A Word about Datum

When asked “what is the reference frame used by GPS?”, one will answer of course: “WGS 84”. But when asked which revision of WGS 84, there might not be an answer at all. Furthermore, GPS, SBAS (WAAS, EGNOS, MSAS, GAGAN, etc), Omnistar, radiobeacon, local base stations, etc, all use different datum or reference frames. It is very important to understand the differences between the various implementations within a specific datum and make sure that the maps and GPS data are in the same revision of the datum used.

GPS and the History of the WGS 84 Evolution
(The text in italic comes from the following NGS source: http://www.ngs.noaa.gov/faq.shtml#WGS84)


WGS 84 is the World Geodetic System of 1984. It is the reference frame used by the U.S. Department of Defence (DoD) and is defined by the National Geospatial-Intelligence Agency(NGA) (formerly the National Imagery and Mapping Agency) (formerly the Defence Mapping Agency). WGS 84 is used by DoD for all its mapping, charting, surveying, and navigation needs, including its GPS “broadcast” and “precise” orbits. WGS 84 was defined in January 1987 using Doppler satellite surveying techniques. It was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987.

This initial version of WGS 84 is now commonly referred to as WGS 84 (Original).

At 0000 GMT January 2, 1994, WGS 84 was upgraded in accuracy using GPS measurements. The formal name then became WGS 84 (G730) since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994.


At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) 94. It was then called WGS 84 (G873). WGS 84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997.

On January 20, 2002, the latest realization of WGS 84, called WGS 84 (G1150) was adopted and is the current version. This realization is based on the ITRF 2000.

Between the first implementation of WGS 84 (Original) and the current revision, the differences can be 1-2 meters. For North America, the following online datum transformation tool from the NGS can be used:
http://www.ngs.noaa.gov/cgi-bin/HTDP/htdp.prl?f1=4&f2=1

 

The following section explains why the reference frame change for GPS orbits.

GPS and the International Terrestrial Reference Frame (ITRF)

GPS orbits are computed from data collected by a global network of receivers coordinated by the International GPS Service for Geodynamics (IGS). The accuracy of the GPS orbits depends on many factors, including the accuracy of the coordinates of the data collection sites. The earth’s surface is not fixed and rigid like an egg shell. It consists of many sections, or plates, which move slowly over time in various directions and rates in a process called crustal motion. Scientists have been studying this movement for several reasons. This includes wanting to know where land masses are with respect to one another and where they will be in the future. Since IGS sites are located on these crustal plates, we must be able to estimate where the sites are when the data are collected.


The International Earth Rotation Service (IERS) periodically computes the positions of the sites for a given date. The sites define the IERS, International Terrestrial Reference Frame (ITRF) and the date defines the epoch. IERS also computes the movements (or velocities) of the sites to estimate where the sites will be in the “near” future with some degree of accuracy. The ITRF is an internationally accepted standard, and is the most accurate geocentric reference system currently available. The longer the sites operate, the better the positions and velocities can be determined and the more accurate the orbits will be.

Both GPS and SBAS are based on the ITRF 2000 reference frame, with the following difference:

 

  • GPS has fixed the epoch in January 2002 (in other words, for GPS, the crustal motion of the earth has stopped since 2002 and until the next revision)
  • SBAS (WAAS, EGNOS, MSAS, etc) on the other end, recalculate the location of the ground stations on a regular basis (almost every year) and predict (or project) the velocities a few months in the future or to the middle of the following year.

GPS Coordinates and Map datum

When using a GPS receiver in autonomous mode (with no differential correction), the coordinates output are in WGS 84 (G1150), the current GPS reference frame. When using differential corrections from:

 

  • SBAS: coordinates output by a GPS receiver are in ITRF 2000 (current epoch)
  • Omnistar VBS Service in North America: coordinates output are in Nad 83 (Original). (OmniSTAR has elected to keep its North American VBS system on the NAD83 datum to stay consistent with prior VBS survey data.)
  • Omnistar VBS Service Worldwide (excluding North America): coordinates output are in ITRF 2005 (current epoch)
  • Radio Beacon: verify with you local Coast Guard for the datum used
  • A local base station (realtime or Post-processed): the rover will follow the datum of the coordinates used at the base station
  • Canadian Precise Point Positioning (PPP) Online post-processing: choice of Nad83 (CSRS98) or ITRF 2005

Around the world, local datums have also evolved. For example in the United States, the North American Datum of 1983 (NAD 83) was originally equivalent to WGS 84. Since then it has followed its own path and now comes in various flavours: NAD 83 (CORS 96) or HARN. The same for Canada migrating from NAD 83 (Original) to NAD 83 (CSRS 98)
http://www.geod.nrcan.gc.ca/publications/papers/abs6_e.php

Fortunately, the newer revisions of local datums are being tied to some ITRF revision, allowing the use of 6, 7 or 14 parameter transformations. It is important to make sure that the field data collection software or office GIS/mapping software support the various revisions of datums and are up-todate on the transformation parameters.

 

Useful Datum References


International Terrestrial Reference Frame:
http://itrf.ensg.ign.fr/

International Earth Rotation and Reference Systems Service:
http://www.iers.org/


Canada


Canadian Spatial Reference System (Natural Resources Canada, NRCan):
http://www.geod.nrcan.gc.ca/index_e.php

NRCan TRNobs Online transformation tool:
http://www.geod.nrcan.gc.ca/tools-outils/tools_terms_e.php?file=apps/trnobs/trnobs&apps=TRNOBS&info=trnobs
(provides with transform parameters used)

Paper – Realization and Unification of NAD83 in Canada and the U.S. via the ITRF:
http://www.geod.nrcan.gc.ca/publications/papers/abs6_e.php


USA


Horizontal Time Dependent Positioning, NGS online transformation tool:
http://www.ngs.noaa.gov/cgi-bin/HTDP/htdp.prl?f1=4&f2=1

Paper – Transforming Positions and Velocities between the International Terrestrial Reference Frame of 2000 and North American Datum of 1983:
http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf

A List of Papers and Articles from the NGS Website:
http://www.ngs.noaa.gov/CORS/Articles/

 

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