OAC Best Practice Guidelines for Encoded Archival Description
Table of Contents
- 4.4. Linking to Digital Objects
-
- 4.4.1. Terminology
- 4.4.2. Context
- 4.4.3. <dao> Digital Archival Object vs. <daogrp> Digital Archival Object Group
- 4.4.4. Use of the ROLE Attribute
- 4.4.5. Linking from Finding Aids to Digital Objects: Collection or Major Subdivision vs. <c0x> Component Levels
- 4.4.6. Linking from Digital Objects to Finding Aids
- 4.4.7. Implementing Archival Resource Keys (ARKs) for Digital Objects
- 4.5. Linking from Finding Aids to CDL-Hosted METS Digital Objects/Locally-Hosted Digital Objects
- 4.6. Linking from Finding Aids to Legacy CDL-Hosted METS Digital Objects (Metadata Derived From EAD)
This section pertains only to linking from a particular finding aid to digital objects described in that finding aid, using either <dao> Digital Archival Object or <daogrp> Digital Archival Object Group tags. Element statuses (i.e., "Required", "Mandatory", or "Preferred") are therefore relative to this situation. For complete guidelines on preparing digital objects and linking them to their corresponding finding aid, please consult the CDL GDO.
For guidelines on internal linking within a finding aid, and external linking to digital resources or objects that are not part of the materials being described by the finding aid, see Section 3.6.
A digital object may be defined as a resource entity in which one or more content files (e.g., an image file, TEI structured text file, etc.) and corresponding metadata are united, physically and/or logically, through the use of a digital wrapper.
The use of a digital wrapper is what gives technical specificity to the term "digital object." Digital wrappers are pieces of software for binding digital content files and their metadata together and for specifying the logical relationships among the content files. The Metadata and Encoding Transmission Standard (METS), currently implemented by the CDL for digital objects that it manages in its repositories, is the emerging national standard for wrapping digital library materials. For more information, see the CDL METS Repository.
The OAC technical framework and service model is predicated on ingesting and managing digital objects that are associated with finding aids: digital objects should ideally be hosted by the CDL to ensure persistent access to those digital objects and interoperability with other data formats. However, contributing institutions may link to locally-hosted digital objects.
Legacy digital objects in the OAC created before 2003 were generated by the CDL through an EAD extraction process. Metadata for a particular digital object was extracted directly from the <daogrp> Digital Archival Object Group encoding within an EAD finding aid, and united with associated content files (either contributed to the CDL or hosted on the institution's local web server) by compilation into a METS wrapper. (As of 2005, digital objects contributed to the CDL should be prepared and submitted according to the specifications outlined in the CDL GDO).
The subsequent tables in this section therefore distinguish two permissible methods for linking to digital objects in the OAC: one for CDL-hosted METS digital objects (or locally-hosted digital objects), the other for legacy EAD extracted digital objects.
The <dao> tag should be used for general linking to CDL-hosted METS digital objects (or locally-hosted digital objects). The <daogrp> tag should be used exclusively when linking to legacy, EAD extracted CDL-hosted METS digital objects.
The OAC reserves use of the ROLE attribute on <dao> and <daogrp> for particular kinds of data processing purposes, summarized below.
Table 4.4.
| http://oac.cdlib.org/arcrole/link/ | Use when linking directly to a URL for a METS object already ingested and hosted by the CDL, or to a locally-hosted digital object. |
| http://oac.cdlib.org/arcrole/link/grab/ | Use when linking to a URL for a METS object to be ingested and hosted by the CDL (i.e., a URL to a METS file). |
| http://oac.cdlib.org/arcrole/define | Use to indicate that the CDL should continue to generate and host a METS object, extracting metadata for that object from <daogrp> subelements. |
| http://oac.cdlib.org/arcrole/link/search/ | Use when linking to a search for a collection of CDL-hosted METS objects described in the finding aid. Also supply the total number of digital objects in the collection at the end of the URL (e.g., "http://oac.cdlib.org/arcrole/link/search/" for a search of a collection of digital objects). |
The link from a finding aid to an object or group of objects can be made at any level (i.e., collection, series, subseries, file, or item level) in the finding aid, but it should be made at the level where the object(s) is described or implied in the finding aid.
Linking to a group of objects can be done using two different methods:
-
Use a single <dao> Digital Archival Object or <daogrp> Digital Archival Object Group tag, as appropriate, to link to individual CDL-hosted METS objects or locally-hosted objects as defined in Tables 4.5 and 4.7 of this chapter. Note that this is most practical if linking from the file or item level to an individual object or group of objects.
