A RECENT HISTORY OF GEODETIC DATUMS IN BRAZIL

From the early 1900's until the 1950´s, geodetic and mapping work in Brazil was conducted by the Army's Geographical Service. Since then, the responsibility for systematic topographical mapping has been shared with the Brazilian Institute of Geography and Statistics (IBGE), and States and Municipalities have the coordination of cadastral mapping. In terms of geodetic activities, IBGE is the institution responsible for them since then.

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From about the time IBGE started its work in mapping until the 1980's, Brazil adopted a geodetic datum known as Córrego Alegre (the name comes from the geodetic marker used as the origin), based on the Ellipsoid of Hayford. In the 1980's, the South American Datum of 1969 (known as SAD-69), based on the international ellipsoid of 1967, was defined by law as the official datum to be adopted.

In practice, several institutes and agencies kept on using Córrego Alegre (most notably some very important state agencies in the states of São Paulo and Minas Gerais) and still do today, maps are produced in both datums. With GPS era, a network adjustment was carried out, connecting GPS and classical networks. This new realization of the SAD69 has been called SAD-69/96. With this, new maps were created using this realization and causing the change of few metres in a geodetic marker's coordinates.

Today two non-geocentric datums co-exist in Brazil:

    • Corrego Alegre
    • SAD-69

With the last one being realized twice, as mentioned above.

To solve this situation, the moving to a geocentric coordinate system showed to be a solution, which would have the additional benefit that positioning with GPS would be compatible with the maps.

However, moving to a geocentric coordinate system would cause those who have adopted the new realization SAD69/ 96 to change everything again. There are also other questions:

    • What about those who have remained with the old Córrego Alegre? Would they change?
    • How much would this change cost?
    • What complications would this change bring to data bases?
    • What should other agencies, institutes, private companies do to accommodate this change?
    • What guaranties would there be that a new datum would simply become another one?

With the suggestion that IBGE took a piecemeal approach to solving the problem, a project to move towards a new, geocentric coordinate frame was proposed by IBGE to the Brazilian community in October 2000, during a Workshop in Rio. The project was called the Projeto Mudança do Referencial Geodésico no Brasil (PMRG) or Project for Changing the Geodetic Reference System in Brazil.

In September 2004, Canada signed a cooperative agreement with Brazil to assist in the migration to SIRGAS2000 (the chosen one), giving rise to the project entitled the National Geospatial Framework Project or PIGN - Projeto Infra-estrutura Geoespacial National.

The Workshop II on Geocentric Reference System in Brazil, in November 2004, was the first joint event of the Brazilian/Canadian cooperation project and was the sequence of the Workshop I on Geocentric Reference System in Brazil, in 2000, when IBGE presented the proposal of changing the Brazilian geodetic system to a geocentric one. It counted with representatives of relevant government and private companies, universities and users that deal with geospatial information. With the choice of SIRGAS2000 as the new Brazilian geodetic reference system during the first workshop, the main purpose of the second workshop was to discuss with the community about the possible impacts of SIRGAS2000 implementation, to provide them with all the possible support on the move and to identify demonstration projects in the scope of the PIGN project.

In January 2005, a federal law was signed defining IBGE as responsible for defining the horizontal and vertical reference systems for geodesy and cartography in Brazil. As a result, in February 2005 a resolution from IBGE´s President officially established SIRGAS2000 as the geodetic reference system for the country and defined a transition period of no more than 10 years when the old and the new systems can be used at the same time. The signatures of these documents were immediately followed by the availability of a new and friendly geodetic database and geoid model with coordinates in both systems to the users. It was also made available on internet were all the presentations from the Workshop II and a document with information about the reference system change.