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Structural Salience of Landmarks for Route Directions

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This paper complements landmark research with an approach to for- malize the structural salience of objects along routes. The aim is to automat- ically integrate salient objects—landmarks—into route directions. To this end, two directions of research are combined: the formalization of salience of objects and the conceptualization of wayfinding actions. We approach structural salience with some taxonomic considerations of point-like objects with respect to their positions along a route and detail the effects of different positions on the con- ceptualization process. The results are used to extend a formal language of route knowledge, the wayfinding choreme theory. This research contributes to a cogni- tive foundation for next generation navigation support and to the aim of formal- izing geosemantics.
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Structural Salience of Landmarks for Route Directions
Alexander Klippel1and Stephan Winter2
1Cooperative Research Centre for Spatial Information
Department of Geomatics, The University of Melbourne, Australia
aklippel@unimelb.edu.au
2Department of Geomatics, The University of Melbourne, Australia
winter@unimelb.edu.au
Abstract. This paper complements landmark research with an approach to for-
malize the structural salience of objects along routes. The aim is to automat-
ically integrate salient objects—landmarks—into route directions. To this end,
two directions of research are combined: the formalization of salience of objects
and the conceptualization of wayfinding actions. We approach structural salience
with some taxonomic considerations of point-like objects with respect to their
positions along a route and detail the effects of different positions on the con-
ceptualization process. The results are used to extend a formal language of route
knowledge, the wayfinding choreme theory. This research contributes to a cogni-
tive foundation for next generation navigation support and to the aim of formal-
izing geosemantics.
1 Introduction
This paper investigates the structural salience of objects along routes. The structural
qualities considered are induced by embedding the route into a street network. Objects
are called structurally salient if their location is cognitively or linguistically easy to
conceptualize in route directions.
A generic and formal method of assessing the structural salience of objects with
the goal of finding a landmark selection process for route directions is proposed. Such
a measure of structural salience complements a formal model of salience recently de-
veloped (see, e.g., [1,2]), building on an earlier characterization of the nature of land-
marks [3]. This model assumes visual, semantic and structural qualities of objects that
contribute to their salience. Measures for structural qualities have been left out of this
model so far, a gap which will be filled by this paper.
The proposed method in this paper utilizes a conceptual approach to spatial infor-
mation exemplified by route direction elements (e.g., [4,5]), and extends the wayfind-
ing choreme approach, i.e., a formal language for the specification of conceptual route
knowledge, for the inclusion of salient features. This paper classifies structural aspects
of landmarks, especially their location with respect to the re-orientation actions per-
formed at the nodes of the underlying street network, which are frequently called deci-
sion points.
The linguistic complexity of characterizing the relative location of a landmark is dis-
cussed in relation to the conceptual complexity of realizing its wayfinding affordance.
Klippel, A., & Winter, S. (2005). Structural salience of landmarks for route directions. In A. 
G. Cohn & D. M. Mark (Eds.), Spatial Information Theory. International Conference, 
COSIT 2005, Ellicottville, NY, USA, September 14-18, 2005, Proceedings (pp. 347-362). 
LNCS 3693. Berlin: Springer. (pls quote original)
The hypothesis in this paper is that the conceptual complexity influences the selection
of a landmark by direction givers, and hence, the salience of this landmark.
The described approach of the paper brings together two lines of research that
have been unrelated so far: the formalization of salience and the conceptualization of
wayfinding actions. The paper is organized as follows: We start by reviewing work on
landmarks and conceptualization processes relevant for interactions with spatial envi-
ronments. For the conceptualization of landmarks as route elements we start by in-
stantiating a taxonomy that classifies the location of landmarks with respect to routes.
Building on this general classification scheme, we detail the different possibilities of
locating landmarks at decision points. We investigate the conceptual complexity of us-
ing different types of landmarks, and derive a measure of structural salience from their
ease of use. This measure can be combined with the existing measures for visual and
semantic salience [1]. We briefly discuss the extension of the wayfinding choreme the-
ory and the formalization of conceptual route knowledge by relying on the developed
taxonomy, and conclude with a discussion.
