Nearly 3,000 specialized Defense Department aides working on opposite sides of the Beltway are sweating out a new Pentagon study that could mean one - maybe both of the groups in question - will get a new home.

The units are part of the Defense Mapping Agency. One is the 2,200-employee topographic command in Bethesda. The other outfit in the study is the hydrographic office in Suitland that has 700 civilian employees. Both the Suitland and Bethesda sites are already crowded, with parking and traffic problems that consolidation would only make worse.

In addition to considering a shift of all or part of the Bethesda operations to Suitland, or vice versa, the Pentagon will also study moving the units somewhere else. Rumors - and they are just that at this point - say Pennsylvania, Illinois and/or points south are a possibility.

Defense has notified the appropriate Capitol Hill powerbrokers that it has three options:

1) Combine upper level managment from the topographic command and the hydrographic office in one site, either Suitland or Bethesda.

2) Combine the two big operations in either Suitland or Bethesda.

3) Merge them at another locality.

The fourth, obvious option, leave them alone, is not part of the planning.

The idea behind the study, the Pentagon says, is to increase productivity, reduce waste and eliminate jobs. That is the standard reasoning behind every government game of musical chairs. The two now cost $85 million annually and the idea is to trim costs by $4 million a year, mainly by eliminating 150 overlapping jobs.

Defense workers are nervous, for good reason.It was only a couple of years ago that the Navy began a feasibility study of moving the Suitland-based Naval Oceanographic Office. It finally concluded (some believe the fix was in from the first day of the study) that the ideal place for the NOO would be Bay St. louis, Miss.

Bay St. Louis is the backyard of the House Armed Services Committee Chariman F. Edward Hebert (D-La.) and other influencial congressmen. You can now reach most of those workers - or at least their jobs - by writing c/o Bay St. Louis.