Re: Confused by DATUM's

Dominic Sexton wrote:

In article <12mh3glct0pmh04@xxxxxxxxxxxxxxxxxx>, Richard Owlett <rowlett@xxxxxxxxxxxxx> writes

I can "see" rectil-linear (sp?) systems such as UTM having a dependence on presumed shape of earth.

Why/how would this influence a readout in latitude/longitude/elevation(assumed to be distance from earth's center of gravity).

What SIMPLE thing am I missing?


Datums can not only have a different shape for the earth but also different location for the centre of gravity and even a rotation of the co-ordinate frame.

Usually the earth shape will be defined by the ellipsoid used and the different locations will be expressed as shifts in x y and z directions. For higher accuracy transformation between datums there are also rotations around each axis and a scale factor.



I think you may have focused on my *PROBLEM*
EARTH *does have* a "center of gravity"
That's a physical "fact of life"

IIRC a satellite MUST orbit in path defined by focii of an ellipse.
If focii are equal, orbit is a circle.

So what am I missing.

Why would not lat and long ignore orbit? ?????

.

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