As I understand this, a good bit depends on an individual institution's 
decision. See the Oct. 2014 working draft for DCRM--cartographic: 
http://dcrmc.pbworks.com/w/file/87480814/DCRMC20141014.doc . Others here 
will know the schedule for a final release and contentious points that 
may still exist, but from the existing document:

    "X. Precataloging decisions

    Before a bibliographic record can be created for an item or group of
    items awaiting cataloging in an institution’s special collections,
    appropriate decisions must be made regarding the array of
    descriptive options available to the cataloger. These precataloging
    decisions include: determining whether DCRM(C) or AACR2 rules will
    govern the description, choosing the level of cataloging that will
    be applied, and determining the extent to which various options in
    the rules will be exercised.

    Because DCRM(C) was written to address the special needs of users of
    rare materials, it is likely to be the appropriate cataloging code
    for the majority of cartographic items held in special collections.
    However, for some categories of materials, the cataloging objectives
    (see introductory section III) may be met by use of AACR2 or by the
    application of options within the DCRM(C) rules that permit less
    detail in the description. Full-level DCRM(C) records that employ
    all possible descriptive options will not necessarily be the best
    choice for every item.

    The following section provides guidance for catalogers and
    cataloging administrators faced with these decisions and identifies
    some of the institutional and contextual factors that should be
    taken into consideration. It assumes that certain routine choices
    will already have been made, such as whether the encoding standard
    for the description will be MARC 21.


    Institutions may promote efficiency by setting cataloging policies
    for specific categories of materials in their collections rather
    than making decisions on an item-by-item basis. For example, an
    institution may decide to catalog all pre-20th-century cartographic
    materials using DCRM(C), trace printers and mapsellers for all
    pre-19th-century cartographic materials, but give signature
    statements and expansive descriptive notes for pre-18th-century
    materials only. It may choose to catalog all later materials
    according to AACR2, but add selected genre/form or provenance name
    headings. It may decide that collection-level cataloging is
    sufficient for 20th-century road maps. A mechanism for easily making
    exceptions to general cataloging policy is desirable as well. If,
    for example, a curator buys an atlas for its notable cloth binding,
    description of and access to the binding ought to be given in the
    bibliographic record, even if it is not the institution’s usual
    policy to describe bindings."


On 10/16/2014 2:15 PM, Theresa Quill wrote:
> Hi all,
>
> I'm in the middle of revamping what we consider "rare maps" and I was 
> wondering if y'all have policies that spell out which maps are "rare". 
> Do you do it by date? If so, what is your cutoff? Number of copies in 
> the world? Relevance to your institution?
> For example, the USGS 15 and 30 minute topos are OLD but not 
> necessarily RARE. We also have some newer maps that we hold the only 
> cataloged copy for. So maybe there's a hybrid solution? If you do have 
> a policy, would you mind sharing it?
>
> Thanks!
>
> -- 
> Theresa Quill
> Map/GIS Coordinator, Herman B Wells Library
> MLS Candidate, School of Library and Information Science
> Indiana University

-- 
Joel Kovarsky
The Prime Meridian: Antique Maps & Books
1839 Clay Drive
Crozet, VA 22932 USA
+1-434-823-5696
www.theprimemeridian.com