As a general rule you would include the following information when citing a map. The exact information and arrangement will vary according to the type and date of the map and the referencing style e.g. Harvard or MHRA.
Corporate author and/or publisher
Year of publication (in round brackets)
Title (in italics)
Sheet number, Scale
Place of publication: Publisher
Series (in round brackets)
if online: Available at: URL
(Accessed: date)
For more information see any guidelines given to you by your School or Department; The i-Cite website ; or Cite Them Right Online