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Towards a Vertical Datum Standardisation Under the Umbrella of Global Geodetic Observing System Research Article

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Towards a Vertical Datum Standardisation Under the Umbrella of Global Geodetic Observing System Research Article Journal of Geodetic Science • 2(4) • 2012 • 325-342 DOI: 10.2478/v10156-012-0002-x • Towards a vertical datum standardisation under the umbrella of Global Geodetic Observing System Research Article L. Sánchez∗ Deutsches Geodätisches Forschungsinstitut (DGFI), Munich, Germany Abstract: Most of the existing height systems refer to local sea surface levels, are stationary (do not consider variations in time), and realise different physical height types (orthometric, normal, normal-orthometric, etc.). In general, their accuracy is about two orders of magnitude less than that of the realisation of geometric reference systems (sub millimetre level). The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), taking care of providing a precise geodetic infrastructure for monitoring the system Earth, promotes the standardisation of height systems worldwide. The main objectives are: (1) to provide a reliable frame for consistent analysis and modelling of global phenomena and processes affecting the Earth’s gravity eld and the Earth’s surface geometry; and (2) to support the precise combination of physical and geometric heights in order to exploit at a maximum the advantages of satellite geodesy (e.g. combination of satellite positioning and gravity eld models for worldwide unied precise height determination). According to this, the GGOS Theme 1 ”Unied Height System” was established in February 2010 with the purpose to bring together existing initiatives and to address the activities to be faced. Starting point are the results delivered by the IAG Inter-Commission Project 1.2 ”Vertical Reference Frames” during the period 2003-2011. The present actions related to the vertical datum homogenisation are being coordinated by the working group ”Vertical Datum Standardisation”,which directly depends on the GGOS Theme 1 and is supported by the IAG Commissions 1 (Reference Frames) and 2 (Gravity Field), as well as by the International Gravity Field Service (IGFS). This paper discusses some aspects to take into consideration for the realisation of a standardised globally unied vertical reference system. Keywords: GGOS height system • global reference level • global vertical reference system • vertical datum standardisation • W0 reference value © Versita sp. z o.o. Received 01-10-2012; accepted 18-11-2012 1. Introduction (ITRS, Petit and Luzum 2010) guarantee a globally unied geomet- ric reference frame with reliability at the mm-level. An equivalent high-precise physical reference frame is missing. The existing phys- ical height systems and all the geodetic data depending on their Studying, understanding and modelling global change requires realisation (e.g. gravity anomalies, geoid estimations, digital ter- geodetic reference frames with (1) an order of accuracy higher rain models, etc.) are usable in limited geographical areas only and than the magnitude of the effects we want to study, (2) consis- their combination at regional or global levels presents discrepan- tency and reliability worldwide (the same accuracy everywhere), cies with magnitudes very much higher than the accuracy required and (3) a long-term stability (the same order of accuracy at today. They do not provide a reliable frame for consistent analysis any time). The denition, realisation, maintenance, and wide- and modelling of global phenomena related to the Earth’s gravity utilisation of the International Terrestrial Reference System eld (e.g. sea level variations from local to global scales, redistribu- tion of masses in oceans, continents and the Earth’s interior, etc.), and they are not able to support the precise combination of phys- ∗E-mail: sanchez@dgfi.badw.de 326 Journal of Geodetic Science Table 1. Main characteristics and drawbacks of the existing height systems. Characteristic Drawback The reference level (1) There are so many reference lev- (zero-height surface) is els (vertical datums) as reference tide realised by the mean gauges and (2) they are related to dif- sea level measured at ferent reference epochs. individual tide gauges and averaged during Figure 1. Height datum discrepancies. different time periods. The dynamical ocean (1) Equipotential surfaces passing topography at the local through the different reference tide reference tide gauges gauges realise different (local) geoids, ways the same: to refer all existing physical heights (or geopoten- has not been taken into which are lying very close to sea sur- tial numbers) to one and the same reference surface, which must account. face (< ∼2 m) and are practically par- be realised with high precision globally. The fundamental quanti- allel to each