Computational Aspects: How Landmarks Can Be Observed, Stored, and Analysed - Landmarks: GIScience for Intelligent Services - page 152

a b c

Fig. 5.12 Virtual branches demarcating before- and after-region are needed to correctly distin-
guish passing a landmark before or after the turning action. ( a ) An example case where a landmark
is next to the incoming route segment, but actually not directly passed during the turning action;
( b ) the before-region for this situation; ( c ) the after-region

lm - lm -

lm <

lm <

Fig. 5.13 Example of the relation of an extended landmark to the route. For each coordinate,
its relation to the route is determined. Since different relations hold, the overall relation is lm
(from [ 36 ] , modified)

Given that these landmarks are represented by a sequence of coordinates, for
each of these coordinates their relation to the decision point is determined (either
lm < , lm > ,orlm ). If for every coordinate the relation is lm < (lm > ), the landmark
is passed before (after) the turning action and we can assign the relation lm < (lm > )
to the landmark as a whole. In all other cases, i.e., if there is a mix between lm <
and lm > , or the relation is lm for some or all coordinates, then the landmark is not
completely located in either the before- or after-region of the decision point. In this
case, the relation lm should be assigned indicating that the actual location of the
landmark is difficult to communicate precisely (and thus simply 'at' may be used to
indicate the presence of the landmark; see Fig. 5.13 ) .
Dale et al. [ 6 ] presented another approach integrating landmarks into route
directions. Their CORAL system produces natural language instructions for route
following, mimicking human principles of direction giving. It will be further dis-