I figured it was fairly obvious that "sea level" wasn't a real thing because of the Panama Canal situation, but I kind of wondered what "sea level" actually was... I guess this explains that.
On Mon, May 25, 2020 at 8:01 PM <ch...@wbmfg.com> wrote: > In areas of the globe of high gravity, the water piles up. > > -----Original Message----- > From: Ken Hohhof > Sent: Monday, May 25, 2020 6:55 PM > To: 'AnimalFarm Microwave Users Group' > Subject: Re: [AFMUG] Light reading > > I've always been puzzled how the Atlantic and Pacific oceans can be > different heights at the Panama Canal. What about "seal level" and "water > seeks its own level" and all that? Apparently they're the same height down > by Tierra del Fuego. Very confusing. > > -----Original Message----- > From: AF <af-boun...@af.afmug.com> On Behalf Of ch...@wbmfg.com > Sent: Monday, May 25, 2020 7:37 PM > To: af@af.afmug.com > Subject: [AFMUG] Light reading > > So, being confused at ellipsoid heights vs geoid heights and what I am > looking at in my fancy GPS system, etc I bought a book about the subject > and > got smelf smarted. For a terribly dry subject and a book full of > equations, > I really enjoyed the crap out of reading it. > > Several notable thing I learned: > The ellipsoid is a mathematical representation of the planet. That is > referred to as WGS84. At least that is the model we are all currently > using > until they tweak it at some point in the future. > The ellipsoid is a smooth shape calculated from the center of the earth. > They claim it is within 2cm of being accurate. > > The actual axis of rotation of the earth wobbles in about 6 different > directions all with periods from tens of thousands of years to days. So, > the elipsoid (WGS84) is defined by actual earth bound landmarks and an > artificial/derived/ false/imaginary axis of rotation. > > It is like a grid cage pinned at a few locations to a beating heart. Not > only does the earth's real axis of rotation meander around, so do > continents > and plates. > > The Geoid (EGM 96) is a list of points in a grid the covers the planet. A > correction table for elevations. > Each point defines a distance from the geoid to the ellipsoid in the > vertical/elevation direction. > It is generally +/- 100 meters or so of the ellipsoid. > > The geoid (EGM 96) is an artificial representation or model of mean sea > level. As it turns out, water can indeed run up hill. The Atlantic and > Pacific oceans both slope upward going north. This is due to the > variations > of gravity. If gravity is stronger up the hill than at the base, the water > will run up the hill. This is seen in the great lakes. Not likely to be > seen in short distances. > > In any event, the geoid gives us some offsets in altitude/elevation so we > can figure out where mean sea level is at any point on the globe. You can > calculate the ellipsoid with just math, but then MSL/elevation will be > offset from that spheroid in a vertical direction. No calculations > involved > > in the geoid, just a bunch of correction data points. Mostly created by > satellite and aerial radar and laser surveys. > > Down is not down. A plumb bob seeks the center of mass of the planet. > Down > is an angle normal to the tangent of the ellipsoid. If you are in an area > where the center of the mass of the planet is not aligned with gravity > there > will be an angle between the two downs and ups. Just an odd factoid. Only > used if you are involved in long baseline survey work the old fashioned > way. > > When we talk about antenna elevations, we are really talking about height > above (or below, I guess, death valley etc) the geoid. There really is no > such thing as mean sea level. The sea has considerable elevation > differences around the globe. So, with lots of averaging and some laws > being written and international consortiums of folks that study this stuff, > the geoid was created. In some areas the geoid is below the ellipsoid and > in some areas it is above. > > We are currently using geoid EGM96 (Earth Gravitational Model 1996) It was > defined along with WGS84 datum which is the ellipsoid. It is a 15' x 15' > grid, so pretty accurate. > > There is a new one, EGM2008 which will have a grid of 2.5' x 2.5' Seems > funny to me that they are using feet instead of meters to define these > grids. > > > > -- > AF mailing list > AF@af.afmug.com > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > > > -- > AF mailing list > AF@af.afmug.com > http://af.afmug.com/mailman/listinfo/af_af.afmug.com >
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