Subject: Re: Really Arbitrary Points<br/>
Date: Jul 07, 2001 @ 20:59<br/>
Author: bjbutler@bjbsoftware.com (bjbutler@...)<br/>
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<div id="ygrps-yiv-441645341">David, how do you define &quot;vertical&quot;? Surely not gravitationally <br/>
because variations in rock density, elevation, and even geometrical <br/>
latitude would produce &quot;lines&quot; of latitude that were iso-vertical <br/>
contours.  I thought geocoordinates were expressed as angles within <br/>
an ellipsoid (i.e. a datum), a better approximation of the earth&#39;s <br/>
shape but still a mathematically regular object.  I don&#39;t know if <br/>
these data contain local corrections.<br/>
<br/>
BJB<br/>
<br/>
<blockquote><span title="qreply"> --- In BoundaryPoint@y..., David Mark &lt;dmark@g...&gt; wrote:<br/>
&gt; Technically, of course, the 45th parallel is not precisely half way<br/>
&gt; between the equator and the pole.  It would be if the earth were a <br/>
sphere.<br/>
&gt; But since the earth is somewhat flattened pole-to-pole compared <br/>
with the<br/>
&gt; equator, the length of the degree of latitude is not constant but <br/>
itself<br/>
&gt; varies with latitude.<br/>
&gt; <br/>
&gt; Many people mistake the definition of latitude. It is not a solid <br/>
angle at<br/>
&gt; the center of the Earth between a line to the center and the plane <br/>
of the<br/>
&gt; equator. Latitude of a point on the Earth&#39;s surface is the angle <br/>
between<br/>
&gt; the vertical and the plane of the equator. These two definitions <br/>
also<br/>
&gt; would be identical on a sphere.<br/>
&gt; <br/>
&gt; Due to the true definition of latitude and the flatteneing, the <br/>
length of<br/>
&gt; a degree is a little larger toward the poles and smaller near the <br/>
equator.<br/>
&gt; So the 45th parallel is closer to the equator than to the pole, and <br/>
the<br/>
&gt; midpoint between the pole and the equator would be somewhat north <br/>
of the<br/>
&gt; 45th, how far I&#39;m not sure (I have books at the office in which I <br/>
could<br/>
&gt; look this up).<br/>
&gt; <br/>
&gt; David<br/>
&gt; dmark@g...<br/>
&gt; <br/>
&gt; On Fri, 22 Jun 2001, Bill Hanrahan wrote:<br/>
&gt; <br/>
&gt; &gt; And if it wasn&#39;t halfway to the North Pole, it wouldn&#39;t be the <br/>
45th <br/>
&gt; &gt; parallel (Northern Hemisphere of course) in the first place.<br/>
&gt; &gt; <br/>
&gt; &gt; Bill<br/>
&gt; &gt; <br/>
&gt; &gt; At 05:51 PM 6/22/01, you wrote:<br/>
&gt; &gt; &gt;It is also marked in Maine.  I think I have seen it on Route 1.<br/>
&gt; &gt; &gt;There is a monument and a plaque that says something about<br/>
&gt; &gt; &gt;being &quot;halfway to the north pole&quot;.  But then, everywhere is <br/>
halfway<br/>
&gt; &gt; &gt;to the north pole from somewhere!<br/>
&gt; &gt; &gt;<br/>
&gt; &gt; &gt;BJB<br/>
&gt; &gt; <br/>
&gt; &gt; </span></blockquote></div>