- Use a single <dao> Digital Archival Object tag for a group of objects, with the ROLE attribute set to "http://oac.cdlib.org/arcrole/link/search/" as defined in Table 4.5 of this chapter. Within the HREF attribute, encode a link to a search for a collection of CDL-hosted METS objects, using the following search query syntax. Supply the ARK unique identifier for the finding aid in the "relation" parameter of the search query (see bolded "ark:/13030/kt9290094p" below):
Example:
<dao role="http://oac.cdlib.org/arcrole/link/search/" href="http://content.cdlib.org/search?style=oac-img&sort=title&relation=ark:/13030/kt9290094p" title="Online items">
Note that this is most practical if linking from the collection level or major subdivision to the entire group of objects represented by that level of description.
Objects can be linked to the finding aid in two ways:
-
If linking to the collection level, use an external reference in the digital wrapper for the METS object to link to the beginning of the finding aid. (If submitting metadata records and associated content files for compilation by the CDL into a METS object, use an external reference in the object's metadata record).
-
If linking to any other specific part of the finding aid (i.e., series, subseries, file, or item level), encode a unique identifier in the digital wrapper for the METS object. (If submitting metadata records and associated content files for compilation by the CDL into a METS object, encode a unique identifier in the object's metadata record).
At the appropriate <archdesc> Archival Description or <c0x> Component level where the object(s) is described or implied in the finding aid, encode the unique identifier (see bolded "xyzj0098" below) within an ID attribute for that <c0x> Component tag.
Example:
<c02 id="xyzj0098" level="item">
The value of the unique identifier must be unique for the finding aid. The unique identifier must also comply with rules for IDs specified in the W3C's XML Schema Part 2: Datatypes. Namespaces must start with a letter and can include letters, digits, periods, hyphens, and underscores. Colons are not allowed.
<did>
<container type="box" label="Flat file">7</container>
<unittitle>Ushiwaka and Benkei duelling on Gojo Bridge or Gojo Bridge, an episode from the Life of Yoshitsune, Chronicles of Yoshitsune, <unitdate>1881</unitdate>.
</unittitle>
<dao role="http://oac.cdlib.org/arcrole/link/image" href="http://ark.cdlib.org/ark:/13030/kt4p3005qx/"></dao>
</did>
</c02>
As of 2004, links to specific <c0x> Component levels cannot be implemented in the OAC database, but are expected to be implemented by early 2005. However, institutions are encouraged to encode valid <c0x> Component level links in the meantime, as they will be activated upon implementation.
For complete guidelines, see the CDL GDO.
The Archival Resource Key (ARK) identifier is a naming scheme for persistent access to digital objects (including images, texts, data sets, and finding aids), currently being tested and implemented by the CDL for collections that it manages. The ARK is a specially constructed, globally unique, actionable URL. Each ARK persistently links users to three things:
-
Digital object metadata
-
Digital object content files
-
A faceted commitment statement by the CDL to provide persistent access to the digital object.
Given that ARKs are specially constructed and globally unique, their production and management is controlled by the CDL.
Institutions must encode ARKs within a finding aid whenever linking to corresponding CDL-hosted METS digital objects. For information on the use of ARKs at the CDL, and options for OAC contributing members to obtain ARKs for CDL-hosted METS digital objects, please see the CDL ARK service description.
Table 4.5.
| ELEMENTS & ATTRIBUTES | STATUS | REPEAT? | COMMENTS / APPLICATION NOTES | ||||||||||||||
| <dao> | R | N | Use one or more <dao> tags per finding aid, but use only one <dao> tag for each link to a single object. Nest <dao> within <did>. |
||||||||||||||
|
R | CDL systems supply a default value of "http://oac.cdlib.org/arcrole/link/image" within the ROLE attribute for <dao>. To override this default value, use the following guidelines. Use "http://oac.cdlib.org/arcrole/link/" when linking directly to a URL for a METS object already ingested and hosted by the CDL, or to a locally-hosted digital object. Use "http://oac.cdlib.org/arcrole/link/grab/" when linking to a URL for a METS object to be ingested and hosted by the CDL (i.e., a URL to a METS file). Also supply one of the following qualifiers at the end of the URI to specify type of object (e.g., "http://oac.cdlib.org/arcrole/link/text"): Table 4.6.
