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
