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I found out that WGS84 is an ellipsoid with a "geoid component", but I'm unsure exactly how that can be. This is from PostGIS in Action, Second Edition (pg. 64), but it seemed a bit vague.

Fortunately, today the world is settling on the World Geodetic System (WGS 84) and Geodetic Reference System (GRS 80) ellipsoids, with WGS 84 becoming the standard of choice. WGS 84 is what all GPS systems are based on. [...] To call WGS 84 simply an ellipsoid isn’t quite accurate. The WGS 84 GPS systems we use have a geoid component as well. The present WGS 84 system uses the 1996 Earth Gravitational Model (EGM96) geoid and is the ellipsoid that best fits the geoid model for the selected survey points in the set.

How does an elipsoid coordinate system also have a "geoid component"?

  • I would go over to the ESRI boards and tag Melita Kennedy (@MKennedy-esristaff) in a new thread asking the same question. She's pretty active and definitely one of the tops in this area. – Jvhowube Oct 19 '16 at 17:29
  • @MickyT sure, why not? an irregular shape stapled onto earth at a single place? The same way NAD27 was stapled onto Meades Ranch? I could be wrong? but that's how I would picture it. – Evan Carroll Oct 20 '16 at 1:48
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    I do not know if it will help you to read NGA's EGM2008 page. It says originally calculated on a mean-earth model, then adjusted to tie it to WGS84 (ellipsoid, I assume). – mkennedy Oct 20 '16 at 18:18
  • Thanks @mkennedy! I think that helped a lot!! I asked and self-answered one of my own questions with that. gis.stackexchange.com/q/214965/6052 I'm still confused at how the geoid model EGM2008 is placed on the WGS84 elipsoid. – Evan Carroll Oct 20 '16 at 19:18
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WGS84 is natively XYZ, like the International Terrestrial Reference Frames (ITRF), and you can use an ellipsoid model to convert to latitude, longitude, and ellipsoidal height. Ellipsoidal heights aren't very useful. Water can flow up here, and it doesn't reflect the terrain at all.

A geoid, kinda sorta, is the surface you would get if there were tubes running under all the continents so that the oceans could settle equally everywhere (no tides, no rotation, etc.). An ultimate mean sea level.

It's much more useful to convert to geoidal heights using a model that contains the offsets between the ellipsoid surface and the geoid surface. That way your data better reflects the terrain, water flows downhill, etc.

The US GPS system does use WGS84 which makes it interesting because normal consumers don't have access to WGS84 control points or CORS stations to RTK or post-process the data as accurately as it could be. That information's only available to US military members, maybe some NATO or other allied personnel who have security clearances.

Other GNSS systems do not use WGS84. The Glonass system uses PZ-90. BeiDou may use CGJ-02.

