>What is the equivalent of the atn function as shown here in this Access example:
>
>

>UPDATE [TABLENAME] SET XAxis = (cos(((4*((4*atn(1/5))-
>  (atn(1/239))))/180)*[TABLENAME].Latitude)*cos(((4*((4*atn(1/5))-
>  (atn(1/239))))/180)*[TABLENAME].Longitude));
>

Take also a look here SQL Server Zipcode Latitude/Longitude proximity distance search

and also http://blogs.lessthandot.com/index.php/DataMgmt/DataDesign/sql-server-2008-proximity-search-with-th (using new geography types in SQL Server 2008).