Alternatively, use "http://oac.cdlib.org/arcrole/link/search/" if you would like to link to a search for a collection of CDL-hosted METS objects described in the finding aid. |
|||||||||||||||
| href= | R | If the ROLE attribute is set to "http://oac.cdlib.org/arcrole/link/", then encode the URL for a METS object already ingested and hosted by the CDL, or to a locally-hosted digital object. If the ROLE attribute is set to "http://oac.cdlib.org/arcrole/link/grab/", then encode the URL for a METS object to be ingested and hosted by the CDL (i.e., a URL to a METS file). Alternatively, encode the search query for a collection of objects in HREF using the following search query syntax. Supply the ARK unique identifier for the finding aid in the "relation" parameter of the search query (see bolded "ark:/13030/kt9290094p" below):
Example: The corresponding ROLE attribute should then be set to "http://oac.cdlib.org/arcrole/link/search/". Do not use XLINK, XPOINTER, or ENTITYREF in lieu of HREF |
|||||||||||||||
| title= | P | Use an appropriate label to characterize the nature of the digital object version or format (e.g., "Image" or "Online items"). The default label supplied by the OAC is "view attached object". Any label supplied in TITLE will override the default label supplied by the OAC. | |||||||||||||||
| <daodesc> | P | Use to state information about the contents, usage, or source of a <dao> Digital Archival Object, when not sufficiently characterized by information in <unittitle> or other descriptive information. |
Example (link to a new CDL-hosted METS digital object, from an item-level description):
<dsc>
...
<c02 id="xyzj0098" level="item">
<did>
<container type="box" label="Box">7</container>
<unittitle>Ushiwaka and Benkei duelling on Gojo Bridge, an episode from the Life of Yoshitsune, Chronicles of Yoshitsune</unittitle> <unitdate>1881</unitdate>
<dao role="http://oac.cdlib.org/arcrole/link/image" href="http://ark.cdlib.org/ark:/13030/kt4p3005qx/" />
</did>
</c02>
...
<dsc>Example (link to a locally-hosted digital object, from an item-level description):
<dsc>
...
<c03 level="item">
<did>
<container type="box" label="Box">12 : 1</container>
<unitid>11479778</unitid>
<unittitle>Photograph of desserts at Lee's Sandwich</unittitle> <unitdate>July 2002</unitdate>
<dao role="http://oac.cdlib.org/arcrole/link/image" href="http://38.184.1.89/TLCScripts/interpac.dll?LabelDisplay&Config=YSM&Branch=,0,&FormId=-59912&DataNumber=868344&ItemField=1" />
</did>
</c03>
...
<dsc>Example (link to a search for a collection of CDL-hosted METS digital objects, from a collection-level description):
<archdesc level="collection">
<did>
<origination label="Creator">
<corpname source="aacr2">Anaheim Public Library </corpname>
</origination>
<unittitle>Anaheim Public Library photograph collection on Anaheim local history</unittitle>
<unitdate type="inclusive" normal="1860/1970">1860-1970</unitdate>
<unitdate type="bulk" normal="1860/1923">(bulk 1860-1923)</unitdate>
<physdesc>
<extent>1,802 items</extent>
</physdesc>
<abstract>Anaheim Public Library's photograph collection includes images of historical interest of the City of Anaheim and other areas of Orange County from the 1860s to 2002. Images document public, residential and commercial buildings, including businesses, schools, churches, citrus packing houses, fire and police department facilities, theaters (such as the Fox Theater), and the public library; street scenes; neighborhoods; significant individuals, including members of the Los Angeles Vineyard Society which founded Anaheim in 1857 as a wine making colony, early mayors and civic leaders (such as August Langenberger and Charles Pearson); groups and family portraits; annual events, such as the California Valencia Orange Show and the Anaheim Halloween Parade; local geography, including the Santa Ana River, Anaheim Landing and local canyons; rancho families, such as Juan Pacifico Ontiveros and Vicenta Sepulveda Yorba Carrillo; Mission San Juan Capistrano; viticulture and agriculture, including the Anaheim chili peppers; transportation; ethnic communities, including Japanese Americans, Chinese Americans, etc.; natural disasters, such as the 1933 Long Beach earthquake and the 1938 flood; Anaheim Resort area, including Edison International Field (also known as the Big A), Anaheim Convention Center, Arrowhead Pond of Anaheim and Disneyland. Formats include panoramic and aerial photographs. Of particular interest are the large number of photographs which document the development of the Los Angeles Vineyard Society from circa 1860 to 1890. The collection also contains a small number of ambrotypes, daguerreotypes, and tintypes from the 1870s and 1880s.</abstract>
<repository>
<corpname source="lcnaf">Anaheim Public Library.
<subarea>Central Library. History Room.</subarea></corpname>
<address>
<addressline>Anaheim, California 92805</addressline>
</address>
</repository>
<unitid repositorycode="CAna" countrycode="US">Consult repository</unitid>
<langmaterial>Collection materials are in <language langcode="eng">English</language>.</langmaterial>
<dao role="http://oac.cdlib.org/arcrole/link/search/" href="http://content.cdlib.org/search?style=oac-img&sort=title&relation=ark:/13030/kt2199p9w7" title="Online items">
<daodesc>
<p>Selected digitized images from this collection.</p>
</daodesc>
</dao>
</did>
...
