THE WORLD GEODETIC SYSTEM (WGS 1984)

 WORLD GEODETIC SYSTEM (WGS 1984)

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INTRODUCTORY   DEFINITION

A Geodetic datum or geodetic system is a coordinate system, and a set of reference points, used for locating places on the Earth. Datums are used in geodesy (the branch of mathematics dealing with the shape and area of the earth), navigation, cartography and by Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS) and the Global Orbiting Navigation System (GLONASS). Each datum starts with an ellipsoid (stretched sphere), and then defines latitude, longitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor "base-points".

WGS 84 is an Earth-centered, Earth-fixed Terrestrial reference system and geodetic datum. WGS 84 is based on a consistent set of constants and model parameters that describe the Earth's size, shape, and gravity and geomagnetic fields. WGS 84 is a Global Positioning System (GPS) or called as Global reference system for noting geospatial information.The World Geodetic System (WGS) is a standard for use in cartography, geodesy, and satellite navigation including GPS.  It is as compatible with the International Terrestrial Reference System (ITRS).The latest revision is WGS 84 (also known as WGS 1984, EPSG:4326), established and maintained by the United States National Geospatial-Intelligence Agency since 1984, and last revised in 2004.

                             Descriptive Charracteristics of World Geodetic System 1984

Responsible Organization: National Geospatial-Intelligence Agency 

Abbreviated Frame Name: WGS 84

Associated TRS: WGS 84 

Coverage of Frame: Global 

Type of Frame: 3-Dimensional 

Last Version: WGS 84 (G1674) Reference Epoch: 2005.0

The Global Positioning System uses the World Geodetic System (WGS84) as its reference coordinate system.

WGS Outline Frame 

• Origin:  Earth’s center of mass being defined for the whole Earth including oceans and atmosphere. 
• Z-Axis: The direction of the IERS Reference Pole (IRP). This direction corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) (epoch 1984.0) with an uncertainty of 0.005".
• X-Axis: Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis. The IRM is coincident with the BIH Zero Meridian (epoch 1984.0) with an uncertainty of 0.005".
  
  
  
• Y-Axis: Completes a right-handed, Earth-Centered Earth-Fixed (ECEF) orthogonal coordinate system.
• Scale:  Its scale is that of the local Earth frame, in the meaning of a relativistic theory of gravitation. Aligns with ITRS. 
• Orientation:  Given by the Bureau International de l’Heure (BIH) orientation of 1984.0.
• Time Evolution:  Its time evolution in orientation will create no residual global rotation with regards to the crust.

Defining Parameters

WGS 84 identifies four defining parameters. These are the semi-major axis of the WGS 84 ellipsoid, the flattening factor of the Earth, the nominal mean angular velocity of the Earth, and the geocentric gravitational constant as specified below.  

Transformation Parameters
The parameters are defined from the listed reference frame to WGS 84 (G1674) at epoch 2005.0.  

      (Note : -Rotations are for the position vector rotation convention. Units are meters, mas (milliarcsecons) and ppb (parts-per-billion).1 mas = 0.001 " = 2.77778 e-7 degrees = 4.84814 e-9 radians. 0.001 " corresponds to about 0.030 m at the earth's surface.)

Historical Epic Evolution in WGS 84 Sytem

A unified geodetic system for the whole world became essential in the 1950s for several reasons:

• International space science and the beginning of astronautics.
• The lack of inter-continental geodetic information.
• The inability of the large geodetic systems, such as European Datum (ED50), North American Datum (NAD), and Tokyo Datum (TD), to provide a worldwide geo-data basis
• Need for global maps for navigation, aviation, and geography.

In the late 1950’s, the US Department of Defense began to develop a worldwide system, referenced to geodetic datums, which was intended to establish compatibility between the coordinates of widely separated locations. Efforts of the Army, Navy and Air Force were combined and lead to the development of the DoD World Geodetic System 1960 (WGS60).

In 1966, a tri-service World Geodetic System Committee was assigned the responsibility of developing an improved WGS, based on newly available satellite imagery, surface gravity observations and doppler data, to satisfy evolving mapping, charting and navigation requirements. the committee's efforts resulting in WGS66 which was implemented in 1967 and served the Department of Defense needs for about five years.

WGS development and refinement continued through this time period during which new satellite, surface gravity and astrogeodetic data, from both military and non-military sources, were used to develop WGS72.

The deployment of the GPS constellation allowed the surveying process to be updated in 1984 incorporating GPS as the primary method of reference. This significant accuracy update resulted in WGS84, the current geodetic standard. WGS84 was adopted by ICAO as the standard navigation reference system for international civil aviation in 1989. The vast majority of the world's nations currently use the WGS84 standard to publish waypoint coordinates for navigation inclusive of airports, runways, navigation aids and intersections.