2 Landmark research and conceptual approaches to spatial
information
Since the work by Lynch [6] on elements that structure our urban environmental knowl-
edge, the concept of landmarks has inspired multiple research papers. There are some
simple and straightforward facts that can be manifested from this research:
Everything that stands out from a scene can be a landmark [7].
In certain contexts, for example route following, even road intersections can be
landmarks.
Landmarks are pertinent for finding one’s way.
Landmarks are remembered/learned early on (i.e., landmark knowledge) [8].
Landmarks structure environmental knowledge, for example, as anchor points [9].
Landmarks are used to communicate route knowledge verbally and graphically
[10].
Landmarks are integrated in route directions to varying degrees, with greater quan-
tities at origins, destinations and distinguished decision points [11,12].
Landmarks at street intersections (decision points) are more pertinent when a change
in direction is required [13].
Landmarks generally work better than street signs in wayfinding (e.g., [14]).
While we understand how and why, people use landmarks in communication, and
hence in memory and mental processes, the technical basis to automatically construct
wayfinding directions with landmarks is still limited. Suitable formalisms for character-
izing conceptual route knowledge that can be flexibly adapted to canonical and personal
preferences are still missing.
Part of the problem is formalizing the concept of a landmark, such that a service
can identify objects of some landmarkness or salience, i.e., objects that differ from
their background [7]. According to a recent proposal [1,2, 15] the salience of objects
is determined by visual, semantic and structural qualities. These qualities can be char-
acterized to provide an overall measure of salience. The approach proposes measures
for visual qualities (such as the size, form, or texture of an object), and semantic quali-
ties (such as their prominent or labelled use); measures for structural qualities have not
been included so far. Objects that show large visual and semantic salience show good
compliance with cognitively salient objects, i.e., landmarks chosen by people for their
communication of routes.
The measure of salience can be adapted to context [15], and can be weighted by
advance visibility along a route [16], which makes it route specific. The approach has
also been adopted for data mining in topographic data sets [17,18].
An approach orthogonal to the work on the salience of objects is taken by wayfind-
ing choreme theory [19, 4,20]. This approach, however, can provide the missing aspect
of structural salience. It is based on wayfinding events such as re-orientations and turns
at street intersections as primitives; conceptual primitives of turns are derived from this.
The conceptualization of actions and events3and their formal specification is a re-
cently well discussed area of research [21, 22]. Complementary to other computational
formalisms [23] the wayfinding choreme theory stresses the cognitive aspects of route
knowledge by making (cognitive) conceptual primitives the basis of the formalism. Yet,
the focus on conceptualization and the development of a formal language [4] offers
many ties to recent discussions on the formalization of conceptual spaces [24], e.g., for
landmarks [25], and the general approach of integrating cognitive semantics (sometimes
referred to as conceptual semantics) into information systems [26]. The wayfinding
choreme approach seeks conceptual primitives as a foundation for a formal language of
space in which the number of basic elements are restricted and the combinatorial pos-
sibilities are constraint by the represented knowledge, in our case, the linear character
of a route. Additionally, the focus on conceptual aspects of information creates a basis
for multimodality in that the externalization of conceptualizations can be specified in
various output formats (e.g., [12,27]): verbal, graphical or gesture.
The conceptualization of an action at a decision point, however, is not only depen-
dent on the angle of the turn, i.e., the geometric representation of the trajectory of the
movement, but also on the street structure in which the action is embedded [28]. Addi-
tionally, the possibility of relating the turning action to supplementary information—for
example a landmark—has an influence on the conceptualization.
A formal specification of conceptual primitives of landmark locations should there-
fore allow the characterization of different layers of interaction with the environment
and grasp the resulting conceptual primitive adequately. In this paper, we will further
elaborate the conceptual approach and focus on landmarks, specifically their structural
salience induced by the conceptualization of a wayfinding action. Based on this charac-
terization we will extend the rules specified for the higher order route direction elements
(HORDE) [29,30] to allow for different levels of granularity in route directions.
3In this article we do not distinguish between actions and events.
