U.S. Geological Survey Photographic Library
Field 5. is a code derived from photographer or collection name and photo number. This was used as the basis of a file name with the file name repeated in folders containing various resolutions. Field 6. is the description/caption.
Field 1. Records begin with "ID." Followed by a Photographer/Contributer Name or Collection Name (ex. CVO-F for Cascade Observatory) and a number that usually corresponds to a number in the physical album.    Field 2. is the dimensions of the thumbnail (used by the web application to navigate a file structure that is organized by landscape or portrait orientation)
Field 3. is a batch scanning number. Batch numbers eventually show up the file structure of the URL for the image Field 4. is a subject for the batch - subjects could be a bulletin number, geographic area, topic, event or combination of these elements - Not controlled.
ID. Adams Building  1|0075:0100|btch331|Washington, DC- Images|adams001|Adams Building, U.S. Geological Survey, 1333-1335 F. Street, N.W. , Washington, D.C. 1917.  Right photograph page 11 , Images of the U.S. Geological Survey, 1879-1979. Fields delimited by |
To illustrate what a record looks like, this is an example of a Metadata Record for a photo.
About the Collection 
The person I interviewed is Keith Van Cleave, a librarian in charge of the collection for the US Geological Survey at the Denver Library. The collection is extensive, but unfortunately there are no strict controls for the collection. There is, however, a solid system in place for the metadata used as shown here.  The records are all kept in one large text file and each photo is defined by six fields. The U.S. Geological Survey Denver Library maintains a collection of over 400,000 photographs taken during geologic studies of the United States and its territories from 1868 to the present. These images provide a visual history of the discovery, development, and sciences of the United States and its Geological Survey. Some photographs have been used in USGS publications, but most have never been published. Currently, the website represents less than 10 percent of the Library's images with approximately 30,000 photographs on-line.  Users are students and researchers.