Integration of Land and Sea Vertical References: Involved Concepts, Developed Projects and Challenges

Abstract

The Reduction Levels (RLs) of Tide Gauge (TG) are used, in Brazil, as Ocean Vertical Data or Chart Datum (CD) which are references to nautical charts made by the Hydrography and Navigation Board of the Brazilian Navy (DHN- Diretoria de Hidrografia e Navegação in Portuguese). In contrast to that was adopted in Brazil, the International Hydrographic Organization (IHO) recommends that the Datum reference surface for survey reductions in coastal zone may be defined by the LAT (Lowest Astronomical Tide) calculated from at least 18.6 years of tide gauge observations. The unification of the Chart Datum and the vertical reference integration has been aimed for the Brazilian coast through partnerships between several institutions and the Brazilian Institute of Geography and Statistics (IBGE – Instituto Brasileiro de Geografia e Estatística in Portuguese). Adopting a surface given by the LAT would solve the problems presented by adopting the RLs as CD, such as: the local coverage; the fact that some depths may be smaller than those presented in the chart; the question of determining each RL at different times; the lack of standardization about the time series extension to quantify the RL; and, above all, the impossibility of the connection to the Geocentric Reference System for the Americas (SIRGAS – Sistema de Referência Geocêntrico para as Américas in Portuguese). In order to contribute to the discussions concerning the integration of vertical reference in coastal regions, this article presents concepts involved, projects already developed in other regions of the world and some challenges inherent to Brazil for the unification of the oceanic reference and integration of vertical reference.