3 Conceptualizing landmarks as route elements
Some of the observed characteristics of landmarks discussed in Section 2 concern their
structural qualities with respect to a given route: the route structure (co-)determines
which salient objects are selected to give route directions. Hence, this paper develops a
classification schema for point-like landmarks depending on their location relative to a
route and the route’s structural embedding in the street network. For some ideas on other
types of landmarks and their relation to route directions see [20]. We further show that
some locations are conceptually easier and conceptually less ambiguous than others,
especially with regard to building complex route elements, i.e., combining conceptual
primitives into HORDE.
3.1 Landmark locations
The following taxonomy of landmarks induced by a route embedded into a street net-
work is illustrated in Figure 1. Landmarks can occur (i) distant from the route (distant
landmarks), (ii) somewhere along route segments (segment landmarks), or (iii) at spe-
cific route nodes (node landmarks). Segment landmarks and node landmarks can be
grouped as either close to or on-route. Route nodes are also called decision points.
Fig.1. A structural taxonomy of landmarks; the abbreviations are detailed in the text.
Distant and on-route landmarks. With respect to a route, these categories of land-
marks have different degreesof freedom regarding their location. A distant landmark is
not determined in its exact location in two dimensions. The conceptualization details an
area, a region in which the landmark is placed, or a line of sight. A segment landmark,
on the other hand, has its location determined only within the one-dimensional interval
between two route nodes; their exact location along the segment—the linear reference
from start or end node—remains under-specified. In contrast, node landmarks are con-
strained in their location by a node of the street network or decision point. With respect
to their function in route directions and the conceptualization of the action that has to
be performed, both their location and their remaining degrees of freedom in location
have to be reflected, for example, in the type of verbal reference.
Distant landmarks fulfill a variety of functions, for example, global orientation, re-
assurance and confirmation [31]. Their actual location (or distance) is irrelevant as long
as the direction or visibility can be taken as reference. Distance generally is not a crite-
rion for exclusion from route directions. Consider for example the route direction:
Follow the street until you see the castle distantly on your right. (1)
Yet, the effect of distant landmarks on the conceptualization of route parts, for ex-
ample, spatial chunking [32], is rather complex. Many parameters that influence the
conceptualization of these distant landmarks are not primarily spatial (at least not pla-
nar spatial). They are therefore not the focus of this paper.
Within the category of landmarks that are close to or on-route (cf. [33]) segment
landmarks [34] are used for reassurance and confirmation. The influence of segment
landmarks on chunking is discussed in Section 3.3. In contrast, node landmarks may be
used as an anchor for action (re-orientation and turning), and hence, their location with
respect to the route is relevant. But node landmarks can also occur at decision points
where no re-orientation is necessary. Within our taxonomy we write segment landmarks
as lm
, and node landmarks as lm
.
Route directions cannot neglect any necessary re-orientation, but they can neglect
confirmations that may occur either between decision points or at decision points where
no re-orientation is required. That means, node landmarks at decision points with re-
orientation are essential, and other landmarks are less important or optional. This char-
acterization establishes a first indication of structural salience.
Node landmarks. A further common distinction is made between decision points
with direction change, dp
, and decision points without direction change, dp
. This
distinction has to be accounted for in the taxonomy of node landmarks. It has been
shown that landmarks at dp
are more pertinent to route knowledge [11,13]. Within
our taxonomy we coin them lm
, and landmarks at dp
are coined lm
, such that
lm
={lm
, lm
}.
At a more detailed level of spatial granularity, it is of interest where with relation
to any decision point, dp
or dp
, a landmark is placed. Not every node landmark
is equally suited to aid wayfinding and to be integrated into route directions and the
conceptualization of route parts, respectively.
Note that this characterization is based on locational properties, i.e., the location of
a landmark with respect to the physical layout of an intersection. It is not a character-
ization based on the visual or semantic salience of a landmark. This characterization
is also a specification of the locational properties from the perspective of mental con-
ceptualization processes, i.e., the conceptualization of an action performed in a spatial
environment. In experimental settings (e.g., in [32]) it was made sure that at dp
pri-
marily those landmarks that can be integrated into a route direction conceptually easily
are used. This integration is afforded by the landmarks’ location with respect to the
action at the decision point. More specifically, landmarks at dp
were chosen that are
passed immediately before a turning decision. These node landmarks may be located
on the left or on the right side of the route, independent from the direction of the turn.
