Metadata for raster images is discussed in detail in ‘Guidelines for Handling Image Metadata’ (Metadata Working Group 2009) and on the JISC Digital Media page discussing ‘Metadata and Digital Images’. As highlighted in the JISC Digital Media document, image metadata can cover a wide range of areas (structural, descriptive, administrative and technical metadata) and can be either embedded within the image file (e.g. in EXIF, IPTC and XMP) or stored as a separate file (database, spreadsheet or text document).

Technical metadata

Embedded technical metadata, commonly in the EXIF format, generally relates to the creation of the image file and records various camera settings used in capturing the image. There has been much discussion regarding whether automatically generated metadata such as EXIF should, or should not, be considered a significant property. There are obviously scenarios where such metadata is consciously used by a data creator and inherently significant, but commonly this is not the case. There is also the potential that certain metadata elements (e.g. date), where not deliberately used and set up, can actually contain incorrect information. Embedded metadata within images has, however, become common practice and a decision should be made when converting or reproducing images as to whether this metadata should be retained. Generally, if embedded metadata is seen to be valuable, it should be preserved.

Other technical elements may be included in embedded metadata sets but are more commonly recorded as separate files (e.g. text files or spreadsheets) due to the varying requirements of each data creator and archive. Common technical metadata elements include documenting capture device information, software and editing history.

Wider metadata

Other non-technical metadata elements may also be recorded and either embedded in e.g. IPTC or XML formats or recorded in separate files. Such metadata is primarily aimed at recording descriptive and administrative data and documents the image subject, creators and content. The IPTC Photo Metadata set (IPTC 2010), created by the International Press Telecommunications Council for news and stock photography, is a good example of the range of coverage that a single standard can have and includes elements that cover the image’s content, related location, subject and rights. Additionally, data creators and archives may draw upon the wide array of existing metadata standards such as Dublin Core[2], MIX[3], PREMIS[4] and ANSI/NISO Z39.87[5] and specify a set of metadata elements that work best for the images that they are creating and preserving.

The following are suggested metadata elements and have been drawn together from the sources discussed above (namely AHDS Bitmap handbook, Metadata Working Group guidelines and Dublin Core). It is important to note that many of these elements can be applied to an entire collection (e.g. a set of photographs) and would not need to be repeated for each file.

Element	Description
Identifier	Image filename e.g. survey01.tif
Title / Caption	The title of the image or a suitable caption
Description	Description of the image
Creator	Name of image creator
Date	Date on which image was created
Rights	Details of copyright or other rights and holder details
Keywords	Keywords e.g. period, site or feature terms. Use suitable thesauri where they exist
Location	Location information for the image. Where possible use a standardised format e.g. Lat/Long or keywords from a suitable thesauri e.g. Getty Thesaurus of Geographic Names[6]

Additional basic technical metadata

Element	Description
File Format and Version	e.g. TIFF 6.0
File Size	Size of the file in bytes
Spatial Resolution	The resolution of the image measured in pixels per inch (ppi)
Dimensions	Dimensions of the image in pixels e.g. 400 x 700px
Colour Space	The colour space used in the image e.g. RGB or grayscale
Bit Depth	e.g. 24bit or 8bit
Capture Device	Either the camera make and model or details of the scanner
Capture Software	Software used to capture the image (generally used for scanned images) e.g. Adobe Photoshop CS3

When using embedded metadata, it is important to flag up its use in any project level metadata and to also be aware of how embedded metadata is supported by files and how data migrations affect such metadata. For example, EXIF metadata is supported by JPEG and TIFF but not – at least in the same way – by JPEG2000 (LaBarca 2010) and PNG formats. IPTC metadata also exists in two types (the current Photo Metadata set and the legacy IIM set) and is also supported within the XMP format.

[2] http://dublincore.org/

[3] http://www.loc.gov/standards/mix//

[4] http://www.loc.gov/standards/premis/

[5] ANSI/NISO Z39.87 Data Dictionary – Technical Metadata for Digital Still Images. https://www.niso.org/publications/ansiniso-z3987-2006-r2017-data-dictionary-technical-metadata-digital-still-images

[6] http://www.getty.edu/research/conducting_research/vocabularies/tgn/