The foaf:based_near relationship relates two "spatial things" (anything that can be somewhere), the latter typically described using the geo:lat / geo:long geo-positioning vocabulary (See GeoInfo in the W3C semweb wiki for details). This allows us to say describe the typical latitute and longitude of, say, a Person (people are spatial things - they can be places) without implying that a precise location has been given.

We do not say much about what 'near' means in this context; it is a 'rough and ready' concept. For a more precise treatment, see GeoOnion vocab design discussions, which are aiming to produce a more sophisticated vocabulary for such purposes.

FOAF files often make use of the contact:nearestAirport property. This illustrates the distinction between FOAF documents (which may make claims using any RDF vocabulary) and the core FOAF vocabulary defined by this specification. For further reading on the use of nearestAirport see UsingContactNearestAirport in the FOAF wiki.