Since the introduction of XMP in 2001 Adobe published several versions of the XMP specification:

XMP 2001-09: first published version, also called XMP 1.5

XMP 2004-01: basis of PDF/A-1; removed alias mechanism

XMP 2005-06: widely used

XMP 2005-09: many clarifications, but no significant changes; basis for PDF/E-1, will probably also be used as basis for ISO 32000

There have been a few incompatible changes. For example, the type of the property photoshop:SupplementalCategories changed from Text in XMP 2004 to Bag Text in XMP 2005.

Since up to now no XMP validation service has been available, various kinds of invalid XMP can be found in real-world documents including PDFs created with Adobe software. For example:

Creating an Author entry in the document properties panel of Acrobat 8 will insert the dc:creator property with type Bag. This should be type Seq according to XMP 2004.

Adobe applications create various properties outside the list of predefined XMP properties, e.g. pdf:Title, pdf:CreationDate, photoshop:History. This is not per se a violation of the XMP spec (which supports additional schemas and properties), but created confusion, especially for properties where a predefined property is available (e.g. custom pdf:CreationDate vs. predefined dc:CreateDate).

The original PDF/A-1 standard publication also suffers from several problems in the description of its XMP requirements. Most of these problems have been rectified in Technical Corrigendum 1 (TC1). For example:

The crosswalk for mapping document info entries to XMP properties maps the Subject entry to dc:subject instead of the correct dc:description (fixed in TC1).

Embedded font file streams are recommended to include the font name in the xmp:Title property instead of the correct dc:title (fixed in TC1). This problem is related to the alias mechanism described in XMP 2001; however, aliasing is no longer recommended and not supported in XMP 2004 and PDF/A-1.

Similar for xmpRights:Copyright instead of the correct dc:rights (fixed in TC1).

In order to create valid XMP metadata you must ensure that it conforms to the XMP specification, and (depending on context) additional requirements such as those mandated by PDF/A-1. Since XMP is based on XML, the first step in checking your XMP is to make sure that it is indeed well-formed XML. You can use any of the existing XML valdation tools for this step. Taking into account that XMP is derived from W3C's RDF, you must also make sure that your XMP contains valid RDF. This can be tested with the RDF validation service provided by the W3C.

But how do you make sure that your XMP conforms to Adobe's XMP specification, and that it possibly conforms to the additional XMP requirements of PDF/A-1? Having XMP validation results is useful for developers and administrators who want to create their own custom XMP for inclusion in PDF and especially PDF/A-1. Since Acrobat does not supply XMP validation information we provide a free XMP validation service for PDF/A.