A natural language example would be:
Turn right after the post office. (2)
Based on the idea that the action performed (or imagined) at a decision point is the
pertinent factor for the conceptualization process, we introduce further sub-concepts for
landmarks, namely landmarks passed before the action is performed, lmb, landmarks
not directly passed, lmn, and landmarks passed after decision, lma(see Figure 1, and
also Figure 2 for more details). These concepts can be specified for decision points with
a direction change, but also for decision points without a direction change.
Fig.2. Possible locations of node landmarks with re-orientation. The different locations result in
different conceptualizations, and not every location of a landmark functions equally well as an
identifier for the required decision.
At dp
, landmarks passed before decision, lmb
, work equally well for all turning
concepts. That is, they are straightforward to conceptualize as the turning occurs im-
mediately after them and does not conflict with the overall branching structure of the
decision point. Compare the use of a lmb
:
Make a sharp right turn after the post office. (3)
with a lmn
or lma
:
Make a sharp right turn at the intersection where the post office is. (4)
which represents here a more precise, but also more complicated direction ‘make a
sharp right turn at the intersection where the post office is at the opposite corner’. Es-
pecially at more complex intersections, where it is difficult to conceptualize the location
of a landmark, a lmb
is the only unambiguously identifiable one.
3.2 A route direction grammar with node landmarks
Having a taxonomy for the location of landmarks with respect to a route, we can in-
tegrate them into the wayfinding choreme route grammar [4]. Generally, two turning
concepts have been differentiated, standard turning concepts hSTCiand modified turn-
ing concepts hMTCi[4]. Both can be extended to incorporate the different landmark
locations. To this end, the node landmark and its location is added to the wayfinding
choreme grammar as an annotation. We exemplarily detail the notation for the wayfind-
ing choreme of a right turn, wcr, added with a landmark passed before the decision,
lmb
:
hwclmb
ri(5)
Likewise for other turning concepts.
3.3 Spatial chunking with node landmarks
This section exemplifies the influence of structural aspects of landmark positions within
yet another aspect of route directions, the change in granularity by applying higher
order route direction elements (HORDE). We discuss here the possibilities of chunking
with node landmarks at decision points without a direction change, lm
preceding a
dp
.lm
have two functions: First, they are used to identify a decision point resulting
in a verbalization such as ‘go straight at the intersection where the McDonald’s is’.
Second, they are used in a way analogous to a segment landmark lm
, such as ‘pass
the McDonald’s and turn right at the Shell gas station’. Here it is not specified whether
the lm
is placed at a decision point or between two decision points. The second case
might be an example of spatial chunking.
Two distinctions are pertinent for lm
that determine their function in spatial chunk-
ing. First, whether the landmark is passed before (lmb
) or after (lma
) the action of
straight crossing the intersection. Second, whether a landmark is present at the chunk
terminating dp
ahead. We observe that lm
only appear within small chunks (similar
to segment landmarks lm
) if at least one and possibly both of the following condi-
tions are met: (i) the lm
is of the type lma
; (ii) there is an easy to conceptualize node
landmark at the chunk terminating dp
. Figure 3 illustrates these assumptions. The fol-
lowing cases demonstrate some rules that can be distinguished and integrated into the
wayfinding choreme route grammar:
Consider the first example in Figure 3. lm
is passed before orientation: lmb
,
and at the chunk terminating dp
there is a landmark. The resulting concept is:
PASS FIRST LANDMARKAND TURN RIGHT AT SECOND LANDMARK’. This
concept is over-specified when only two decision points are present; the first land-
mark should be left out, even if it is the somewhat more salient one.
Consider the second example in Figure 3. lm
is passed before re-orientation: lmb
,
but no landmark is present at the chunk terminating dp
. This situation has to be
put in a less specific form: AFTER THE INTERSECTION WHERE THE LANDMARK
IS TURN RIGHT AT THE NEXT INTERSECTION, or alternatively, in a more complex
concept: AFTER THE INTERSECTION WHERE THE LANDMARKIS AT THE RIGHT
CORNER TURN RIGHT AT THE NEXT INTERSECTION.