Compliance

ICAO specifies the World Geodetic System 1984 (WGS-84) as the geodetic reference datum Standard for air navigation latitude/longitude coordinates. Although most countries meet the requirement, not all locations have adopted, or have completely converted to, the ICAO standard. Jeppesen has compiled and maintains a WGS84 Status Report based on information found in the Aeronautical Information Publication (AIP) (or equivalent publication) of each country. Note that many countries include only international airport and airspace information in their AIP. Therefore, coordinates associated with domestic and military airports and airspace may not be WGS-84 compliant, even if the status report indicates that the country is "compliant". The Jeppesen Status Report categorises countries as follows:

• Compliant - The country AIP contains a statement that all latitude/longitude coordinates are referenced to the WGS-84 datum
• Partially Compliant - The country AIP contains a statement that latitude/longitude coordinates are referenced to the WGS-84 datum, but there are exceptions or other indication that conversion to WGS-84 is not complete
• Not Compliant - There is a statement in the country AIP indicating that a geodetic reference datum other than WGS-84 is used
• Unknown - There is no statement or an unclear statement in the country AIP indicating the geodetic reference datum used

The most current Jeppesen WGS84 Status Report can be accessed here. Pilot action may be required for a GPS equipped aircraft transiting, conducting an approach or a departure in non-compliant airspace.

Relationship with other reference systems: WGS84 realizations

Both the EPSG database and the NGS website use 'WGS 84' with spaces between 'WGS' and '84'. The EPSG database contains no specific WGS84 datum realizations.WGS 84 (G1674) is aligned to ITRF2008 with the same epoch of 2005.0. The purpose of this alignment is to ensure scientific integrity and follow best practices. The ITRF incorporates multiple methods to realize the reference system such as satellite laser ranging and very-longbaseline interferometry that NGA does not include. Adjusting WGS 84 to ITRF allows the reference frame to take advantage of those methods without directly incorporating them into the coordinate determination software. WGS 84 (G1674) adopted the values of NGA stations coordinates in the ITRF2008 reference frame with the exception of its stations located in Bahrain and Korea. Computations were performed to align the remaining WGS 84 reference stations to this network. For WGS 84 (G1674), all WGS 84 reference stations adopted ITRF2008 velocities of the station or nearby sites. The estimated accuracy of WGS 84 (G1674) is better than one centimeter overall for each 

Geog 2D Code	Datum Code	Short Name	Datum Epoch	Area Code	Area Name	Remarks	Shift
4326	6326	WGS84	1984	1262	World	First realization established by DoD in 1987 using Doppler observations. Also known as WGS84 (1987), WGS84 (original), WGS84 (TRANSIT). For surveying purposes, original WGS84 is identical to NAD83 (1986). WGS84 is connected to ITRF90 by a 7-parameter Helmert transformation.	N/A
		WGS84 (G730)	1994.0			Realization introduced by DoD on 1994-06-29 based on GPS observations. G stands for 'GPS' and 730 is GPS week number. Based on ITRF91.	0.70 meter
		WGS84 (G873)	1997.0			Realization introduced by DoD on 1997-01-29 based on GPS observations. G stands for 'GPS' and 873 is GPS week number. Based on ITRF94.	0.20 meter
		WGS84 (G1150)	2001.0			Realization introduced by DoD on 2002-01-20 based on GPS observations. G stands for 'GPS' and 1150 is GPS week number. Based on ITRF2000.	0.06 meter

WGS84 is standard for GPS The Global Positioning System uses the World Geodetic System (WGS84) as its reference coordinate system. It’s made up of a reference ellipsoid, a standard coordinate system, altitude data, and a geoid. Similar to the North American Datum of 1983 (NAD83), it uses the Earth’s center mass as the coordinate origin. Geodesists believe the error is less than 2 centimeters which is better than NAD83. NAD83 BASICS The NAD83 coordinate reference system is a horizontal adjustment of existing data from previous surveys, Doppler and Very Long Baseline Interferometry (VLBI) data. The geocentric datum is earth-centered/Earth-fixed, utilizes the GRS80 ellipsoid, and is intended to be identical to the original WGS 84 reference frame with the origin at the center of the mass of the Earth. SYSTEM COMPARISON The concept of a world geodetic system is to provide a globally dedicated reference system and to minimize or eliminate the need for local systems. The usual reason for a local coordinate system was to meet the needs for an area before the implementation of a larger system was possible. So often, the worst part of having and maintaining a horizontal system separate from a world system is the means and methods of transformation/translation of data. In the meantime, here are a few of the main differences between WGS 84 and NAD83: While both use a similar ellipsoid, they differ slightly and thus create different results. The coordinate system for WGS 84 is geographic, and the NAD83 system is projected. WGS 84 values are points in space, while NAD83 coordinates are physical locations on the Earth. WGS 84 is based upon the NAVSTAR satellite system, and the NAD83 system is based upon a network of ground points, observation data and CORS. WGS 84 ellipsoid is defined as a geocentric, equipotential frame, whereas NAD83 considers GRAV-D data collection and tectonic plate velocities. While the original WGS 84 system aligns with the NAD83 (1986) adjustment, further refinement of WGS 84 has been completed to maintain similarity to ITRF realizations. Future Plans: NGA plans to conduct a WGS 84 reference frame network adjustment in early 2013 to incorporate IERS Conventions 2010 Technical Note 36 (TN 36). Note that WGS 84 will retain its four defining parameters of which some do not comply with TN 36. When completed, the newest realization of WGS 84 will be published, and NGA (formerly NIMA) Technical Report 8350.2 will be updated.  Visit the NGS website for more information. The initial framework sounds very robust and user-friendly, so keep your eyes and ears open for more details as they develop.