Subject: Re: [BoundaryPoint] Re: Dutch provincial North sea<br/>
Date: Dec 18, 2001 @ 00:02<br/>
Author: Brendan Whyte (&quot;Brendan Whyte&quot; &lt;brwhyte@...&gt;)<br/>
<a href="5159.htm">Prev</a>&nbsp;&nbsp;&nbsp;&nbsp;Post in Topic&nbsp;&nbsp;&nbsp;&nbsp;<a href="5161.htm">Next</a>&emsp;[<a href="topic5083.htm">All</a> Posts]<br/>
<a href="5159.htm">Prev</a>&nbsp;&nbsp;&nbsp;&nbsp;Post in Time&nbsp;&nbsp;&nbsp;&nbsp;<a href="5161.htm">Next</a><br/>
<br/><br/>
<div id="ygrps-yiv-1084496408">The oceans are not flat. There is a hollow of sea in the Bay of Bengal which <br/>
is about 100m lower than theoceans in other places. One must also take into <br/>
account the shape of the geooid used in various countries. This will have <br/>
much greater impact in China vs India, than in Belgium vs Neth. So if you <br/>
triangulate from the Bay of Bengal to Everest, and from Shanghai, you will <br/>
get two different heights even if  both are 100% error free. With such <br/>
distance involved the curvature of the earth is vital.<br/>
There is a 5km wharf in South Austrlaia which dips 2m along the earth&#39;s <br/>
curvature.<br/>
I would say Nepal and India&#39;s datums are similar, while China&#39;s will be <br/>
totally different. So different heights may be based on one Nepal and one on <br/>
China&#39;s datums (i won&#39;t use the correct plural, data, as it confuses the <br/>
issue!).<br/>
<br/>
So: you must allow for :<br/>
-different datums, ie where each country measures 0m a.s.l. to be.<br/>
-differences in oceanic surface height<br/>
-the natural shape of the earth<br/>
- the different models of the shape of the earth used by each country in its <br/>
surveys.<br/>
-errors in measurement over the distances from sea to Everest<br/>
<br/>
BW<br/>
<br/>
<br/>
<blockquote><span title="ireply"> &gt;From: &quot;ps1966nl&quot; &lt;<a rel="nofollow" target="_blank" href="mailto:smaardijk@...">smaardijk@...</a>&gt;<br/>
&gt;Reply-To: <a rel="nofollow" target="_blank" href="mailto:BoundaryPoint@yahoogroups.com">BoundaryPoint@yahoogroups.com</a><br/>
&gt;To: <a rel="nofollow" target="_blank" href="mailto:BoundaryPoint@yahoogroups.com">BoundaryPoint@yahoogroups.com</a><br/>
&gt;Subject: [BoundaryPoint] Re: Dutch provincial North sea<br/>
&gt;Date: Mon, 17 Dec 2001 21:30:32 -0000<br/>
&gt;<br/>
&gt;Which brings us to the relativity of highest and lowest points (we&#39;re<br/>
&gt;pointing aren&#39;t we?). In the Netherlands, NAP is used, and in Germany<br/>
&gt;it is NN. But the Belgian benchmark, Oostends Peil (Ostend Level) is<br/>
&gt;reportedly 2m and 34cm below these. And other countries use other<br/>
&gt;mean sea levels, probably.<br/>
&gt;<br/>
&gt;How is the height of mountains measured (taking which level as 0 m, I<br/>
&gt;mean)? Say that, for example, the &#39;level zero&#39; of Nepal is different<br/>
&gt;from that of China, then Mt. Everest has two heights. But I have seen<br/>
&gt;more than two on various maps over the years...<br/>
&gt;<br/>
&gt;Peter S.<br/>
&gt;<br/>
<br/>
<br/>
<br/>
<br/>
 </span></blockquote>_________________________________________________________________<br/>
Join the world�s largest e-mail service with MSN Hotmail. <br/>
<a rel="nofollow" target="_blank" href="http://www.hotmail.com">http://www.hotmail.com</a></div>