Subject: Re: [BoundaryPoint] Re: caus straight or irregular
Date: May 05, 2001 @ 13:51
Author: michael donner (michael donner <m@...>)
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exquisitely done maestro

can you also say which of the 3 choices gps survey technology adopts
& whether this points to a most consensual usage

or is it equally at home geocoordinating in all 3 versions

m


>
>Re Peter's point about approximations to the shape of the earth.
>The sphere is a first approximation. After that come various second
>approximations, all oblate ellipsoids. Your choice depends on whether
>you're mapping the whole Earth (in which case the WGS84 ellipsoid is
>the best fit) or some part of it (in which case you can choose the
>local best fit - Airy in the U.K., Clarke in the U.S.) WGS84 is the
>shape the Earth would be if it was a spinning lump of completely
>homogenous rock. The third and most accurate level of approximation
>is the *geoid* which reflects local variations in density (and
>therefore surface gravity). It's the real mean sea level that water
>would settle at if it were allowed to seek a level through tiny
>frictionless channels bored out under the Rockies, Himalayas etc.
>Corresponding to these levels of approximation are various ways of
>determining latitude: Geocentric latitude is the angle between your
>current location, the centre of the Earth and the equator. Geodetic
>latitude is the angle between the normal to the ellipsoid at your
>location and the equatorial plane. Astronomical latitude is the angle
>between your local vertical (ie the normal to the geoid) and the
>equatorial plane.
>I labour through all this to respond to Brendan's guess that the line
>of sight is equivalent to a great circle. I think more or less yes,
>but precisely no. The great circle segment connecting any two point
>on the Earth's surface is defined by a plane passing through those
>two points and the Earth's centre - it therefore corresponds to the
>(rather poor) spheroidal approximation. *But* I think the line of
>sight corresponds to a local-vertical sort of situation - you set
>your theodolite vertically over one point, and sight on a flag held
>vertically over another point. And the local vertical at 49 degrees
>latitude is (using the second-level approximation of the WGS84
>ellipsoid ... scribble, scribble) tilted by about a fifth of a degree
>relative to the great circle - the vertical plane at that latitude
>misses the centre of the Earth by close to 30km.
>So the line of sight along the 49 deg section of ca-us lies just a
>hair north of the great circle, which lies a way or so north of the
>parallel of latitude.
>Whatever - a gently scalloped edge to the U.S., like a Victorian lace
>doily.
>
>Grant
>
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