ancestor namespace

Hi, all
  Can you help me some body with my problem.
  I do not know when I must use all ancestor namespace for signed and
digested nodes.

  For example:
     <a:root xmlns:a="some.namespace">
	<element xmlns="default.namespace" Id="signedElement">
	   .....
	</element>
     </a:root>

  Question is when I must use namespace from ancestor elements for
canonicalization signed element.

  Thank you
	Martin