<para>
Adds a constant value to every altitude. You can specify negative numbers to subtract the value.
</para>
<para>
The units may be specified by appending a suffix to the supplied number:
</para>
<simplelist type="vert">
<member>'m' for meters, e.g. 3.5m</member>
<member>'ft' or 'feet' for feet, e.g. 11.483ft</member>
<member>'k' or 'km' for kilometers, e.g 0.0035km</member>
<member>'nm' for nautical miles, e.g. 0.0018898nm</member>
<member>'mi' for miles, e.g. 0.0021748mi</member>
<member>'fa' for fathoms, e.g. 1.9138fa</member>
</simplelist>
<para>
If no units are specified, the units are assumed to be meters.
</para>
