Brazilian Geodetic System
On October 13 of 1945, the Leveling Division (SNi) started the High Accuracy Spirit Leveling activities, beginning, in turn, the establishment of the Brazilian Geodetic System (SGB) Altimetric Network. In the District of Cocal, Municipality of Urussanga, Santa Catarina, where the RN 1-A Level Reference is located, the staff, including the engineers Honório Beserra – SNi Director –, José Clóvis Mota de Alencar, Péricles Sales Freire and Guarany Cabral de Lavôr, performed the first spirit leveling in IBGE.
In December of 1946, the connection with the Tide Gauge Station of Torres, Rio Grande do Sul, was accomplished, enabling the altitude estimates of the already implemented Level References. As a result, the objective of Professor Allyrio de Mattos came true: provide Brazil with a fundamental altimetric structure, aimed at supporting mapping and huge engineering constructions. This altimetric structure is of vital importance to projects of basic sanitation, irrigation, roads and telecommunication.
In 1958, when the Altimetric Network covered more than 30,000 kilometers of leveling lines, the Torres Datum was substituted for the Imbituba Datum, defined by the tide gauge station of the port of Imbituba in Santa Catarina. Such substitution rendered a significant improvement of definition in the system of altitudes, considering the Imbituba station had at the time nine years of observation, much more than the Torres station.
The High Precision Altimetric Network (RAA) of the Brazilian Geodetic System (SGB) had several manual adjustments of the leveling observations (1948, 1952, 1959, 1962, 1963, 1966, 1970 and 1975), according to its development and calculus tools at each epoch. The last adjustment, named Preliminary Global Altimetric Adjustment (AAGP), ended in 1993 and corrected some problems of the previous adjustments, such as the application of the pseudo-orthometric reduction, which only takes into account the non-parallelism effect of the equipotential surfaces of the normal gravity field. However, due to the limitations of the programs, the AAGP was conducted in order to divide the RAAP in several macrocircuits (MMCC) and independent adjustments.
Only in the beginning of 2005, it was possible to start the process that led to the simultaneous adjustment, finished in May and made available on June 20 of this year. The organization and preparation of all the data from the RAAP (observations and descriptive records) demanded the generation of specific programs of data analysis to identify and correct inconsistencies. Thus, it was possible to include stations that previously received preliminary values and around 12,000 that had not been estimated yet. In addition, areas that demand new measurements were identified, considering the necessity of the maintenance of several Geodetic stations and the construction of new ones.
The Canadian software, GHOST (Geodetic adjustment using Helmert blocking Of Space and Terrestrial Date), was used for the adjustment calculus. It allows for the simultaneous adjustment of large Geodetic networks. In this adjustment all the RRNN, measured and not calculated, before and after the AAGP, and the RRNN belonging to the leveling line “branches” were included. As a result, adjusted heights of approximately 69000 RRNN were made available, together with their standard-deviations, propagated from the origin of the Network, in Imbituba/SC tide gauge.
Due to the impossibility of establishing Level References in the surroundings of the Amazon River lows, the small portion of the Altimetric Network in the state of Amapá could not be connected to Imbituba. That fact led to the use of the mean sea level in Porto de Santana between 1957 and 1958, originating the Santana Datum.
It is worth highlighting that the introduction of new observations, the used methodology and the corrected inconsistencies resulted in changes in the heights of the old stations.