Question about XML

Hello:
      I'm a student from Taiwan, I'm writing a program about XML. After
reading the document(http://www.w3.org/TR/xmldsig-core/#sec-References), I
don't understand the digest value element. I use SHA1 to hash the file at
first , and then using BASE64 to encode the result after hashing. The
digest value I get is still wrong and the length of digest value is much
longer. Could you tell me what wrong
 about my step? Thanks for your time!