> Somehow I had the idea that the current transaction log can be used with a
> point in time restore operation.

Yes.

> But I now have the idea that point in time restore can only be used when a
> transaction log backup has taken place.

Transaction log keeps all changes made to the db from the time it's been
last backed up (i.e truncated). Now imagine following: You've made a full db
backup (this includes log as well) at 10 pm, used your db happily up to
11:30 PM when your HDD crashes and your DB is gone. Since you're a smart guy
you've kept your transaction log on separate device. Your situation is as
follows: You've got the state from 10 PM on your tape + all the changes made
between 10 pm and 11:30 pm in your log. All you have to do is
1) Backup your existing log (one that is still available) with NO_TRUNCATE
option
2) Restore your DB from the tape
3) Restore all your log backups (in this case just one that ypu've made at
point 1)

You've got the state from 11:30 PM. (Or if you want to you may restore it to
the state it was at 10:30)

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HTH

Marcin