Tagging photos for your own personal use is both a great idea and a must do. Danny shares plenty of examples why in his SearchDay article: Photo Search: Google Picasa 2 Vs. Adobe Photoshop Album 2.
It's one thing to create a set of terms for your own personal use. However, when creating and using tags to build a shared database of images or text, problems can quickly come into view. Synonyms are a problem (ie. pop vs. soda, torch vs. flashlight, bag vs. sack) Another example, if I tag an item "Treo" does it mean images taken by a Treo camera? Images of Treos? What do the tags "From" or year mean? As categories grow larger, can they be subdivided? What are the subdivisions? What about pluralization? Vocabularies should quickly bring like things together, help the user conduct a thorough search, and hopefully help save the searcher time.
I believe controlled vocabularies still have plenty of value especially when trying to add verbal subject access to materials (both image, video, print).
Are controlled vocabularies the perfect solution? No. First, they can be expensive to build and maintain. Second, they take time and effort to update (add new terms, remove old terms, map old to new, etc.) Second, localization can still be an issue. Third, scalability can pose problems. Finally, it's one thing to build a vocabulary, but it's another thing to apply the terms it contains properly. Deciding the "aboutness" of an item in just a few words can be a real challenge.
Ask working librarians and library school students what they found the most challengeing part of their MLIS education, and you'll likely here that cataloging (aka adding metadata) was it.
With that out of the way, I thought some of you might be interested in learning about a few of the many controlled vocabularies out there.
Let's begin with a look at the Thesaurus for Graphic Materials II: Genre and Physical Characteristic Terms (TGM II) from the Library of Congress. This searchable thesaurus contains more than 600 agreed upon terms to describe graphic materials.
The terms in the TGM II provide the cataloger (who chooses to uses this thesaurus) with agreed upon terms to describe both the genre and physical characteristics of an image or other type of graphic material (posters, line art, etc.) For example:
+ What does a "birds eye view" mean?
Spend some time browsing and searching TGM II and you'll see how terms relate to one another plus find "scope notes" to explain what certain terms mean.
Finally, if you want to see the TGM II in action, check out the Prints & Photographs Online Catalog. All of the records contain subject access and more than 65% of the holdings in this database have been digitized.
Much more on controlled vocabularies (both pluses and minuses) and many more examples of tools to visit, coming soon. Again, my reason for sharing is not to say that one specific tool or method (controlled vocabulary vs. folksonomy) are better than the other but rather to introduce resources you might not know about and perhaps promote some thought and discussion.