<?xml version="1.0"?>
<sch:schema xmlns:sch="http://www.ascc.net/xml/schematron">

   <sch:pattern name="Aircraft Altitude">

       <sch:rule context="Flight">
           <sch:assert test="Aircraft/Altitude &gt; Obstacle/Height">
             The Aircraft Altitude must be greater than the Obstacle Height.
           </sch:assert>
       </sch:rule>

   </sch:pattern>

</sch:schema>