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that are subdivided into regions and towns). Hierarchies can also emerge from
functional dependence, due to the asymmetry of social or economic relations. The
various studies presented in this volume refer to hierarchies in different scenarios.
The interactive visualisation DAGMap referred to two different hierarchies,
with one governing how subsidiaries organise and the other governing how they
distribute over world regions. The two hierarchies were merged into a single visual
structure, a TreeMap displaying the organisational hierarchy and colour-coding
world regions. Examining how the hierarchy of subsidiaries was globally distributed
supported the analysis of patterns reflecting the potential impacts of globalisation.
Specific interactions were designed to allow the analyst to navigate through the
different levels of the organisational hierarchy and identify colour-coded patterns
(see Figs. 7.11 and 7.13 ).
In Chap. 6 , the edge bundling method produced a hierarchy that integrated
physical distances and graph topology. Although the goal of the method was to
improve the readability of a map, edge bundles also reduced edge cluttering in
graphs by routing edges along Bézier curves. Similar to creeks and smaller rivers
that flow into a larger stream, the edges were partly merged. The original graph
connections were combined into higher-level edges that described overall flows on
the map, which created a new hierarchy in the graph by aggregating closely edged
segments. This hierarchy of bundles of routes provided an original and novel method
for visualising a hierarchy in space.
The order hierarchies present in networks are often used to compare or reveal
node roles, as mentioned above. Node degree remains an essential node statistic
that captures the overall structure of a network and often reveals the graph's
scale-free structure. Although not systematically or explicitly stated in the various
studies presented in this volume, most of the networks studied were scale-free. This
property is not surprising because places are also hierarchised based on economic
dependency. For example, the Christaller model ( 1933 ), which is the root of the
settlement system, is hierarchised by access to services as well as economic and
political functions. Further theoretical research into geographical organisations with
different types and scales of hierarchies would be worthwhile ( Castells , 1996 )and
would detect the specific scales that optimally represent a phenomenon's spatial
organisation.
5
Detecting Critical Structural “Levels” in Global
Organisations
In summary, the methodology adopted in the SPANGEO project consisted in
comparing and contrasting the network topology of the regional partitioning of
nodes (places). The network's modular and multilevel organisation was based on
identifying key nodes or edges that decomposed the network into smaller, denser
components. Various connectivity statistics were used to identify the minimal
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