GEOG 862
GPS and GNSS for Geospatial Professionals

The International Terrestrial Reference System (ITRS)

map showing the International Terrestrial Reference System velocity field from 2008
ITRF Velocity Field
Source: ITRF

As NAD83 has aged, there has been constant improvement in geodesy. When NAD83 was created, it was intended to be geocentric. It is known that the center of the reference ellipsoid of NAD83, GRS80, is about 2.24 meters from the true geocenter. However, there is a reference system that is geocentric. It is known as the International Terrestrial Reference System, ITRS. The reference frame derived from it is the International Terrestrial Reference Frame (ITRF). Its origin is at the center of mass of the whole earth, including the oceans and atmosphere. The unit of length is the meter. The orientation of its axis was established as consistent with that of the IERS’s predecessor, Bureau International de l’Heure, BIH, at the beginning of 1984.

Today, the ITRF is maintained by the International Earth Rotation Service (IERS), which monitors Earth Orientation Parameters (EOP) for the scientific community through a global network of observing stations. This is done with GPS, Very Long Baseline Interferometry (VLBI), Lunar Laser Ranging (LLR), Satellite Laser Ranging (SLR), the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and the positions of the observing stations are considered to be accurate to the centimeter level. Recognizing that the several hundred control stations worldwide for which it publishes yearly coordinates are actually in motion due to the shifting of approximately 20 tectonic plates worldwide, the IERS also provides velocities for them. The International Terrestrial Reference Frame is actually a series of realizations. The first was in 1988. In other words, it is revised and published on a regular basis.

The vectors that you see on this illustration indicate the motion of the tectonic plates. You can see the legend there, the length of two centimeters per year. It shows the directions that these tectonic plates are in motion relative to the center of mass of the Earth.