Computational Aspects: How Landmarks Can Be Observed, Stored, and Analysed - Landmarks: GIScience for Intelligent Services - page 151

a b c

Fig. 5.11 Three functionally different locations of a point landmark relative to a decision point:
( a ) turning action after passing the landmark; ( b ) turning action before passing the landmark; ( c )
landmark not at a functionally relevant branch (from [ 36 ] , modified)

landmark may be next to the incoming route segment, next to the outgoing route
segment, or next to any of the other branches of the intersection. Accordingly, the
task now is to determine this next to relationship.
A next to relationship corresponds to a neighborhood relation between a
landmark and a branch, which gets us back to the circular ordering. By introducing
a virtual branch that connects the landmark with the decision point we include the
landmark into the branches' circular ordering. With that, we can determine whether
the virtual branch is a neighbor to one of the functionally relevant branches, simply
by checking whether one succeeds the other in the ordering. If the landmark is
direct successor or predecessor of the incoming route segment, the turning action
is performed after the landmark is passed (denoted with lm < ). Accordingly, if
the landmark is a neighbor to the outgoing route segment, the turning action is
performed before passing the landmark (lm > ). If the landmark is not directly
neighbored to either of the functionally relevant route segments, all we can say is
that a turning action is performed at the decision point with this particular landmark
(denoted by lm ).
One further restriction is illustrated in Fig. 5.12 . A landmark may be next to
the incoming route segment, but still only be passed after the turning action or
not directly passed at all (e.g., at a T-intersection). Therefore, we need to further
restrict the neighborhood region for incoming and outgoing route segment, such that
only the area before or after the turning action, respectively, is considered. Again,
this is solved by introducing virtual branches demarcating these regions. These
virtual branches start at the decision point and are perpendicular to the incoming
(or outgoing) route segment. We call the area next to the incoming route segment
before-region , and the area next to the outgoing route segment after-region [ 40 ] .
Using this combination of virtual branches and reasoning with ordering informa-
tion we can determine where a point landmark is located relative to the route and,
consequently, what its functional role is in terms of the wayfinding process (see
Chap. 6 ) . However, as stated above, landmarks are not always represented as points.
They may also be extended objects, i.e., a line or polygon. Richter's approach can
be extended to handle these objects as well in a straightforward manner [ 36 ] .