At risk of taking the discussion further off-topic, the Flickr API provides a way to find boundaries determined by how people have tagged photographs. It's not exactly definitive, but still intriguing. The API is explained here - http://www.flickr.com/services/api/flickr.places.getShapeHistory.html but you can see the results more easily here - http://boundaries.tomtaylor.co.uk/#15726 (the link shows Chatham).
I found that Yahoo "Where on earth" bounding boxes vary a lot in their precision - some are quite tight around the place, but others are huge. In my other attempts to find an automagic way of working out the extent of a settlement I've been experimenting with drawing polygons around places tagged "IS_IN" (not, I hasten to add, on the map itself - on an extract of the database). It works some of the time (up to a point), but I need to be a bit more clever at untangling the different forms of description that are used for "is_in". If it is of interest to anyone else I will post some results once I clean it up a bit.
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