Subject: Re: caus straight or irregular<br/>
Date: May 05, 2001 @ 14:21<br/>
Author: granthutchison@cs.com (granthutchison@...)<br/>
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<div id="ygrps-yiv-1503809798">Michael:<br/>
<blockquote><span title="ireply"> &gt; can you also say which of the 3 choices gps survey technology adopts<br/>
&gt;   & whether this points to a most consensual usage<br/>
&gt; <br/>
&gt; or is it equally at home geocoordinating in all 3 versions<br/>
<br/>
 </span></blockquote>The basis for GPS is the WGS84 ellipsoid. But my (rather elderly) GPS <br/>
receiver provides conversion to many local mapping systems, based on <br/>
many locally-defined best-fit ellipsoids. <br/>
Traditionally, horizontal mapping has been done using an ellipsoid <br/>
model (because the sums are easier to do than if you treat the Earth <br/>
in all its lumpy reality), but vertical positioning must always be <br/>
referred to the geoid, since we&#39;re interested in height above mean <br/>
sea level. In the old days, this was all done with tide gauges and <br/>
sight lines.<br/>
So my old GPS gives horizontal data relative to the ellipsoid, which <br/>
match the mapping nicely, but also *heights* relative to the <br/>
ellipsoid, which can be adrift by several metres from MSL. There&#39;s no <br/>
reason in principle why the geoid model couldn&#39;t be built into a GPS <br/>
receiver (it&#39;s basically just a contour map of MSL relative to the <br/>
ellipsoid) although it&#39;s intensive in memory and calculation, I would <br/>
guess that such things do exist, but I haven&#39;t seen them.<br/>
<br/>
So: ellipsoid maths is sufficiently simple that there&#39;s no reason to <br/>
use the spherical approximation. It also transfers GPS data <br/>
accurately to the horizontal detail of a map, which is what most <br/>
people want for navigation. But for accurate height surveying <br/>
information you need a geoid model built into your GPS receiver <br/>
software.<br/>
<br/>
Grant</div>