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336A. T. LEMOS & R. D. GHISOLFITable II. MSL discrepancies between that measuredlocally and the reference level transported mechanicallyby geometric operations from Imbituba (SC). Adaptedfrom Alencar (1990).Tide gauge stationsGapTorres (Rio Grande do Sul) +0.0584Itajaí (Santa Catarina) +0.1399Paranaguá (Paraná) +0.0010Rio de Janeiro (Rio de Janeiro) +0.1237Vitória (Espírito Santo) +0.2840Fortaleza (Ceará) +0.2923Belém (Pará) +0.8808Positive values indicate that the local MSLis above the MSL referenced to Imbituba. Accordingto Alencar (1990), the differences were accountedfor by the errors in measuring sea level locally aswell as by the instrumental, operational andgravimetric errors associated with the procedure oftransporting the reference from Imbituba. Othercauses may be due to meteorological andoceanographic factors, such as sea-surfacetemperature and salinity, and the lack of accurateinformation about MSL.To achieve the main PTNG goal, it isnecessary to repeat the leveling operations betweenstations using gravimetric information, and controlthe horizontal and vertical movements using CGPSestimates. These procedures will allow for ameticulous determination of the tide-gauge RRNNaltitude as well as help in identifying the crustalmovements, because any movement introduces errorin the MSL estimates.The PTNG is also a part of the SIRGASProject (Reference System to the Americas(‘Sistema de Referência para as Américas’), a studythat began in 1993. The project aims to define aunique reference system for the whole of SouthAmerica, to establish and maintain a referencenetwork among the South American countries, aswell as to define a geocentric Datum. The readerscan refer to SIRGAS (1997) for more details aboutthe project.Until 1993, every measurement assigned tothe Datum and to the RRNN were (and most, or all,of them still are) topocentric. According to Freitas etal. (2002), the geocentric positions of the tidegauges serve as an initial condition to associate theMSL with global geoids. Thus, the contribution ofSIRGAS to the studies on MSL comprises producingaccurate positioning of the tide gauges and RRNNs,and their respective referencing to the newgeocentric Vertical Datum.The MSL topography, defined as thedistance between the MSL and the geoid, shouldalways be used to correct the tide gaugeobservations (Luz et al. 2008). The MSL topographyresults from the almost continuous action of manymeteorological and oceanographic factors active onthe sea surface, mainly on the shallow coastalregions. Thus, it is necessary for each tide gaugestation to determine a specific value for the MSLtopography, which was not possible until after 1995,making it difficult to correlate the MSL of theVertical Datum region with that measured by thetide gauge stations along the coast. The technologythat enables accurate estimates of MSL topographyis altimetry satellite. The altimetry information wasavailable since after the launch of a Franco-American mission carrying the TOPEX/Poseidon(T/P) sensor in 1992 (Dunbar & Hardin 1992). In2005, the T/P mission finished its operations, whenit was substituted by the Jason-1 satellite, launchedin 2001. Despite the importance of the altimetrysatellite in determining MSL topography, theestimates along the coastal regions are subject tosome atmospheric and geophysical corrections oflow accuracy. Nevertheless, Bosch & Savcenko(2007) used T/P and Jason-1 observations between2002 and 2005 to estimate MSL topography inglobal coastal areas, using one-dimensional spectralfiltering. Luz et al. (2008) applied the samemethodology to the south-southeast Brazilian coast,with the purpose of solving the problem ofintegrating the RMPG results with the RRNNs. Theresults obtained showed some problems inestimating the MSL topography, mainly due to thespatial extension of the filter used to make thesatellite and geoid model estimates compatible.However, the authors suggested a further studyusing the EGM-2008 (Earth Gravitational Model –US – National Geospatial-Intelligence Agency –NGA) model, which allows for a better spatialresolution of the geoid model in shallow waters.Matching deep-water sea level estimates and in situtide gauge measurements, obtained along thesatellite tracks, is also one of the objectives of thePTNG.Present state of sea level measurements along theBrazilian coastDuring the last century, some effort has beenmade to collect sea level information along the Braziliancoast. This aspect has been briefly reviewed inPan-American Journal of Aquatic Sciences (2010), 5(2): 331-340