
Representation of a Movement as a Spatial Interaction
Movements can be simplified in three basic categories: freight, people
and information. Representing a movement as a spatial interaction involves
the following considerations:
- Locations. A movement is occurring between a location
of origin and a location of destination. i generally
denotes an origin while j is a destination. The representation
of origins and destinations commonly involves centroids. A centroid
is an abstraction of a the attributes of a zone at a point. This
is of particular relevance when the attributes of the movements
considered are zonal (e.g. ZIP codes, cities, states, etc.) while
the graphic representation of these movements requires specific
origins and destinations. For instance, showing flows between ZIP
codes on a GIS would implicitly require the generation of one centroid
for each ZIP code.
- Flows. Flows are generally expressed by a valued vector
Tij, representing an interaction between locations
i and j.
On the above figure, two geographic zones, zone i and zone
j, are expressed by two centroids, i and j. A
vector Tij links two centroids and has a value assigned
to it (50) which can represents movements such as tons of freight, numbers
of passengers per day, or number of phone calls.