[Geotiff] DEM in a GeoTIFF

[Mike]

We're looking to optionally encode DEM data in a GeoTIFF but have not 
found much in the way of information on this.  Encoding the data 
itself isn't a problem but encoding the tags correctly is a concern.  
We also have DEMs where the horizontal and vertical units are 
different and want to preserve that data.

Can anyone point us to example GeoTIFF DEMs or any "standards" for 
encoding this type of data in a GeoTIFF?

Thanks,
--Mike