Now turn to the third example in Figure 3. lm
is passed after re-orientation: lma
,
and a landmark is present at the corresponding dp
, for example, a lmb
. The re-
sulting concept is similar to the first case: PASS FIRST LANDMARKAND TURN
RIGHT AT SECOND LANDMARK’. When only two decision points are involved,
the first landmark is left out.
Finally, consider the fourth example in Figure 3. lm
is passed after re-orientation:
lma
, but no landmark is present at the corresponding dp
. Here, lm
is used
similarly to a lm
:TAKE A RIGHT TURN AFTER LANDMARK’.
Fig.3. Landmarks at a decision point without a direction change, and their influence on spatial
chunking in examples 1-4.
So far, distance has not been considered. There are, however,similarities to another
approach to integrate distance in term rewriting as a method to extract conceptually
connected primitives [35]. A more detailed characterization of spatial situations to dif-
ferentiate, for example, the following two concepts is ongoing research: (a) MAKE A
RIGHT AND THEN ANOTHER QUICK RIGHT, versus (b) MAKE A U-TURN.
4 Structural salience of landmarks
The previous sections discussed the conceptual approach to characterize the positions
of landmarks with respect to their relevance for route directions. This section extends
the salience model for landmarks to include structural aspects derived from the findings
above.
4.1 The salience model
The salience model [1, 2] provides a measure of salience for all identified objects within
a street network. These measures enable choosing the most salient objects along a spe-
cific route, for example, at decision points with direction change, to enrich route direc-
tions. For static objects the measures can be calculated once, and stored as parameters
of the objects.
The original model of salience was based on three qualities: visual, semantic and
structural [3]. Each quality can be characterized by a normed measure of salience
(with values 0. . . 1), resulting in visual salience sv, semantic salience ss, and structural
salience su. These measures can be combined to a weighted average of joint salience,
so:
so=wvsv+wsss+wusuwith wv+ws+wu= 1 (6)
So far, structural salience has been considered in the model, but it was not developed.
Saliences svand ssare determined by comparing visual and semantic properties of
objects with the properties of other objects in their neighborhood. The more distinct a
property of an object is, in its neighborhood, the higher the object’s salience measure
is. By this way, the figure-ground relation mentioned in Section 2 is quantified. This
means:
Visual and semantic salience is dependent on the properties of objects nearby; it is
a relative property of an object, not an absolute one. For instance, a red facade in
an area where all facades are red will not stand out. But the same facade in a grey
neighborhood stands out.
Joint salience is quantitative, i.e., it can be represented by a real number between 0
and 1. It is not qualitative (e.g., ‘landmark’, ‘no landmark’), as supposed so far in
the previous sections.
It was shown further that weighting individual visual and semantic criteria differently
allows for adapting the salience measure to different wayfinding contexts [15].
This original model of salience was investigated for one class of objects: facades
of buildings. Consider for example Figure 4, which shows eight street facadesof build-
ings at a street intersection, and their visual and semantic salience represented by grey
intensity. According to the original salience model we would choose the object of high-
est salience (in Figure 4, one of the two facades of building c) as a landmark for route
directions.
Fig.4. A street intersection with eight street facades; each individual facade has a salience indi-
cated by grey intensity.
4.2 Advance visibility
Visual and semantic salience characterize properties of objects that are independent
from routes and the street network. Route dependent properties of objects can be con-
sidered additionally. Each object can be related to an infinite number of routes, but lo-
cally the number of combinations, i.e., the number of approaching directions, is small.
This means local route-dependent properties can be represented by a small number of
fixed parameters. This number is rarely larger than four; extreme cases, such as the
Arc d’Triomphe in Paris, extend already the concept of an intersection to a large circle
which can be considered as comprising of several intersections.
