Computational Aspects: How Landmarks Can Be Observed, Stored, and Analysed - Landmarks: GIScience for Intelligent Services - page 150

Fig. 5.9 Important concepts
in modeling wayfinding in
street networks: a decision
point with two functionally
relevant branches, the
incoming and outgoing
route-segment. From [ 36 ] ,
modified

decision
point

Fig. 5.10 Circular order of
objects at an intersection.
Starting with A, the order is
A<D<C <church <
B<A(from [ 36 ] , modified)

C

D

B

A

Importantly, the representation is coordinate-based, i.e., each node has a coordinate,
which then allows to talk about distances and directions. Considering movement
through this network, an intersection becomes a decision point, i.e., a point along the
way where alternative continuations of movement direction exist. Only the branches
one enters and leaves this decision point on are functionally relevant. They are
crucial in finding the (right) way. The former is called incoming route segment ,
the latter outgoing route segment (Fig. 5.9 ) .
Landmarks are integrated into this representation by using the same reference
frame as the network elements. They are represented either as point, (poly)line or
polygon using the same coordinate system as for the decision points. This way, it
becomes possible to relate landmarks to the route. In Richter's approach, to this end
circular ordering information is exploited. Figure 5.10 illustrates this idea. Ordering
information derives from the linear, planar, or spatial ordering of features [ 44 ] .
This kind of information is a powerful structuring mechanism as it only requires
knowledge about a neighborhood relation between relevant objects. No further
metric knowledge is needed.
First, let us consider landmarks represented as points. Determining the location of
linear and area-like landmarks works accordingly as will be explained below. There
are three qualitatively different cases that need to be distinguished when looking
at point landmarks. The turning action at the decision point may happen after the
landmark has been passed, before it is passed, or the landmark may not be located
at a functionally relevant branch at all (Fig. 5.11 ; see also [ 26 ] ). In other words, the