[FWTools] How to distinguish a DTED tile that's accidentally absent, from one that's always absent because it covers only ocean?

[Mikael Rittri]

Hello,
I have a problem that isn't specific to FWTools, 
but I don't know a better place to ask. 

As I understand the DTED format, it comes in 
one-degree tiles, one file per tile, but tiles
that cover only ocean - not a single island - 
are omitted, because they would contain only zeros.

That may seem reasonable, but of course a file 
can also be absent because my particular set of
DTED data does not have worldwide coverage.

Am I right in my understanding of ocean files? 

If so, how do I distinguish missing data from
sea-level data?  

Any advice would be welcome.

Best regards, 
--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com