A method for determining rotational offset between first and second gravity measurement devices deployed on a downhole tool is disclosed. The method includes positioning the tool in a previously surveyed section of a borehole that provides a historical survey including at least three previously surveyed...http://www.google.com.au/patents/US6882937?utm_source=gb-gplus-sharePatent US6882937 - Downhole referencing techniques in borehole surveying
