Talk:World Geodetic System
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- 1 Text from Geodesy for the Layman
- 2 Ellpsoids not Geoids?
- 3 system,datum
- 4 EGM06
- 5 WGS 64??
- 6 Link to Mercury
- 7 Zero meridian
- 8 relationship to TRANSIT / NAVSAT / NNSS
- 9 Notation
- 10 Link for "collocation", please
- 11 Accuracy of a and b ?
- 12 EGM96 is NOT a Geoid
- 13 history/helmert's book
- 14 WGS84 invalid after 2010?
Text from Geodesy for the Layman
This text is taken from the public domain resource Geodesy for the Layman at http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003E.HTM#ZZ11 -- please Wikify as necessary. This document was written in 1984 and may need to be updated.
Ellpsoids not Geoids?
Aren't these all ellipsoids and not geoids? These WGSxx ellipsoids seem like the best fitting ellipsoids for the geoids, but they are still theoretical surfaces and not gravity based. DavidForrest 01:56, 4 April 2006 (UTC)
- The WGS84 and EGM96 define both an ellipsoid and a geoid, with the ellipsoids essentially "first order" geoids. The EGM96 geoid is definitely gravity based. Unfortunately, the ellipsoid was not changed from the earlier WGS84 parameters because this would have seriously impacted the operation of GPS. MFago 14:05, 4 April 2006 (UTC)
geodetic system == geodetic datum, need to clatify that. The word "datum" is used in GPS geceivers, so it is very important.
"This new model will have a geoid with a resolution approaching 10 km, requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84)."
The resolution of 10 km is wrong. Perhaps it is 10 cm? Also, it would be good to include the maximum spherical harmonic degree of the EGM96 and EGM06 models (360 vs. 2160?). Lunokhod 19:44, 13 November 2006 (UTC)
In the second paragraph was that previous versions of WGS were wgs 72, wgs 64 and wgs 60. But in the rest of the article and also in other sources in not any wgs 64, but wgs 66 - so i will correct it, ok? --Stardust 00:11, 2 April 2007 (UTC)
Link to Mercury
The the astronautic Mercury datum links to Mercury (planet). I'm fairly confident the planet is irrelevant here so I'll remove the link. I assume it should link to Project_Mercury but I'm not confident enough fill that link in. EdDavies 20:10, 30 April 2007 (UTC)
The longitude positions on WGS84 agree with those on the older North American Datum 1927 at roughly 85° longitude west, in the east-central USA. By contrast, the zero meridian of WGS84 is about 100 metres east of the Prime Meridian at Greenwich, UK.
North American Datum 127 says it is based on a geodetic base point in Meades Ranch, Kansas. So what I don't understand is - why happens WGS84 to be approximately 100 meters apart from the traditional Prime Meridian in Greenwich? What is the definition of the WGS84's zero meridian? --Abdull 11:27, 3 May 2007 (UTC)
- Found it out myself and put it in the article. --Abdull 12:11, 3 May 2007 (UTC)
WGS 84 uses the zero meridian as defined by the Bureau International de l'Heure, which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.
The WGS84 Zero Meridian (IRTF) link incorrectly points to the unrelated Prime Meridian page.
Is it not the case that a discrepancy in originally defining the Meades Ranch datum's longitude, was then propogated into the WGS84 TRF, resulting in the differences between Greenwich & WGS84 ? http://www.ngs.noaa.gov/faq.shtml#WhatDatum
relationship to TRANSIT / NAVSAT / NNSS
After reading through this page, and also looking through the Transit (satellite) page, I noticed that while 'Navy Navigation Satellite System' is mentioned on the WGS84 page, no link is provided to the TRANSIT page. I notice that searching wikipedia for 'NAVSAT' or 'TRANSIT' will take you to the appropriate page, but searching for 'NNSS' does not. I don't know quite enough about wikipedia to know how to make the NNSS acronym take you to that page, but perhaps it should at least be linked from this page? Also, I have found a lot of great background information on both WGS84 and TRANSIT / NAVSTAR / NNSS in the following source:
Seeber, G. (2003). Satellite Geodesy (2nd ed.). Berlin, Germany: Walter de Gruyter.
Could any kind person with more knowledge of WGS 84 explain the notation of a coordinate in WGS 84? (with °, ′ and ″ symbols). I really miss this part in the article. Cristan (talk) 12:33, 4 March 2009 (UTC)
Link for "collocation", please
The word "collocation" is used in the article in the section called "A new World Geodetic System: WGS 84". I was going to Wikilink "collocation" but I am not qualified to decide if the link should be to the "Collocation (remote sensing)" article or the "Collocation method" article. I request that someone with expertise wrap some braces around "collocation" and point the link in the proper direction. Thank you. O'Dea (talk) 00:04, 27 August 2010 (UTC)
Accuracy of a and b ?
Why is "a" (major radius) quoted as = 6,378,137 m (which implies an accuracy +/- 1m) and "b" (minor radius) quoted as 6,356,752.314 245 m (which implies an accuracy of +/- 0.001 mm)? Surely they should both be quoted to the same accuracy? Is "a" supposed to be 6378137.000000 m, or is "b" supposed to be 6356752 m?
- "a" is exact; "b" is exactly (297.257223563/298.257223563) times a. Tim Zukas (talk) 02:11, 14 November 2011 (UTC)
EGM96 is NOT a Geoid
Presently WGS 84 uses the 1996 Earth Gravitational Model (EGM96) geoid, revised in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution). The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about -105 m to about +85 m. EGM96 differs from the original WGS 84 geoid, referred to as EGM84.
EGM96 is wrongly referred to as a geoid.
It is a model of geo-potential (gravimetric) variation, and, as such, may be used to correct the ellipsoidal height as determined by the GPS system, to an orthometric (geoidal) height, commonly referred to as 'above mean sea level'. Other than in this context, it has nothing to do with WGS.
The error values quoted have nothing to do with EGM96, but refer to the error between the ellipsoidal height derived by the GPS system and the geoidal height. The EGM96 parameters are commonly used to remove this error, but leave a residual error in the region of 150 to 750mm, depending on the actual height above mean sea level, local topography, etc. The higher order EGM08 database, which has superceded EGM96, claims a residual error somewhere in the region of 20mm.
If you decide to retain this section, I'd recommend removing the meaningless part about 'spherical harmonics of degree 360'. The ACTUAL definition is included in the linked page if anyone feels it is worth keeping. Although (once corrected) it is a true statement, referring to the method of computation of the mathematical model defined by the EGM96 data, I fail to see what it adds to anyone's understanding of WGS. --126.96.36.199 (talk) 13:06, 13 March 2011 (UTC)
the original title is "Die mathematischen und physikalischen Theorien der Höheren Geodäsie" not "Mathematische und Physikalische Theorien der Physikalischen Geodäsie" —Preceding unsigned comment added by 188.8.131.52 (talk) 18:32, 15 May 2011 (UTC)
WGS84 invalid after 2010?
'The latest revision is WGS 84 (dating from 1984 and last revised in 2004), which will be valid up to about 2010'