The notion of urban unit is based on the continuity of built up land mass and the number of inhabitants. We call urban unity a municipality or a group of municipalities which includes a continuously built up zone (no cut of more than 200 meters between two constructions) and at least 2,000 inhabitants.
If the urban unit is situated on a single municipality, it is called an isolated town. If the urban unit extends over several municipalities, and if each of these municipalities concentrates more than half of its population in the continuously built up zone, it is called a multi-municipal agglomeration.
The municipalities are considered as rural when they go into constitutionare not included in an urban unit: the municipalities of 2000 inhabitants with no continuously built up zone, and the municipalities in which less than half of the municipal population is settled in a zone of built continuously built up zone.
These thresholds of 200 metres for the continuity of habitat and a population of 2,000 inhabitants are the result of internationally-adopted recommendations.
In France, the calculation of the space between two constructions is realized by the analysis of databases on the built of the National Geographical Institute (IGN).
It takes into account cuts of the urban infrastructure such as waterways in absence of bridges, gravel pits, important difference in level. Since the cutting of 2010, certain public places (cemeteries, stadiums, airfields, car parks...), industrial or commercial land (factories, industrial estate, shopping malls) are treated as built up with the rule of 200 meters to connect inhabited zones of construction, unlike the previous cuttings where these spaces were only cancelled in the calculation of the distances between built up.
The urban units are periodically redefined. The current zoning dated 2010 was established in reference to the population of the census of 2007 and on the territory geography on January 1st, 2010. A first demarcation of towns and agglomeration was realized whith the census of 1954. New urban units were then constituted during the censuses of 1962, 1968, 1975, on 1982, 1990 and 1999.
The urban units can extend over several departments, even cross the national borders (see : international urban unit).
The cutting in urban units concerns all the municipalities of Metropolitan France and French overseas departments. The cutting of 2010 integrates the municipalities of the new department of Mayotte.