geography Data Type Method Reference
SQL Server 2008 supports a set of methods for the geography spatial data type, including methods on geography that are defined by the Open Geospatial Consortium (OGC) standard and a set of Microsoft extensions to that standard.
The error tolerance for the geography methods can be as large as 1.0e-7 * extents. The extents refer to the approximate maximal distance between points of the geography object.