The salience model can be extended by advance visibility [16]. This measure char-
acterizes the visibility of the object from an approaching direction, and hence, is differ-
ent for each approaching direction. The rationale for this additional component is that
the most salient object at a decision point can form a poor reference in a route direction
if it is not visible in advance but an alternative salient object is. Thus advance visibil-
ity sahas to be balanced with joint salience to characterize total salience st, e.g., by
multiplying the two measures:
st=sosa(7)
Note that sais also a normed value between 0 and 1. Multiplication favors objects that
are at the same time at least to some extent jointly salient and to some extent visible
in advance, compared to salient but not visible, or visible but not salient objects. This
behavior seems to be reasonable.
Consider for example Figure 4 again. The four facades facing towards the street
the wayfinder is approaching are all visible to the wayfinder, but from the four facades
facing the cross-road two are only partially visible (4(a) and (b)), and two are not at all
visible (4(c) and (d)). Hence, the total salience stis largest for the facade of the building
cthat faces towards the street the wayfinder approaches. This facade should be used for
route directions if we consider visual and semantic salience and advance visibility.
At this stage, we have a model that ranks objects by salience and advance visibility,
but remains indifferent to the structural characteristics of the relation between objects
and street network, or to the relation between objects and routes. However, in Section 3
we saw that the relationships between landmark, route and street network influence the
selection process of landmarks. The integration of the structural properties of objects in
the salience model still needs to be done. This means we have to develop
normed salience measures for the identified structural properties of objects (su);
an adaptation of higher order route direction elements (HORDE) for quantitative
measures of landmarkness.
4.3 Structural salience
The discussion of structural properties of landmarks in Section 3 showed that
1. structural properties of objects co-determine their suitability as a landmark;
2. structural properties are, if not quantitative, at least ordered, such that a specific
weight of at least an ordered scale can be attached to each situation;
3. structural properties are determined by the structure of the underlying street net-
work, and locally route dependent, which means they are countable and constant.
The set of weights should reflect the hierarchy of Figure 1, and the distinctions of Fig-
ures 2 and 3. The order reflected in these figures is motivated by the previous dis-
cussions, and partially validated by cognitive, behavioral or linguistic experiments.
Presently, we convert the ordered scale measures of structural salience into ratio scale
by matching an equally partitioned interval from 0 to 1. However, determining more re-
alistic ratio scale weights needs careful human subject testing and is beyond the scope
of this paper.
The third aspect—countable and constant measures—means that the measures can
be stored as properties with each object. They are route- and street network dependent,
and hence their combinatorial complexity is higher than, for example, for advance vis-
ibility. For node landmarks, for example, the complexity is n(n1), with nbeing the
node degree of the street intersection, because the structure requires consideration of
not only the incoming direction, but also the one going off. Note that this includes the
distinction between node landmarks with re-orientation, lm
, and landmarks without,
lm
. This means with street intersection degrees of rarely larger than four the com-
plexity is rarely larger than twelve.
The measure of structural salience can be integrated into the original model of
salience (Eq. 6). It is still one of three components that add up, i.e., an object is salient
if it is visually, semantically or structurally distinct.
If we survey people for measures of structural salience with the one-dimensional
configurational relationships, the results might be mixed up with expectations of ad-
vance visibility. However, in the motivation for structural distinctions we only argued
for cognitive and linguistic simplicity. Hence, advance visibility is different and remains
a component of total salience (Eq. 7).
For an illustration, consider the situation in Figure 5. The situation shows a route at a
decision point with direction change, and some facades with their measures of salience.
According to the discussion in Section 3, landmarks at decision points are more perti-
nent than those along route segments, and at decision points distinctions can again be
made in relation to the action (here: turning right). The structural salience measures for
the given facades reflect this hierarchy. Note that the structural salience measures recur
for buildings (as point-like landmarks), not for facades individually. Advance visibility
is assumed to be equal for all facades facing the street the wayfinder approaches to the
decision point, less for the facade on the cross-road facing the advancing wayfinder,
and zero (not visible in advance) for the facade on the cross-road facing away from the
advancing wayfinder.
Fig.5. A route at a decision point with direction change, and measures of salience for some
facades (svs: visual and semantic salience, su: structural salience, sa: advance visibility).