  • I means, that great but I'm still confused as to how the geoid fits in. I know what a geoid /is/. How does it fit in though? Is the Z coordinate in WGS84 the offset from the geoid? Is that all the geoid is for, the Z coordinate? How does the Geoid get pegged to the elipsoid? How does that datum work without an initial point? – Evan Carroll Oct 19 '16 at 21:40
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    The z value from the grid, at a coordinate referenced to the reference frame (converted to lat/lon, usually), is subtracted from the ellipsoidal height to produce an orthometric height. The ref frame and the ellipsoid have a coincident origin -- the centre of the earth. The grid has no origin, it's just a database. – Rob Skelly Oct 19 '16 at 22:16
  • If the Z value is from the grid then what is the geoid? Is a Geoid just represented as a raster over the ellipsoid? Clearly, you're very knowable, but I'm somewhat confused at all the acronyms you've given. Moreover, I don't even understand their relevance to the question to want to look them up GNSS/PZ-90/CGI-02,CORS/ITRF. There is something to be said for either simplicity or explaining the relevance. Specifically, how is WGS84 an ellipsoid and how and why does it incorporate geoid elements. I like the form of @o_7O's answer much more. – Evan Carroll Oct 20 '16 at 17:33
  • Take a look at this, @EvanCarroll: esri.com/news/arcuser/0703/graphics/geoid2_lg.gif In a nutshell, with GNSS you'll measure your height above the ellipsoid (because it's mathematical, it can't tell you your height above sea level). On this surface water doesn't necessarily flow to lower ellipsoid heights (because gravity). Surveying uses a relative height, based around gravity (setting up an instrument level, i.e., tangential to gravity). The geoid approximates this and is, ideally, a surface of gravitational equipotential. On this surface water flows to lower elevations. – Alex Leith Oct 25 '16 at 0:04
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When measuring heights, there should be an initial point or zero point as reference for measuring. That is where the Geoid and Ellipsoid fits in. The Ellipsoid and Geoid are considered the initial points(zero), where heights are referenced from. Heights derived from satellites(ie.the use of RTK GPS, handheld gps e.t.c) are with reference to an Ellipsoid. Since satellites hover around the world, The principle can be basically explained as ellipsoid reference is global,but the situation is ellipsoid does not fit best globally in all zones and countries, so other zones or countries uses the geoid(which is the mean seal level as their zero point). Yes they are both Datum and they relate, but not the same. SO Orthometric Height( i.e height from Geoid) work best for some zones and countries, and Ellipsoid also the same. A cordinate system can have both components(i.e Geoid and ellisoid). The Geoid(h) and ellipsoid(h) run parallel, and the difference or separation or offset(N) between them help them relate. So for instance, if WGS84 zcordinates were derived using Handheld GPS, I can process the z cordinates to orthometric Height(with reference to Geoid) if know the offset(N). So yes, the spatial Reference system like WGS84 can have an ellipsoid and geoid component, if "N" parameters are known.

  • So when we say WGS-84 has an geoid component we mean it offers a feature that is totally an aside from the traditional elipsoid that provides orthometeric height? So by geoid what we really mean is again it's a lookup table of lat-long on the elipsoid with a value for the orthometric height (which is determined by gravity using a satellite)? That's the only connection between the two that I get from your answer, but in T. Allison's answer below he says the geoid is used for centering the elipsoid too. – Evan Carroll Oct 20 '16 at 17:41
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The text is not accurate. GRS80 and WGS84 describe really the ellipsoid only, they are not directly linked to any geoid as the text suggests.

But as the elevation above WGS84 elipsoid is not really useful, it is automatically converted by most GPS devices into height above sea level using some geoid model. EGM96 is one of them (there is also newer version EGM08).

  • This makes soo much more sense then the other answers. So how is the elipsoid pegged to the geoid? – Evan Carroll Oct 20 '16 at 15:10
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The geoid is the gravity model, the ellipsoid is a reference shape for purposes of constructing coordinates.

The geoid is a way of specifying the gravitational field of the earth. With the geoid, you can compute the force of gravity on, say, a satellite.

The ellipsoid approximates the shape of the earth in a mathematically tractable manner. It doesn't have mountains or anything, but it is the best fit of an ellipsoid to the shape of the earth. The ellipsoid provides you with the variables with which you can express the position of the satellite.

If the Earth had a uniform density, the two would essentially be the same thing. Imagine, however, an earth filled with two substances of very different densities, even though it is a perfect ellipsoid.. If the northern hemisphere had all of the less dense substance, and the the southern hemisphere the more dense substance, then the satellites would not orbit about the center of the ellipsoid.

Because the Earth's shape is largely driven by its gravitational field, the two are concepts are easily confused, but they are not the same.

  • So the geoid would tell me where the center of earth is. And, the ellipsoid is the shape but without the geoid there would be no easily discernible center? Why does the center matter to me? Is that just for the purposes of satellites? How is the geoid and the elipsoid tied together? – Evan Carroll Oct 20 '16 at 2:03
  • I don't think the geoid has anything to do with the center of earth, because I don't think ellipsoid is pegged to the center of earth. This answer needs a lot of work, but thus far it's just simple wrong. Please explain more. I'm professing just enough information to think that o_7O and Terah Antwi's answer is right. – Evan Carroll Oct 20 '16 at 18:55

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