There are significant challenges to digital annotation systems, particularly shared annotation systems. While server space was an issue with Mosaic in the 1990s, this isn’t the critical problem any longer, although at scale an annotation system does require some significant hardware support. The real problem associated with public sharing of annotations is getting the model to work across different devices and systems. Locating a reference point is also a particular challenge when working with reflowable text. In such a context, referring to page 164 doesn’t mean anything, because page numbering often isn’t used and, even if it were, one could size up the font to such a large scale that only three words might appear on a “page”. Similarly, one can’t rely on specific character count within the file, since that could change with minor editorial corrections. Creating a hash string of text characters before and after a reference point also has problems, since text could be repeated (as in song lyrics, or a recurring dream sequence). Also matching the point in a text between the first edition of a book and its annotated, fourth version, for example, creates tremendous matching problems, since there isn’t a widely adopted work identifier to tie together various manifestations of a work. This problem expands as one moves away from simply annotating text to comparing different media expressions. Those who remember showing up at a literature class with a different edition of a book than the teacher was using can relate to these challenges. NISO has a working group that is exploring standards to address the location problem, as a jumping off point for work on this topic, which we hope will feed into the IDPF EPUB3 specification.

Other non-technical problems arise that need to be addressed, such as copyright concerns and privacy. If an annotation system allows the capture of a selection of the referenced text, one might be able to collect all of those disparate snippets to recreate the work in its entirety. Realistically, this is among the least likely of pirate scenarios, but some publishers who have engaged in these discussions have noted it with concern.