With Equation 7 we derive the (route- and network dependent) measures of total
salience for the considered facades shown in Figure 6. In the given spatial configuration,
and for the given visual and semantic salience, the facade with st= 0.72 is the most
salient one. This is particularly interesting as it is not the most visually or semantically
salient one. Hence taking into consideration the route- and street network dependent
aspects can change the priorities significantly, a behavior that was sought for.
4.4 Selection process in HORDE
The original salience model did provide a comparison between objects, but did not look
into selection. It still assumed a superordinate selection process that exploits salience
measures to select references to salient objects where needed. In contrast, structural
salience prioritizes visually and semantically salient objects at specific locations along
a route. It establishes a selection process by weighting objects between decision points
against objects at decision points and so on.
Compared to the discussion in Section 3.3 the situation at this stage has changed.
Objects along the route now have more or less salience, and are no longer categorically
considered as ‘landmarks’. The measures of salience along a route form a distribution,
Fig.6. The total salience stfor the facades.
which can further support selection. Let us study the distribution of values with two
examples:
Imagine a route through a green suburb of one-family houses. Salience measures
of the objects (facades) along the route differ slightly, but no object stands sig-
nificantly out. The distribution of salience measures has a small variance and no
outliers. Selecting the most salient object along a route segment is possible, but not
really helpful.
Imagine a route along Vienna’s Ringstrasse. There are frequent salient objects (the
parliament, the Burgtheater, the city hall, the university, the stock exchange, and
so on), and the measures of salience vary largely. The wayfinder is attracted, and if
the route description only indicates to ‘walk straight to the Danube’ and does not
mention the attractions, she might feel uncomfortable and wonder if she is on the
right track4.
In other words, in an environment with one or a few outstanding object(s) these objects
can be used as ‘landmarks’ in the categorical sense of Section 3.3. In an environment
with no outstanding objects it is better to refer to other (structural) properties, such as
the number of intersections. The appropriate method to distinguish these two cases is
outlier detection, i.e., basing the decision on the standard deviation of salience in that
environment, not on an absolute threshold value.
An object with a large salience has probably, but not necessarily, structural salience
as well. This means that objects with large salience measures have a high probability of
being at decision points dp
, or in another salient structural relationships to the route.
5 Conclusions and outlook
In this paper we have combined two approaches on formalizing route knowledge rel-
evant to the selection of landmarks and for integrating them into route directions: on
4An option currently investigated relies on recursion to higher levels of abstraction, such as
‘walk straight along the attractive Ringstrasse to the Danube’.
the one hand the salience of landmarks as dominant objects in route knowledge and
route directions, and on the other hand the conceptualization of wayfinding actions in
relation to landmarks, i.e., the integration of landmarks in the formal specification of a
conceptual route language, the wayfinding choreme theory.
Both approaches on their own are well established and the combination of them re-
sults in efficient formalisms addressing several unsolved research questions. Combined,
they allow for the specification of structural salience and will complement the basis for
an automatic, cognitive adequate generation of route directions in wayfinding assistance
systems.
The approach of defining the structural salience of landmarks through the applica-
tion of a conceptual approach also offers answers to research questions in the area of
geosemantics; especially, their formalization, standardization and automatization, for
example, for mobile navigation systems. The application of conceptual (cognitive) se-
mantics for geographic information science has recently gained attention through re-
search on ontologies [26].
Other approaches that aim to formally characterize spatial structures have to be
considered in greater detail. Especially the approach of space syntax provides several
concepts that relate to the topics discussed in this paper [36,37].
With the precisiation of location of landmarks at intersection the next step in the
formalization of conceptual knowledge, especially with respect to different modi of ex-
ternalization, has been achieved. The dual approach of a generic concept that in general
specifies the presence of a landmark and the possibility of a more detailed analysis of-
fers a means to model different levels of granularity in route directions. It also offers
a means to contribute the structural salience to models finding salient features by data
mining in text documents [38,39].
6 Acknowledgements
This work has been supported by the Cooperative Research Centre for Spatial Infor-
mation, whose activities are funded by the Australian Commonwealth’s Cooperative
Research Centres Programme. We would like to thank Kai-Florian Richter, Bremen,
who saw an earlier version, and the anonymous reviewers for invaluable comments.
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