### Terrestrial Reference Frames (TRF)

As mentioned, a reference frame is the materialization or realization of a reference system. It usually consists of a network of monumented geodetic control points with precise coordinates. Coordinate velocities may also be included for a dynamic system. A TRF is usually a realization of a TRS at a specified epoch in time.

### International Terrestrial Reference Frame (ITRF)

The International Terrestrial Reference System (ITRS) was proposed and implemented over two decades ago. It is physically realized by a global network of a few hundred stations observing one or multiple fundamental geodetic techniques such as Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS), or the Global Navigation Satellite System (GNSS). Known as the International Terrestrial Reference Frame (ITRF), it provides a set of stable, accurate and globally consistent coordinates for the study of earth dynamics at most spatial scales. ITRF is a dynamic global reference designed to support all geodetic and earth science applications. It underpins national geodetic datums, supports satellite navigation, and enables the monitoring of tectonic plate motion. Since it is not fixed to any specific tectonic plate, ITRF station coordinates will change over time. Therefore, ITRF coordinates are valid for specific dates (called epochs) and are accompanied by velocity estimates that reflect station motion and are useful to propagate coordinates over time. Individual realizations of ITRF are denoted by ITRFyy, where yy represents the last year for which data was included in a particular solution. Since 1990, ITRF realizations have been the most accurate and stable terrestrial reference frames available. Details regarding ITRF realizations, including datum definition and transformation parameters, can be found at https://itrf.ign.fr/en/solutions/.

### World Geodetic System 1984 (WGS84)

WGS84 is a three-dimensional ECEF reference system that was originally developed to serve as the official GPS reference system. Unlike ITRF, the WGS84 definition includes the parameters of a reference ellipsoid supporting both Cartesian and ellipsoidal coordinate representations. WGS84 ellipsoidal parameters are identical (up to a very small difference in flattening) to the Geodetic Reference System of 1980 (GRS80). The origin is defined at the center of mass of the whole Earth, including its oceans and atmosphere. Its *z*-axis corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) at epoch 1984.0. The *x*-axis of the system is at the intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the *z*-axis. The original WGS84 agreed with the original realization of NAD83 at the meter-level. However, WGS84 has been updated several times since and has now been aligned to ITRF with centimeter level accuracy. The latest version called G1762 (G stands for GPS and 1762 is GPS week number) was introduced on October 16, 2013. More details about WGS84, including transformation parameters between WGS84 (G1762) and past WGS84 realizations as well as some ITRF realizations, can be found at https://confluence.qps.nl/pages/viewpage.action?pageId=29855173&src=contextnavpagetreemode.

### North American Datum of 1983 (NAD83)

Also called NAD83 original, it is the spatial reference system used as national standard for georeferencing in both Canada and the United States. NAD83 system definition was based on a global reference system known as the BIH Terrestrial System 1984 (BTS84) and the reference ellipsoid of the Geodetic Reference System 1980 (GRS80). This means that the size and the shape of both NAD83 and WGS84 are nearly identical. However, solutions from space-based positioning and ITRF comparisons revealed over the years that NAD83 was offset by about 2 m from the true geocenter. NAD83 is defined as a static reference system (for coordinate values to remain constant over time), anchored to and rotating with the North American tectonic plate. It is mainly used for horizontal control and affected by errors of about 0.3 m on average across Canada, although distortions exceeding 1 m are present in many regions. Several updates to this original realization have occurred since it was first introduced in 1986 (see Table 1), mainly to include new GPS-based solutions. The table shows how NAD83 has been transformed from a traditional terrestrial horizontal to a space-based 3D reference frame called NAD83(CSRS). More details about NAD83 can be found at https://confluence.qps.nl/pages/viewpage.action?pageId=29855153.

### NAD83(CSRS)

NAD83(CSRS) is the three-dimensional modernized realization of the original NAD83. It has been rigorously tied to the ITRF since 1996 and regularly updated since the turn of the century. On average, the horizontal components of NAD83(CSRS) are at least two orders of magnitude (100 times) more precise than those of NAD83. NAD83(CSRS) is supported and maintained by sparse networks of active and passive control stations. NAD83(CSRS) is also tied to ITRF by a subset of core and regional ACS forming a subset of the ITRF network. These stations effectively act as both ITRF and NAD83 reference points, linking both systems and ensuring that our national frame is globally integrated.

NAD83(CSRS) has had several updates and closer alignment with ITRF has been made possible with the improvement of the transformation parameters that link the two systems. A summary is shown in Table 1. As the ITRF uses dynamic coordinates, reference years are assigned to the different frame realizations.

Table 1. Summary of NAD83(CSRS) realizations

Version | Epoch | Frame | Adopted | Description |

v0 | 1986 | Original | 1986-1993 | Horizontal Adjustments |

v1 | 1988.0 | CSRS96 | 1996 | Transformed from ITRF93 |

v2 | 1997.0 | CSRS98/CSRS | 1998 | Transformed from ITRF96 |

v3 | 1997.0 | 2000 | Transformed from ITRF97 (the first complete CBN) | |

v4 | 2002.0 | 2002 | Transformed from ITRF2000 | |

v5 | 2006.0 | 2009 | Transformed from ITRF2005 | |

v6 | 2010.0 | 2012 | Transformed from ITRF2008 | |

v7 | 2010.0 | 2018 | Transformed from ITRF2014 (within a few mm of v6) |