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  {block name=title}GPSBabel development:Manipulate altitudes (height){/block}
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  <div xmlns="http://www.w3.org/1999/xhtml" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Manipulate altitudes (height)</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="filter_interpolate.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Data Filters</th><td width="20%" align="right"> <a accesskey="n" href="filter_track.html">Next</a></td></tr></table><hr /></div><div xmlns="http://www.w3.org/1999/xhtml" class="section"><div class="titlepage"><div><div><h2 class="title" ><a id="filter_height"></a>Manipulate altitudes (height)</h2></div></div></div><p> 
The height filter allows the correction of altitude values.

At least one popular gps logger does store the ellipsoidal height (sum of the height above mean see level and the height of the geoid above the WGS84 ellipsoid) instead of the height above sea level, as it can be found on maps. 

The height filter allows for the correction of these altitude values. This filter supports two options:   

<code class="option">wgs84tomsl</code> and <code class="option">add</code>.  
At least one of these options is required, both can be combined.  
</p><div class="example"><a id="height_wgs84tomsl"></a><p class="title"><strong>Example 4.6.  This option subtracts the WGS84 geoid height from every altitude. For GPS receivers like the iBlue747 the result is the height above mean see level.</strong></p><div class="example-contents"><p><strong class="userinput"><code> gpsbabel -i gpx -f in.gpx -x height,wgs84tomsl -o gpx -F out.gpx</code></strong></p><p>The coordinates and altitude vales must be based an the WGS84 ellipsoid for this option to produce sensible results</p></div></div><br class="example-break" /><div class="example"><a id="height_add"></a><p class="title"><strong>Example 4.7.  This options adds a constant value to every altitude.</strong></p><div class="example-contents"><p><strong class="userinput"><code> gpsbabel -i gpx -f in.gpx -x height,add=10.2f -o gpx -F out.gpx</code></strong></p><p>You can specify negative numbers to subtract the value. If no unit is specified meters are assumed. For feet you can attach an "f" to the value.</p></div></div><br class="example-break" /><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="fmt_height_o_add"></a>add option</h3></div></div></div><p>
      Adds a constant value to every altitude (meter, append "f" (x.xxf) for feet).
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Adds a constant value to every altitude. You can specify negative numbers to subtract the value.
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If no unit is specified, (m)eters are assumed. You can override this by attaching a "f" for feet to the number.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="fmt_height_o_wgs84tomsl"></a>wgs84tomsl option</h3></div></div></div><p>
      Converts WGS84 ellipsoidal height to orthometric height (MSL).
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  Subtracts the WGS84 geoid height from every altitude.
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  For GPS receivers like the iBlue747 this corrects the logged altitudes to height above mean sea level.
</p></div></div><div xmlns="http://www.w3.org/1999/xhtml" class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="filter_interpolate.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="Data_Filters.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="filter_track.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Interpolate between trackpoints (interpolate) </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Manipulate track lists (track)</td></tr></table></div>
  
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