SQL & PL/SQL

I'm having some trouble writing this query using a decode statement within a join. First, lets say I have two tables. Table A has the fields 'Baseline', 'NAD', 'Null', and 'Harn'. Table B has the fields 'Null', 'NAD', and 'Harn'. I want to join these tables together by these fields to return their ID's. Because table B doesn't have 'Baseline' to even have a match between the two tables, in Table A, we want to treat 'Baseline' as 'Harn', hence a decode statement. My query is below

Select
...
...
...
From
...
inner join oracle_srid_xref B on A.horizontal_datum = B.horizontal_datum and
decode(A.horizontal_epoch, 'BASELINE', 'HARN', loc.horizontal_epoch ) = B.horizontal_epoch

I'm not getting the ID's where the horizontal_epoch was initially 'Baseline'

Any help on the matter would be greatly appreciated. Thanks in advance.