bash scripts: how to determine directory of 'source'd file

[michael <linux@networkingnewsletter.org.uk>]

Presuming I have a file setEnvVars.sh that I wish to source, 
  . path/setEnvVars.sh
How do I, within the script, determine the actual directory within which
the setEnvVars.sh file sits? The sourcing seems to disallow me access to
$0 etc

thanks, M