Dave

I think the functionality is that you can input the altitude in any units that you happen to know the altitude in. 

ie You might have a topo map that tells you the altitude in feet.

But the output is always converted to meters, however the text ‘m’ or ‘meters’ is never displayed.

If you want to add a custom definition you could try Thomas Holders contribution http://therion.speleo.sk/wiki/doku.php?id=metapost  near the bottom of the page which guides one through the necessary customization process.

Bruce