How can I generate the right DigestValue for my xml body?

Question

As far as I know, I am supposed to get the DigestValue by taking the canonical form of the XML element that the tag's URI attribute is pointing to, perform a sha1 hash on that, and than base 64 encode it.

this is the reference opening tag: <ds:Reference URI="#id-8199334E5D75D0579B149267885542664"> so I assumed it points to this element:

<soapenv:Body wsu:Id="id-8199334E5D75D0579B149267885542664" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity- utility-1.0.xsd">
      <ns:WTR_ReadRecordRQ Version="0.1">
         <ns:POS>
            <ns1:Source AirlineVendorID="MM"/>
         </ns:POS>
         <ns:RecordID>
            <ns2:RecordType>DELAYED</ns2:RecordType>
            <ns2:RecordReference AirlineCode="MM" ReferenceNumber="10703" 
             StationCode="ATL"/>
         </ns:RecordID>
         <ns:AgentID>RFFTEGRNT</ns:AgentID>
      </ns:WTR_ReadRecordRQ>    
</soapenv:Body>

the digest value I am supposed to get is this one:

<ds:DigestValue>WiMBTNpMAdsACap6eUn7O1IYXG4=</ds:DigestValue>

but no matter what I do I keep getting a different DigestValue.

what do I need to do In order to successfully generate it correctly?! Please I really need help here!!