Geocentric reference system

SIRGAS as geodentric reference system is defined identical  to the International Terrestrial Reference System (ITRS). Its realization is a regional densification of the global International Terrestrial Reference Frame (ITRF) in Latin America and the Caribbean. The reference station positions are associated to a specific (reference) epoch and their variation with time is taken into account by discrete station velocities or by a continuous velocity model, which comprises tectonic plate movements and crustal deformations. Realizations or densifications of SIRGAS associated to different reference epochs and referring to different ITRF solutions materialize the same reference system and, after reducing their coordinates to the same frame (ITRF) and reference epoch, they are compatible at the mm-level. The extension of the SIRGAS frame is carried out by national densifications of the continental network, which serve as local reference frames.

The geographical coordinates are derived by applying the parameters of the GRS80 ellipsoid.

The activities related to realization and maintenance for the geocentric reference system are coordinated by the SIRGAS-WGI: Reference System. The activities related to the densification and application of SIRGAS at national level are coordinated by the SIRGAS-WGII: SIRGAS at National Level.

Vertical reference system

The vertical datums presently used in Latin America refer to different sea levels and different epochs, they do not take care of the height and reference level variations with time, and in most of the countries, the vertical networks were processed without including the corresponding gravity reductions in the levelled heights. These datums present big discrepancies between neighbouring countries, they do not permit data exchange neither in continental nor in global scale, and they are not able to support practical height determination with GNSS techniques.

In contrast, the new SIRGAS vertical reference system shall:

  1. refer to a unified global reference level W0,
  2. be realized by proper physical heights (derived from spirit levelling in combination with gravity reductions or from gravity field modelling of high-resolution),
  3. be connected to the SIRGAS geocentric reference frame, and
  4. be associated to a specific reference epoch, i.e. it should consider the coordinate and referential changes with time.

The official recommendation of SIRGAS about the physical heights is to introduce normal heights; however, since some countries want to adopt orthometric heights, it was decided to define the new vertical reference system in terms of geopotential quantities (W0 as a reference level and geopotential numbers as primary coordinates), and then, in the realization, each country can introduce the desired physical heights with their respective reference surface: the geoid or the quasigeoid.

The definition of the new SIRGAS vertical reference system is identical with the definition of the International Height Reference System (IHRS) given by the resolution No. 1, 2015, of the International Association of Geodesy (IAG). The realization of the new SIRGAS vertical reference system must be a regional densification of the International Height Reference Frame (IHRF).

The activities related to the definition and realization of the new vertical reference system for SIRGAS are coordinated by the SIRGAS-WGIII: Vertical Datum.

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