Reference Frame for South and Central America – SIRGAS

International Association of Geodesy Sub-commission 1.3b

Reference Frame for South and Central America (SIRGAS)

Report July – April 2005

Published on IAG-Commission 1 Bulletin No. 19, pp. 22-23 (2005)

Chair: L.P. Fortes (Brazil)
Vice-Chair: E. Lauría (Argentina)
SC1.3b-WG1: Reference Frame – C. Brunini (Argentina)
SC1.3b-WG2: Geocentric Datum – A. Hernandez (México)
SC1.3b-WG3: Vertical Datum – L. Sánchez (Colombia)

Sub-commission 1.3b (South and Central America) encompasses the activities developed by the "Geocentric Reference System for the Americas" project (SIRGAS). As such, it is concerned with the definition and realization of a unified reference frame for South and Central America, consistent with ITRF, besides promoting the definition and establishment of a unique vertical reference system in this region.

After July 2003, when the term of the elected project president and vice-president was initiated, they contacted the countries and institutions represented in the SIRGAS executive committee (10 – out of a total of 13 – from South America, two from North America – Canada and Mexico – and three from the sponsoring entities – International Association of Geodesy, Pan-American Institute of Geography and History, and National Geospatial-Intelligence Agency) in order to select and/or confirm the names of the Working Groups (WG) presidents. Based on this contact, Claudio Brunini, from the University of La Plata, Argentina, was appointed to the position of president of the Working Group 1 "Reference Frame" (SC1.3b-WG1), Antonio Hernandez, from the National Institute of Statistics, Geography and Informatics (INEGI), Mexico, was appointed as president of the Working Group 2 "Geocentric Datum" (SC1.3b-WG2), whereas Laura Sanchez was confirmed as president of the Working Group 3 "Vertical Datum" (SC1.3b-WG3). The SIRGAS executive committee would like to thank the former WG1 and WG2 presidents, respectively Melvin Hoyer, from the University of Zulia, Venezuela, and Rodrigo Barriga, from the Military Geographic Institute, Chile, for their contribution and dedication to the project.

Concerning the SIRGAS 2000 GPS campaign, the official coordinates of 184 GPS stations covering the entire Americas, referred to ITRF2000, epoch 2000.4, along with their covariance information, had been released in February 2003. A velocity field for South America was released in November 2003, after combining least-squares collocation and finite elements solutions (Drewes and Heidback, 2003). All information is available at the project website (http://www.ibge.gov.br/sirgas). It must be mentioned the work that has been carried out by DGFI, Germany, as the IGS Regional Network Associate Analysis Center RNAAC-SIR for SIRGAS in terms of computing weekly coordinate solutions for the continuous GPS stations in the region as well as velocity values for those stations.

During the last project meeting, held at the INEGI facilities, in Aguascalientes, Mexico, in December 2004, the status of each working group activities was discussed. Representatives from El Salvador and Costa Rica attended the meeting, as an effort to involve countries of that region in SIRGAS. It terms of SC1.3b-WG1, it was decided to establish regional processing centers in the region to carry out a task analogous to that currently performed by the IGS RNACC-SIR. Preliminarily, University of La Plata, Argentina, the Brazilian Institute of Geography and Statistics (IBGE), Brazil, and INEGI, Mexico, were identified as potential candidates to carry out this task. It was also decided to coordinate efforts towards starting a pilot project on ionospheric mapping in the continent. Regarding SC1.3b-WG2, the status of the connection of the national geodetic networks to SIRGAS was surveyed amongst the representatives attending the meeting (Table 1). This table is going to be complemented with information related to the remaining countries of the region. Recommendations for integrating new geodetic stations of each country to SIRGAS frame have also been issued during the meeting. Concerning SC1.3b-WG3, the status of the ongoing efforts for establishing a unified vertical reference system in the continent has been presented. In terms of computing physical heights, every country continued with the compilation of the spirit leveling and gravity data in digital format in order to enable a unique continental adjustment of geopotential numbers. In addition, the connection of the first order leveling networks between neighboring countries was almost completed. For the estimation of the unified height reference surface, there were considerable improvements in the combination of terrestrial gravity data with those coming from the new satellite missions in order to obtain a highly precise (quasi) geoid model. In this sense, the computation of W0 has been evaluated using the Mean Sea Surface (MSS) Model CLS01 in combination with different gravity models, such as EGM96, TEG4 and GMM01. As a complement for the connection of the classical height systems with the new vertical system, several tide gauges in South America are being observed continuously or periodically with GPS. The variations of the ellipsoidal heights are compared with the variations coming from the tide gauge registrations to determine the actual sea surface during the definition period of the classical height systems. For details about discussions and presentations during this last SIRGAS meeting, please see the SIRGAS Newsletter #8, available at the project webpage. The project president and vice-president would like to thank the International Association of Geodesy and the Pan-American Institute of Geography and History, for sponsoring the meeting, and INEGI, for kindly hosting it.

Table 1. Current status of SIRGAS adoption in South and Central America

The next SIRGAS meeting will be held from November 17 to 18, 2005, in Caracas, Venezuela.

Reference

Drewes, H; Heidbach, O. (2003): Deformation of the South American crust estimated from finite element and collocation methods. IAG Symposia (128), A Window on the Future of Geodesy, pp. 544-549, Springer.

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