As far as I understand the EGM96 defines the Geoid, where as the WGS84 Standard defines the Ellipsoid.
Is the ellipsoid defined in the WGS84 standard defined in a way to maximize the congruency with the the geoid defined in the EGM96 standard?
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1Actually Ian's answer is correct. WGS84 includes both a reference ellipsoid and a geoid, but the geoid was updated/replaced by the more accurate EGM96. Therefore, people often mean the reference ellipsoid when they talk about WGS84.– user41733Dec 9, 2014 at 17:09
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Actually, that is not the way I understand it the Earth Gravitational Model 96 contains Harmonics to the 360 degree, I've seen an actual file, and it starts with index 2 2, 3 0, 3 1, 3 2, 3 3, 4 0 ... 4 4,..., 359 0... 359 350, and finally 360 0... 360 360 with each line having coefficients! these coefficients are then used with another program to GENERATE the 15 minute interpolated GRID of GEOIDs!– user129775Oct 11, 2018 at 22:14
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2 Answers
Ian's answer is incorrect.
WGS84 approximates Earth by an elipsoid, which is basically a deformed sphere. EGM96 is a more complex model based on the gravitational force of the Earth (which is not constant) that defines what "sea level" or "up/down" mean, a smooth but irregular shape called "geoid". WGS84 is the elipsoid that best fits that geoid, and this fit has been updated as more accurate measurements of the geoid have been carried out over the years. WGS84 is not outdated; it's just a simplified mathematical model used by positioning systems like GPS, even if a geoid is technically more accurate when it comes to define the height over the sea level (since this is different from GPS altitude). You just have to translate coordinates when you need such distinction.
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7Good explanation. WGS84 (elipsoid) and EGM96 (geoid) are two different models, with different applications. When you see a height in a map, it is usually a height over the geoid (orthometric height), but when you get one from a GPS device, it is usually a height over the elipsoid. There are online tools to transform between them as needed, using what it is called
geoid undulation. E.g. geoid-height-calculator Aug 2, 2014 at 11:48
a quick google of the two will lead you to http://en.wikipedia.org/wiki/World_Geodetic_System
From that page: Updates and new standards The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).
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I've read this but didn't really understand it, maybe because English isn't my first language or maybe because I'm missing domain specific knowledge, hence my question. So both standards define a geoid and an ellipsoid (with EM96 superceding WGS84). Ok. For me the question that remains is then, that if both ellipsoids have the goal to maximize congruence with its geoid (being defined in the same spec).– oschrenkApr 24, 2012 at 12:33
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Ok. After some more reading I get it. The ellipsoid is in fact defined in a way to maximize the congruency (being defined as the theoretical equipotential lines of gravity/the geoid). It's always stated that "the geoid differs from the ellipsoid" in such and such way, which for me is kind of backwards. I think that is what confused me. So in conclusion: The WGS84 Standard defines both, the geoid and the reference ellipsoid. EGM96 also defines both. The geoid in EGM96 though has a better resolution. The changed geoid didn't result in a change of the reference ellipsoid though.– oschrenkApr 24, 2012 at 14:33