Clarification on the relation between the SGB´s Vertical Datum (Imbituba and Santana) and the Reduction Levels and Hydrographic “Zeros” in the Brazilian Shoreline
By means of the Geodetic Database - BDG, the IBGE´s Coordination of Geodesy - CGED provides geodetic information used in engineering projects, mapping and scientific studies for all the Brazilian technical-scientific community. The most frequently asked information is about the height of the vertical stations (Level References, RRNN) of the Brazilian Geodetic System - SGB, for use in studies in which unevenness or declivity are important – for example, in the implementation of a system of water supply and basic sanitation.
The heights of the SGB come from two particular values of the average sea level (NMM), called Datum of Imbituba and Datum of Santana. The former was established from reports of the sea level carried out in the port of the same name (SC) between 1949 and 1957. To establish the Datum of Santana (AP), observations made in 1957 and 1958 were used (Alencar, 1990; FEMAR, 2000; Luz and Guimarães, 2003).
The use of heights from the SGB shows specific difficulties for the vertical positioning in coastal regions. The level of the local sea is a critical element for successful coastal works, though its relation with the vertical data of the SGB is often unknown. The most common situation also involves the characterization of the local tidal regime, since users desire a minimum and maximum reference level rather than an average value.
In addition to the most used reference levels in the Brazilian shore and their approximate relation with the data from Imbituba and Santana, the text presents the Permanent Geodetic Tide Gauge Network - RMPG, conceived and operated by the IBGE to refine the SGB heights.