Important geographical information may be embodied in images. A number of questions commonly arise regarding the best means of providing geographic access, partly because the MARC format offers a number of fields in which geographic designations may be encoded. Use of geographic subdivisions has been expanded in this edition of TGM I because in a MARC record environment it is, at present, one of the best methods for (1) providing hierarchical access to places, enabling keyword retrieval by specific place name as well as by the larger geographic entity in which the place is located and (2) making a clear linkage between topic and place, so as to avoid confusion when multiple topics and multiple places are recorded in a single catalog record. P&P does not employ a technique found in LCSH for subdividing place names by topics (e.g., Chicago (Ill.)--Commerce) because this does not offer as predictable a form of hierarchical place name access; moreover, constructing such headings requires additional indexing effort with little added benefit in systems featuring keyword retrieval and for a collection that comprises all areas of the world. The [place]--[topic] technique may be of more value in manual systems or automated systems that do not have keyword searching capability. It may also be useful in geographically oriented collections where it is important to be able to browse under place name for all aspects of a particular geographic place. Institutions that are trying to match headings that are used on records for books and other materials cataloged using LCSH may also find it desirable to follow this practice. 
A facet indicator for geographic subdivision is added to terms for natural geographical features, structures, and objects which have fixed locations; to most activity terms; as well as to other terms where locale is deemed both a distinguishable and a distinguishing feature. The geographic subdivision is understood to indicate place of depiction (as opposed to, for example, place of origin), unless a cataloger's note specifies otherwise. (Users of TGM I may find terms that do not have a geographic facet indicator, although a geographic subdivision may seem appropriate. To make sure geographic subdivisions are used consistently, indexers may wish to annotate their copies of the thesaurus if they decide to use a geographic subdivision with a heading where the geographic facet indicator does not presently appear.)
Following LCSH practice, geographic subdivisions are constructed in indirect order (i.e., broader place name preceding narrower place name). The facet indicator [country or state]-- [city] merely suggests the general pattern; as appropriate, catalogers substitute names of regions or specific geographic features. Geographic names are taken from LCSH and the LC Name Authority File, or are formulated according to guidelines on choice of name found in the Anglo-American Cataloguing Rules. The Library of Congress' Subject Cataloging Manual: Subject Headings (Section H 830) specifies subdivision practice for various types of geographic entities. For instance, sites in the United States are subdivided first by state (not by country) and then by the lowest appropriate geographic jurisdiction or feature (other than named sections of cities). Geographic features that span more than two jurisdictions are entered without interposing a broader geographic name.
Example: [Thesaurus] Marine terminals --[country or state]--[city] [Indexing string] Marine terminals--California--San Diego [Thesaurus] Steamboats --[country or state]--[city] [Indexing string] Steamboats--Mississippi River
MARC coding: Geographic subdivisions are assigned subfield code "z."