We'll begin by looking at the current state of play as far as metadata in XHTML documents is concerned. A typical example of what people already do is this (we'll pretend that everyone uses XHTML):
<head>
<meta name="author" content="Mark Birbeck" />
</head>
Nothing wrong with that you'll say, and if this snippet of mark-up is in a web page at my blog's address (http://internet-apps.blogspot.com), then we have in effect 'created' the following triples:
<http://internet-apps.blogspot.com> _:author "Mark Birbeck" .
(If you're not familiar with this N3 syntax don't worry, we won't use a lot in this discussion--it just means that some resource has a property called 'author', and the value of that property is "Mark Birbeck". You can think of it as the same as a name/value pair, like those you use to send data to a server, i.e., author="Mark Birbeck".)
So far so good; we've made a statement about my blog, and we could easily have added other information, such as when it was written, when it was last updated, and so on.
The problem comes next though; what if we now want to add that I am currently on holiday in the south of France (if only...)? There are many systems that are making use of 'geo tagging' and there are a number of recommended ways to do this, but they all boil down to doing something like this in the head of your document:
<head>
<meta name="author" content="Mark Birbeck" />
<meta name="geo.position" content="43.95;4.833333" />
</head>
It might appear obvious, but we should ask what exactly this means, since we may be surprised by the answer.
The convention is that it means that 'Mark Birbeck' is currently located in Avignon, France. The technique is not actually limited to people though, and it could also give us the location for a company, a conference, a meeting, and so on.
But there's a problem--in 'proper' RDF terms putting this metadata in the head of a document means that the document itself is in the South of France!