding to mean sea level and its imagined extension under land areas (Pugh, 1987)
The difference between the geoid undulation obtained from the global model and that obtained from the monograph is calculated in each Check points (CPs)
WORKS The results demonstrate that a local geoid can be produced on a little area adapting global geoid by means of geodetic network points (GNPs)
Summary
A geoid undulation model (GUM) describes the vertical separation between the WGS84 ellipsoid and a hypothetical surface corresponding to mean sea level and its imagined extension under (or over) land areas (Pugh, 1987). The orthometric heights can be determined using trigonometric levelling which involves measuring a vertical angle from a known distance with a theodolite and computing the elevation of the point In this case, vertical angle can be measured at the same time horizontal angles are measured for triangulation: this method results more economical but less accurate than spirit levelling (National Geodetic Survey, 2007). GPS/levelling surveys allow to acquire both ellipsoidal and orthometric heights and the differences between those values in a good number of points can be interpolated for determining a GUM. Ad hoc survey based on spirit levelling and GPS techniques for acquiring very accurate orthometric and ellipsoidal height values is to prefer, but the existing data concerning GNPs can be a good starting point. The availability of a global geoid model and the adaptation of it to a local area based on GNPs and interpolation process can be a good integrated approach to increase the result accuracy.
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