SIRGAS (Sistema de Referencia Geocentrico para Las Americas)

Background

The SIRGAS Project was established during the International Conference on the Definition of a South American Geocentric Datum, October 4-7, 1993, in Asuncion, Paraguay, by an invitation of the sponsoring entities: International Association of Geodesy (IAG), Pan-American Institute of Geography and History (PAIGH), and United States Defence Mapping Agency (DMA), now National Imagery and Mapping Agency (NIMA). Representatives of each sponsoring entity and of almost all South American countries participated in that Conference and created the SIRGAS Committee and the Scientific Council. 

The objectives established for the project were the following: 

  • to define a reference system for South America, 
  • to establish and maintain a reference network, and to define and establish a geocentric datum. 
Objectives related to the definition of the reference system and of the geocentric datum for the continent have been achieved in the Asuncion Conference, the plenary having chosen the following: 
  • SIRGAS reference system: IERS Terrestrial Reference Frame (ITRF), 
  • Geocentric datum: coordinate axes based on the SIRGAS reference system and parameters of "Geodetic Reference System 1980 (GRS80)" ellipsoid. 
 There Working Groups were installed to accomplish the objectives:
Working Group I:      Reference System
Working Group II:     Geocentric Datum
Working Group III:    Vertical Datum
At the beginning, SIRGAS was restricted to South America (Sistema de Reference Geocentrico para America del Sur). The reference system was realized by a frame of 58 stations observed by a GPS observation canpaign from May 26th to June 4th, 1995 (ten days of continous measurements). The final set of station coordinates in the International Terrestrial Reference Frame (ITRF 94) was derived from two network adjustments performed by Deutsches Geodädisches Forschungsinstitut (DGFI) and by NIMA and was released during the IAG Scientific Assembly in Rio de Janeiro, August 1997.
The geocentric datum is realized in the individual countries by basic control networks observed by GPS and connected to the existing triangulation networks. These national networks are densifications of the continental SIRGAS frame based on the same reference system.
The vertical datum is classically defined by tide gauges (one for each country) and realized by spirit levellings of individual points. In future, it will be observed by GPS techniques and refer directly to the geoid. The relation between tide gauges and the geoid as well as between leveled heights and ellipsoidal heights observed by GPS has to be determined. For this purpose from May 10th to May 19th, 2000, a GPS campaign was carried out in order to observe all the datum defining tide gauges and some base stations forming the vertical reference frame . This campaign also was the first repetition measurement of the SIRGAS geocentric reference frame. A network of contiuously observing GPS stations is installed in South America in the frame of the International GPS Service (IGS) processed weekly by the RNAAC-SIR.

In January 2001, the 7th United Nations Regional Cartographic Conference for the Americas recommended to use the SIRGAS reference frame in all American countries. During the General Assembly of the International Union of Geodesy and Geodynamics (IUGG) the project was extended to all the Americas (North, Central, South America and Caribbean). The GPS campaign May 2000 included the entire area (Fig.2).

Fig.1 SIRGAS 1995 stations
Fig.2 SIRGAS 2000 stations
(You may click on these figures to get a properly sized pdf-version)

For more details see the SIRGAS Bulletin No.6 (2002). More documents about SIRGAS on the http server of IBGE (Brazil)
ds - 06/2000 Comments & suggestions