From Cartographic Citations: A Style Guide:
Basic form:
Author. Title. Edition. Place of production: Producer, Date of copyright or production.
Example:
U.S. Bureau of the Census. TIGER/Line: Redistricting Census 2000 on DVD. Washington, D.C.: Bureau of the Census, 2001.
Here are some additional guides on citing geospatial data and maps from other institutions and organizations: