Subject: Re: Dutch provincial North sea<br/>
Date: Dec 17, 2001 @ 22:32<br/>
Author: granthutchison (&quot;granthutchison&quot; &lt;granthutchison@...&gt;)<br/>
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<div id="ygrps-yiv-329157927">Peter:<br/>
<blockquote><span title="ireply"> &gt; But the Belgian benchmark, Oostends Peil (Ostend Level) is <br/>
&gt; reportedly 2m and 34cm below these.<br/>
 </span></blockquote>Yes, the Belgian IGN 1:50000 maps carry a legend stating: &quot;Foreign<br/>
heights should be increased by about two metres in order to adjust<br/>
them to the belgian levelling.&quot; Most countries use mean sea level, but<br/>
I guess Belgium uses some low-water datum.<br/>
There are no zero contours on my Oostende sheet, but several loops of<br/>
2,5m, which means there are probably areas below mean sea level along<br/>
this coast. I&#39;ve seen atlases mark such areas at small scale, but I<br/>
don&#39;t have any information about exact location.<br/>
<br/>
<blockquote><span title="ireply"> &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/>
 </span></blockquote>In theory, all these heights should be related to the local mean sea<br/>
level geoid - height above mean sea level if the sea could be<br/>
miraculously ducted into the area. The old 8848m height for Everest is<br/>
probably wrong, since it relied on a very extended trigonometrical<br/>
survey of India, effectively &quot;carrying&quot; sea level from the coast to<br/>
the mountains. <br/>
But there have been two GPS surveys of Everest in the last few years.<br/>
In 1998 it was nailed at 8838m. In 1999 it was nailed at 8850m. Both<br/>
surveys claim accuracy to a metre or so. Take your pick!<br/>
<br/>
Grant</div>