</archdesc>
Example (link to a search for a collection of CDL-hosted METS digital objects, from a series-level description):
<dsc>
...
<c01 level="series">
<did>
<container type="box" label="Drawer : Folder ">1 : 56 - 4 : 7</container>
<unitid>Series 2. </unitid>
<unittitle>Correspondence</unittitle>
<unitdate>1840-1947</unitdate>
<physdesc><extent>5.5 cubic ft.</extent></physdesc>
<dao role="http://oac.cdlib.org/arcrole/link/search/" href="http://content.cdlib.org/search?style=oac-img&sort=title&relation=ark:/13030/kt9290094p" title="Online items">
<daodesc>
<p>Selected digitized images from this series.</p>
</daodesc>
</dao>
</did>
<scopecontent>
<head>Series Scope and Content Summary</head>
<p>This series contains correspondence between Ellen Browning Scripps and her family, friends, and acquaintances. Most of the excerpts of correspondence located at the beginning of this series were compiled by J. C. Harper and are arranged by subject. Excerpts may also be found in correspondence with Edward Wyllis Scripps (E. W.), 1868-1926.</p>
<p>This series is arranged alphabetically by correspondent or subject.</p>
</scopecontent>
...
</c01>
...
</dsc>
Table 4.7.
| ELEMENTS & ATTRIBUTES | STATUS | REPEAT? | COMMENTS / APPLICATION NOTES | ||||||||||||||
| <daogrp> | R | N | Use one or more <daogrp> tags per finding aid, but use only one <daogrp> tag for each link to a single object. | ||||||||||||||
| role="http://oac.cdlib.org/arcrole/define/" | R |
CDL systems supply a default value of "http://oac.cdlib.org/arcrole/define/image" within the ROLE attribute for <daogrp>. This is used to indicate that the CDL should continue to generate and host a METS digital object, extracting metadata for that object from subelements. Override the default ROLE attribute value, if necessary, using one of the following qualifiers at the end of the URI to specify the type of object (e.g., "http://oac.cdlib.org/arcrole/define/text"): Table 4.8.
|
|||||||||||||||
| <daoloc> | R | Y | |||||||||||||||
| role="thumbnail | med-res | hi-res" | R | Use one of the following values to indicate the nature of the digital object version or format:
|
|||||||||||||||
| href= | R | Use to encode the URL for the location of each content file associated with the digital object. Do not use XLINK, XPOINTER, or ENTITYREF in lieu of HREF. | |||||||||||||||
| title= | P | Use an appropriate label to characterize the nature of the content file version or format. (e.g., "panoramic view," "enlargement,"
etc.). A default label for the content file will be supplied by the OAC based on the information encoded in ROLE:
Any label supplied in TITLE will override the default label supplied by the OAC. |
Example (link to a legacy CDL-hosted METS digital object, from an item-level description; metadata derived from EAD):
<dsc>
...
<c02 id="gcga_1991.38.5" level="item">
<did>
<origination>
<persname>Tsukioka Yoshitoshi, 1839-1892</persname></origination>
<unittitle>The ferocity of Tametomo driving away the small-pox demons, from New Forms of Thirty-six Ghosts</unittitle>
<unitdate>1890</unitdate>
<physdesc>
<genreform>Oban</genreform>
<physfacet>Color woodcut</physfacet>
<physfacet>Inscription: Recto, signed Yoshitoshi and Taiso seal at l.r.</physfacet>
<dimensions>14 3/8 x 10 inches</dimensions>
<bibref>References: Keyes 509.19</bibref>
</physdesc>
<repository>Grunwald Center for the Graphic Arts, UCLA Hammer Museum</repository>
<unitid>1991.38.5</unitid>
<daogrp role="http://oac.cdlib.org/arcrole/define/image">
<daoloc role="thumbnail" href="http://www.oac.cdlib.org/affiliates/images/grunwald/gcga_1991.38.5_1_3.jpg" />
<daoloc role="hi-res" href="http://www.oac.cdlib.org/affiliates/images/grunwald/gcga_1991.38.5_1_2.jpg" />
</daogrp>
</did>
<admininfo>
<p>
<custodhist>The Eugene L. and Davida R. Trope Collection.</custodhist>
</p>
</admininfo>
<controlaccess>
<head>Subjects</head>
<subject source="local">rulers (people)</subject>
<subject source="local">samurai</subject>
<subject source="local">weapons</subject>
<subject source="local">demons</subject>
<subject source="local">disease<s/subject>
</controlaccess>
</c02>
...
<dsc>
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