Geoscience Reference
In-Depth Information
process of dilatation/retraction
11
, the areas closer than 200 m are connected between
themselves and aggregated. That way the information thus created is compatible
with the usual meaning of built area continuity that underlies the definitions of urban
areas. Thus, a set of geometric envelopes that constitute zones is obtained, and a
map of the urban spots built in this manner can be established.
Identifying the objects
At this stage, the issue of identity is crucially raised. If we are talking in a
general way of the city of London, everyone has a representation of it. Faced with
a map of the urban spots performed according to the principles described above, a
person with a minimum of experience will know how to easily identify London in
this set of zones, as soon as she/he knows approximately its localization, and as
she/he knows that it is the most extended city of the region [BRE 13]. The question
would be yet more difficult for a smaller and less “unique” city. As they have been
defined, the areas thus constructed constitute a statistical population whose identifier
has no meaning. The second step of this operation consists of identifying the areas
according to the nomenclature of cities' names that each country uses. This is a
stage that gives semantics to these areas. In order to achieve it, the LAU2 database is
used that records for each LAU2, its name, its geometry and the resident population.
The operation, which consists of transferring the information to the UMZ database is
not unequivocal and needs to define a model to assign the name of a LAU2 to a
UMZ on the one hand and a model to calculate the UMZ population from
appropriate LAU2 on the other hand. We will not explain the method in detail, but
we illustrate the fact that the relationship between UMZ and LAU2 with regard to
the name assignment is not a simple bijection. This is not surprising, since the
toponymy is the result of historical usages and events. The model relies necessarily
on a standard type of process like “the city is built around a center which remains
the most populated”, but the result will depend on the shape of the city, and
therefore on its history (monocentric and polycentric). Figure 2.5 illustrates the
diversity of cases corresponding to the combination of these two sources. The
model, which was proposed, is based on an index of symmetrical coverage
computed on the intersection between the UMZ and LAU2. It is based on what the
intersection surface represents in the respective surfaces of the UMZ and each of the
concerned LAU2. The UMZ receives the name of the LAU2 entity with which it is
most “linked”. It receives as population the sum of those of the LAU2 that intersect
it at the pro rata of the intersected surfaces.
11 This procedure consists of “expanding” all the built-up areas with a thickness of 100 m, so
that all the areas closer than 200 m to each other connect and form a single zone. All the areas
are then reduced to 100 m in order to recover the initial morphology, not including those areas
that will remain connected.
