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    Re: Ecef
    From: Herbert Prinz
    Date: 2001 Jul 24, 11:15 PM

    Lu Abel asks:
    > 1.  What are the units of measurement?  I think I could grasp
    > 1368950/-4603950/4182550 more easily with a unit of measurement.
    As I said in my previous message and below in the back quote, the units are meters.
    The reason why they don't show up in the coordinates is that coordinates are
    dimensionless scalars that are multiplied with the base vectors. The unit is hidden
    in the base vector, which is understood by convention.
    Lu Abel writes:
    > 2.  I don't necessarily agree that it might make determination of our
    > centroid easier. An average in ECEF coordinates would easily give us a
    > point in the middle of the earth!   Don't know about you, but I prefer
    > sailing to digging :-)
    The centroid is the centroid and is where it is. If it is buried deep down in the
    earth, it cannot be helped. This is the reason why R. van Gent projects it back up
    to the surface of the Earth, which is often a good place for sailing. But it is
    also the reason why I had my doubts that we should be looking for the centroid at
    all. (As a matter of fact, I am now sure that we should not, because it does not
    fulfil any relevant minimum condition.)
    Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    > Lu Abel
    > At 10:31 PM 7/24/2001 +0000, you wrote:
    > >Earth Centered Earth Fixed Cartesian coordinates.
    > >
    > >The origin is at the center of the earth. The positive z-axis goes through the
    > >N-pole. The positive x-axis goes through the equator and the Greenwich
    > >meridian. The y-axis completes the orthogonal system.
    > >
    > >Roughly speaking, ECEF coordinates are what you get when you use the formula
    > >given by  R. van Gent and multiply with the radius of the Earth in meters.
    > >
    > >   x = R * cos(lat) * cos(long)
    > >   y = R * cos(lat) * sin(long)
    > >   z = R * sin(lat)
    > >
    > >Strictly speaking, you must account for the spheroidal shape of the earth.
    > >
    > >These coordinates are easier to use in computers because all trigonometry
    > >becomes algebra. Your GPS reciever uses them internally, astronomers need them
    > >for exact topocentric reductions, or surveyors, for instance when they have to
    > >transform maps from one datum into another.
    > >
    > >There goes:
    > >
    > >Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    > >
    > >
    > >Dan Allen wrote:
    > >
    > > >
    > > > Just what are ECEF coordinates?  I've never heard of them.

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