other; but no one coin- ties of interest are the geopotential differences (δW i = W - W i , cides with a global geoid. (2) Relation- 0 0 0 ship between local geoids and between called also height datum discrepancies or vertical datum pa- them and the global one are unknown. rameters, Fig. 1) between a conventional global reference level i The vertical control (1) Vertical networks have been (W0) and the equipotential surface (W 0) realised by the mean has basically been adjusted piece-wisely, (2) systematic sea level estimated at each local tide gauge (i.e. Heck and Rum- extended by means errors significantly growth with the mel 1990, Rapp 1994, Rummel and Teunissen 1988, Sánchez 2007). of spirit levelling distance from reference tide gauge, i during many years and (3) vertical coordinates refer to dif- Most of the proposals oriented to the calculation of δW 0 are and possible vertical ferent epochs. based on the comparison of physical heights (orthometric HO or displacements have normal heights HN ) derived from levelling (reduced by gravity ef- not been taken into fects) with those computed from gravimetric (quasi)geoid models account. (N, ζ) and ellipsoidal heights (h), i.e. (cf. Fig. 1) Different gravity reduc- (1) Vertical coordinates realise dif- tions (sometimes no re- ferent physical height types (orthome- δW ( ) ( ) duction) have been ap- tric, normal, normal-orthometric, etc.) ≈ h − HO − N ≈ h − HN − ζ (1) plied to the measured and (2) the corresponding reference γ level differences. surfaces do not coincide with a proper geoid or quasi-geoid. (3) These sur- see, for instance, Hirt et al. 2011, Kotsakis et al. 2012, Tenzer et faces are height-dependent. al. 2011, Pan and Sjöberg 1998, etc. However, the combination Heights at the border (1) Reference levels and vertical co- of these variables ”as they are” partially reects the inconsisten- between datum zones ordinates are usable only in limited cies included in the determination of the different height types (Ta- present discrepancies geographical areas; (2) their combi- at the m-level. nation at regional or global level is ble 2), misrepresenting the ”best possible” values of the wanted i unsuitable. level discrepancies δW 0. This misrepresentation limits the relia- bility of the global vertical reference system realisation to the dm- level, being insufficient to support geodetic activities of high pre- ical and geometric heights (i.e. combination of satellite position- cision. In this context, the establishment of a precise global gravity ing with gravity eld models) for worldwide unied precise height eld-related vertical reference system is still an unresolved prob- determination. Table 1 summarises the main drawbacks of the ex- lem. The Global Geodetic Observing System (GGOS) of the Interna- isting height systems. During the last four decades, the unication tional Association of Geodesy (IAG), taking care of providing a pre- of the existing local height systems in a global one has been inten- cise geodetic infrastructure for monitoring the System Earth (Plag sively discussed. Even though different names have been used for and Pearlman 2009), promotes the standardisation of height sys- this theme, e.g. world height system (Rapp and Balasubramania tems worldwide. With the purpose to bring together existing ini- 1992), global or world vertical datum (Rapp 1983a, Balasub- tiatives and to address the activities to be faced, the GGOS Theme ramania 1994, Rapp 1995a), global vertical network (Colombo 1 ”Unied Height System” was established during the GGOS Plan- 1980), global height datum unification (Ardalan and Saffari ning Meeting 2010 (February 1 - 3, Miami/Florida, USA). Starting 2005), global unification of height systems (Rummel 2001), point are the results delivered by the IAG Inter-Commission Project height or vertical datum problem (Heck and Rummel 1990, 1.2 ”Vertical Reference Frames” (IAG ICP1.2, Ihde 2007), which are Sacerdote and Sansò 2001, Sacerdote and Sansò 2004), vertical compiled in the document Conventions for the Definition and datum connection (Xu and Rummel 1991, van Onselen 1997), Realisation of a Conventional Vertical Reference System global unified height reference system (Ihde and Sánchez -CVRS- (Ihde et al. 2007). These CVRS describe the fundamen- 2005, Sánchez 2007, Kutterer et al. 2012), etc., the objective is al- tal aspects to be taken into consideration for the denition and Journal of Geodetic Science 327 Table 2. Inconsistencies making unsuitable the precise combination of physical and geometric heights. Requirement Present status Ellipsoidal heights h and (quasi)geoid heights N must be - Different ellipsoid parameters (a, GM) are applied in given with respect to the same ellipsoid, i.e. the same geometry and gravity. ellipsoid has to be used (1) for the transformation of geo- - h and N are given in different tide systems: centric coordinates into ellipsoidal
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