Using one global document to provide descriptions for multiple instances of the same image is a powerful, portable, re-usable, technique. It provides efficiency and scalability making authoring and maintenance easy wherever instances of the same image are used in multiple locations. It is analogous to the power of external style sheets.
The technique saves time and money by:
Global long descriptions can be applied across multiple sites, or across an entire site, or across a subset of pages, and doing so means that you only need to update one file if you need to make a change.
As documented by the use cases and evidenced by real world
examples a description available in a separate document provides
easy reuse of the targeted description from multiple sources (i.e.
ability for an image to appear in multiple documents throughout a
site or throughout multiple sites or from an HTML email while at
the same time linking to one
Other "solutions" such as
implemented lack this functionality. They are hamstrung to on-page
Design, the Lambda
Theta Phi Latin Fraternity, the Texas State
Library , and the University of
Minnesota Duluth use images in separate documents that share
the same long description page. This technique is
key to the logo description use cases and the lightbox description use case as
evidenced through examples in the wild, Geoff Freed has
indicated that professional content producers (such as those
involved in the ePub initiative, and the US federally-funded
DIAGRAM Project) are interested and ready to use
longdesc to externally store descriptions to facilitate
etext descriptions and describe
Of long description techniques, using a separate document is a prevalent design pattern/cowpath with authors who supply descriptions. Less than two-dozen on-page long descriptions via anchor were found out of over two thousand examples in the wild.
The code and UI patterns for how to navigate to an external page
longdesc are well established (and used for
example by JAWS and Opera).