Atlases can be referenced in the same format as a book. If no author is available then the title of the Atlas can be used.
Format for Atlas with Author:
FAMILY/SURNAME, Initials. (Year of publication – in brackets) Title of Atlas in italics. Series title and volume – if available. Edition – if not the first. Place of Publication: Publisher.
Format for Atlas – no Author:
Title of Atlas – in italics. (Year of publication – in brackets) Series title and/or volume – if available. Edition – if not the first. Place of publication: Publisher.
WAGNER, S. A. (2004) Color Atlas of the Autopsy. Boca Raton: CRC Press. The Times Concise Atlas of the World. (2000) 8th Ed. London: Times Books.
In text example:
As can be seen in The Times Concise Atlas of the World (2000)