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Hello<br>
<br>
Le 04/05/11 19:56, Aaron Braeckel a écrit :
<blockquote cite="mid:4DC1935C.1070301@ucar.edu" type="cite">
<pre wrap="">Are there any authorities (ex: EPSG) that define a general-purpose
ellipsoidal vertical CRS analogous to EPSG:5714?</pre>
</blockquote>
<p>I'm not aware of any code in the EPSG, CRS, AUTO or AUTO2
namespace. Actually the ISO 19111 specification explicitely
forbives "ellipsoidal height" as a standalone vertical CRS, for
conceptual reason. They said that an ellipsoidal height makes
sense only when associated to a geographic CRS (at the difference
of other kind of height which can be used alone). Geotoolkit.org
can handles standalone ellipsoidal heights for pragmatic reason,
but this is a departure from ISO 19111 specification.<br>
</p>
<p>ISO 19111 handles ellipsoidal heights in a very special way. We
can get an ellipsoidal height only as a component of a 3D
Geographic CRS. There is a EPSG code for a 3D Geographic CRS
(WGS84) with vertical ellipsoidal height: EPSG:4327.<br>
</p>
<p>For other kind of CRS (projected...), an alternative may be to
use WKT. The WKT specification has been created before ISO 19111,
at a time where standalone ellipsoidal height were not considered
"bad". So it is possible to express them:<br>
<br>
</p>
<blockquote>
<pre>VERT_CS["Ellipsoidal height",
VERT_DATUM["Ellipsoidal height", 2002],
UNIT["metre", 1.0],
AXIS["Ellipsoidal height", UP]]</pre>
</blockquote>
<p><br>
Except for the above WKT, if we want to be strictly ISO/OGC
compliant, the main alternatives I can see are:</p>
<ul>
<li>
<p>Help OGC to restart the referencing SWG, and propose to allow
standalone ellipsoidal height in some cases.</p>
</li>
<li>
<p>Finish the "ellipsoidal to geoidal" transformation in Geotk.
But even if this functionality was implemented, it would have
significant cost at runtime, which may not be always desired.<br>
</p>
</li>
</ul>
<p><br>
Martin<br>
<br>
</p>
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