The worldwide physical height datum project

Abstract

The definition of a common global vertical coordinate system is nowadays one of the key points in Geodesy. With the advent of GNSS, a coherent global height has been made available to users. The ellipsoidal height can be obtained with respect to a given geocentric ellipsoid in a fast and precise way using GNSS techniques. On the other hand, the traditional orthometric height is not coherent at global scale. Spirit levelling allows the estimation of height increments so that orthometric heights of surveyed points can be obtained starting from a benchmark of known orthometric heights. As it is well known, this vertical coordinate refers to the geoid, which is assumed to be coincident to the mean sea level. By means of a tide gauge, the mean sea level is estimated and thus a point of known orthometric height is defined. This assumption, which was acceptable in the past, became obsolete given the level of precision which is now required. Based on the altimetry observation, one can precisely quantify the existing discrepancy between geoid and mean sea level that can amount to 1 ÷ 2 m at global scale. Therefore, different tide gauges provide biased estimates of the geoid, given the discrepancy between this equipotential surface and the mean sea level. Also, in the last years, another vertical coordinate was used, the normal height that was introduced in the context of the Molodensky theory. In this paper, a review of the existing different height systems is given and the relationships among them are revised. Furthermore, an approach for unifying normal height referring to different tide gauges is presented and applied to the Italian test case. Finally, a method for defining a physical height system that is globally coherent is discussed in the context of the definition of the International Height Reference System/Frame, a project supported by the Global Geodetic Observing System of the International Association of Geodesy (IAG). This project was established in 2015 during the XXVI IAG General Assembly in Prague as described in IAG Resolution no. 1 that was presented and adopted there.

First considerations on post processing kinematic GNSS data during a geophysical oceanographic cruise

“…The ellipsoidal elevations measured with the GNSS receiver were converted into orthometric elevations (H in Figure 9) with respect to the national altimetric system using the Geoid model ITALGEO2005, which is accredited with 2.5 cm of accuracy [5]. However, it should be noted that the reported accuracy is estimated on land where geoid-ellipsoid separation values measured on altimetric benchmarks are used to improve the estimate of geoid-ellipsoid separation itself.…”

“…The ground altimetric reference system is generally based on the definition of a specific equipotential surface of the gravitational field, identified by the mean level of a reference tide gauge. This method has been used in Italy both historically [2] and currently [3] and various national terrestrial altimetric systems have been defined, for this reason the possibility of unifying them at a regional [4] and global level [5] is currently subject of research. GNSS systems have made it possible for the first time to obtain highly accurate plano-altimetric measurements (and therefore also elevation measurements) both on land and at sea; however, these measurements do not refer to a physical reality but to a mathematical surface, the ellipsoid of rotation.…”

Epoch-Based Height Reference System for Sea Level Rise Impact Assessment on the Coast of Peninsular Malaysia

“…A resolution in defining and realising the International Height Reference System (IHRS) was issued by the International Association of Geodesy (IAG) in July 2015. The IHRS coordinates are defined in this resolution as potential differences with respect to the equipotential surface of the Earth's gravity field, which is realised by the conventional value of W 0 = 62 636 853.4 m 2 s −2 [25,26].…”