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Semantic Annotations Part 1: an introduction

This week's post is about a topic I'm really interested in, that is semantic annotations. It is so interesting for me that in the last two years, despite having jobs not directly involving this subject, I have tried to learn more and work on this topic anyway, using my spare time. Why is that so important for me? Well, it might become the same for you if you like its basic concept, that is allowing anyone to write anything about anything else. Moreover, in a perfect (or at least well designed ;)) world semantic annotations would also allow anyone to only get the information written by people they trust/like (or that authorized them), without being overwhelmed by unuseful, wrong, or bad data.

A big problem about semantic annotations is that the subject itself, from a researcher's point of view, is really broad and any kind of breadth-first approach on the topic tends to leave you with pretty shallow concepts to deal with. At the same time, going depth-first while ignoring some aspects of annotations will only make your approach seem too simplistic to people who are experts in other aspects (or who already tried the breadth-first approach ;)). I think this is the main reason why my work on semantic annotations has become more and more like the development of Duke Nukem Forever... Which, in case you don't know, is a great example of how trying to reach absolute perfection -especially in a field where everything evolves so quickly- keeps you more and more far from having something simply done. For anyone who wants to read something about this, I'd really suggest you to give a look at this article, which I found really enlightening.

So, trying to follow the call to "release early, release often", I'll post a series of articles about semantic annotations here. I have decided to skip scientific venues for a while, at least till I have something that is at the same time deep and broad enough. And if I never reach that... Well, you will have read everything I've done in the meanwhile and I hope something good will come out of there anyway.

What is an annotation?

To start understanding semantic annotations, I guess I should first clarify what an annotation is. Annotations are notes about something: if you are reading a book, you can write them in the page margins; if someone parks a car out of your garage, you can leave one on her windshield (well, better if you directly write that on the windshield, so she'll remember it next time ;)); or, for example, you can add an annotation to food in the fridge with its expiration date. What is common between these annotations is that they are all written on a medium (paper, windshield, whatever!) and they are physically placed somewhere. Moreover, they have been written by someone in a specific moment in time, and they comment something in specific (some text within the book, the act of leaving a car in the wrong place, the duration of some food).

What is a computer annotation?

This is what happens "in real life", but what about computer annotations? As everything is data (some time ago I would have said "Everything is byte"), annotations become metadata, that is data about data. For them we would like to be able to maintain some of the characteristics of the "physical" annotations. They are useful if we can see them in the context of the piece of data they are annotating (what use is an expiration date if we don't know which food it refers to?) and if we can know their authors and dates of creation. There is no "physical medium" for them, but nothing prevents us from adding some other meta-meta-data (that is, data about the annotation itself)  that customizes it to become some sort of electronic post-it, a formal note, an audio file, or whatever else we can imagine.

Computer annotation systems are far from new: think, for instance, about the concept of annotations in documents. However, they get a completely new meaning when a medium like the Web becomes available: in this case we talk about Web annotations, every resource with a URL can be uniquely identified and using XPath it is also possible to access specific parts of a Web page. Collecting Web annotations makes it possible, whenever a web page is opened, to check whether metadata exist for it and display them contextually. Systems like Google Sidewiki allow exactly this kind of operations, but they are not the only ones available: tags are nothing else than simple annotations added to generic URLs (such as in delicious), photos (Flickr), and so on; ratings are typically associated to products, but which are often associated with unique URLs within a website, and systems like Revyu allow you to rate basically anything that has a URI. Finally, there are even games exploiting the concept of Web annotations like The Nethernet.

What is a semantic annotation?

A semantic annotation is a computer annotation that relies on semantics for its definition. And here's the rub: this definition is so wide that it can actually cover many different families of annotations. For instance:

• semantics can be used to define information about the annotation itself in a structured way (i.e. the author, the date, and so on). An example of such an annotation system is Annotea;
• semantics can be used to univocally define the meaning of the content of the annotation. For instance, if you tag something "Turkey" nobody will be ever able to know if you were talking about the animal or the country, while if you tag it with Wordnet synsets 01794158 and 09039411 you'll be able to disambiguate;
• semantics can be used to (also) define the format of the contents of the annotation, meaning that the "body" of the annotation is not a simple unstructured text, but it contains RDF triples or some kind of structured information (the semantic annotation ontology I co-developed last year at Hypios follows this idea).

In the next episodes...

Ehmm... I guess I might have lost someone here, but trust me... there's nothing too difficult, it is just a matter of entering a little more into the details. As the "semantics" part requires a more in-depth description, I'll leave it to the next "episode" of this series. My idea, at least for now, is the following:

• Semantic Annotations Part 2: where's the semantics?
• Semantic Annotations Part 3: early prototypes for a semantic annotation system
• Semantic Annotations Part 4: the SAnno project

I'm pretty sure there will be changes in this list, but I'll make sure they will be reflected here so you will always be able to access all the other articles from every post belonging to this series.