You may be right, vertical may be defined by an ellipsoid approximating
the geoid. I am sticking to my point that the degree of latitude varies
systematically, and is LARGER near the poles (latitude as solid
earth-center angle plus a polar-flattened earth would yield SHORTER
degrees near the pole. And if mid-way between pole and equator is defined
by distance, 45 latitude is not exactly the answer.
David
On Sat, 7 Jul 2001 bjbutler@... wrote:
> David, how do you define "vertical"? Surely not gravitationally
> because variations in rock density, elevation, and even geometrical
> latitude would produce "lines" of latitude that were iso-vertical
> contours. I thought geocoordinates were expressed as angles within
> an ellipsoid (i.e. a datum), a better approximation of the earth's
> shape but still a mathematically regular object. I don't know if
> these data contain local corrections.
>
> BJB
>
> --- In BoundaryPoint@y..., David Mark <dmark@g...> wrote:
> > Technically, of course, the 45th parallel is not precisely half way
> > between the equator and the pole. It would be if the earth were a
> sphere.
> > But since the earth is somewhat flattened pole-to-pole compared
> with the
> > equator, the length of the degree of latitude is not constant but
> itself
> > varies with latitude.
> >
> > Many people mistake the definition of latitude. It is not a solid
> angle at
> > the center of the Earth between a line to the center and the plane
> of the
> > equator. Latitude of a point on the Earth's surface is the angle
> between
> > the vertical and the plane of the equator. These two definitions
> also
> > would be identical on a sphere.
> >
> > Due to the true definition of latitude and the flatteneing, the
> length of
> > a degree is a little larger toward the poles and smaller near the
> equator.
> > So the 45th parallel is closer to the equator than to the pole, and
> the
> > midpoint between the pole and the equator would be somewhat north
> of the
> > 45th, how far I'm not sure (I have books at the office in which I
> could
> > look this up).
> >
> > David
> > dmark@g...
> >
> > On Fri, 22 Jun 2001, Bill Hanrahan wrote:
> >
> > > And if it wasn't halfway to the North Pole, it wouldn't be the
> 45th
> > > parallel (Northern Hemisphere of course) in the first place.
> > >
> > > Bill
> > >
> > > At 05:51 PM 6/22/01, you wrote:
> > > >It is also marked in Maine. I think I have seen it on Route 1.
> > > >There is a monument and a plaque that says something about
> > > >being "halfway to the north pole". But then, everywhere is
> halfway
> > > >to the north pole from somewhere!
> > > >
> > > >BJB
> > >
> > >
>
>
>
>
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