- The document describes a new location-based service called terraVX that uses a 3D model of the landscape to determine which landscape features are visible from a user's location and prioritizes providing information about those features. - Existing location-based services use a 2D model and can only provide information about nearby points of interest, but terraVX uses a 3D analysis of visibility to match how people naturally orient themselves using sight. - A prototype of terraVX has been developed for heritage tourism sites, allowing visitors to get information about landscape features visible from their current location tailored to their interests.

1. 3D Intervisibility: The Key to Taking the Wondering out
of Wandering
Neil Stuarta
Fraser MacLeodb
a
School of Geosciences
The University of Edinburgh, Scotland
ns@geo.ed.ac.uk
b
terraVista ltd
Scotland
fraserm@terravistaltd.com
Abstract
The increasing choice of location-based service (LBS) solutions available to the tourist
replicate the traditional map and guidebook paradigm, using a two dimensional geometric
model of the landscape. We argue that this does not reflect the instinctive ways by which most
people orientate themselves to features of interest within their environs, or navigate between
them. To overcome the shortcomings of existing LBS, the terraVX(TM)
system uses a true three
dimensional data model, allowing the delivery of information to be prioritised on the basis of
whether a feature is visible rather than whether it is simply nearby. The 3-D representation
within terraVX(TM)
can also deliver navigational guidance across open terrain irrespective of
whether a network of roads or tracks exists. We discuss how the extended functionality offered
by the terraVX(TM)
system design particularly suits the queries posed by tourists and illustrate
this using a prototype database generated for visitors to a heritage tourism site.
Keywords: location based services; terrain; intervisibility; heritage tourism.
1 Background
From wondering what we can see in the distance to identifying landmarks to help us
navigate we all have an innate curiosity about our surrounding landscape. This
curiosity, or Naive Geography “... is probably the most common and basic form of
human intelligence” (Egenhofer and Frank 1995:4). It is an instinctive skill that we
use daily, whether in a familiar geographic space or as a visitor to an unfamiliar
space. Using “... our senses – mainly our sight ...” (Freundschuh and Egenhofer
1997:362) and our experience we build personal models and cognitive maps to help
orientate ourselves and navigate using features of interest within our immediate
surroundings (Brown and Chalmers 2003, Jonsson 2002, Freundschuh and Egenhofer
1997).

2. In unfamiliar surroundings, we seek to enhance the detail of our cognitive map by
adding information from other sources about both the layout of the landscape and the
identity of key features within it. Traditionally this information would come from
maps, guidebooks, and other published literature. Increasingly, alternative sources of
information are now available from hand held GPS receivers, to electronic
guidebooks, and the Internet. These technological developments are converging into
the market sector that we broadly call LBS.
“There is a general consensus ... that LBS will be amongst the most widely used
mobile services ...” (Beinat 2001:6), with estimates suggesting a market of $20 and
$40bn by 2006 (US Department of Commerce 1998). Given this potential it is
unsurprising that there is a growing choice of LBS products available to the
consumer. In the tourism market several LBS have been developed recently that target
specific tourism locations, such as WebPark (Krug et al 2003), Da-Mobil (Jasnoch
2003), ReGeo (Frech and Koch 2002), TellMaris (Coors et al 2002), CRUMPET
(Schmidt-Belz at al 2002), PARAMOUNT (Loehnert 2002), comMotion (Marmasse
and Scmandt (2000), and GUIDE (Cheverst et al 2000).
Beinat (2001) identifies three classes of LBS: Information services to find nearby
facilities, Interaction services for communication between user groups, and Mobility
services to help users to navigate from A to B. However, we argue that present LBS
do not offer the tourist an effective means to orientate themselves to the landscape,
identify features of interest, or navigate to them. This is partly because of the
limitations in functionality that result from systems based on a two-dimensional
model of the landscape. Further, our instinctive dependence on sight in conceiving
geographic spaces suggests there is an additional overhead of relating a 2D map view
to to the 3D world that surrounds us. We argue that improvements are now required in
LBS design to provide “... a better match with people's thinking” Freundschuh and
Egenhofer (1997).
Despite the potential for LBS in tourism, empirical evidence supports the need for
design innovation. A recent study of tourists found that “we did not observe any use
of digital maps or guidebooks in our observations” (Brown and Chalmers 2003:9).
Many tourists found the hard copy guide book a more accessible solution, leading to
the conclusion that LBS need to offer further benefits before they will be widely
adopted. “Simply copying content into an eBook is unlikely to be successful” (Brown
and Chalmers 2003:16)
Using a 2D landscape model as a basis, existing LBS services can only provide
information on points of interest that are close to the user in terms of distance or time
(Beinat 2001). By contrast, the user's ability to identify landmarks and thus to orient
themselves and navigate is based primarily on what is visible from their present
location. A second limitation that follows from the use of a 2D representation of
landscape is that navigational advice is restricted to route finding along existing
networks of paths and lines (Laurini and Thompson 1992); by contrast, particularly in

3. rural areas the user may need to be guided using any reasonable route across the
landscape, not just where a path exists.
These limitations with existing LBS solutions have recently been recognised
particularly for services to the tourism sector, leading to the exploration of alternative
spatial representations such as 3D landscape visualisation in products such as
TellMaris (Coors et al 2002), and Da-Mobil (Jasnoch 2003). While 3D visualisation
can provide a more intuitive means for tourists to orientate themselves to landscape
features and to identify features of interest, the challenges of the intensive processing
required to generate landscape visualisations and to reduce the volume of data when
these are transmitted to mobile devices remain. Until these challenges are overcome,
interactive visualisation of landscape on mobile devices is not a practical proposition.
The terraVX system also goes beyond existing LBS solutions by using a true 3D
geometric data model. Rather than generating views of the terrain from the data we
adopt an analytical approach to determine which features of the landscape should be
visible from the tourist's present position. Further analysis of the 3D terrain model
can then be used to generate navigational advice across the terrain, irrespective of
whether a network of roads exists.
2 terraVX System Design
The terraVX system is made up of six distinct elements: user, session, service,
features, intervisibility, and guidance subsystems. Firstly, the user subsystem provides
a facility to create user accounts. Each account has an associated user profile where
language preferences, themes of interest, and the type of mobile device are recorded.
In addition, the user profile also records location bookmarks for later reference. This
latter function meets the needs for post-visit functionality identified by Brown and
Chalmers (2003).
A user will subscribe to a service package that gives access to varying levels of detail
on Features of Interest (FOI). It is possible to think of each service being associated
with a content provider. For example it would be possible to offer a Lonely Planet
service, a National Trust for Scotland service, or a ski resort service. By using the
ownership of the actual content as a filter it is possible to restrict access to the detailed
media records. In addition, there is a default service offering that will give access to
FOI names and limited information as a taster for the user. Each service package also
has the option to set charging schema, whether by connection time, subscription,
download volumes, etc.
A FOI is any feature within the landscape that is capable of being geo-referenced, and
is of interest to the potential service subscriber. Thus, a FOI could be the city of
Edinburgh, a historic battlefield, the location of a rare plant species, a shop, etc. For
each FOI a set of media records (text, audio, graphic, or video), and their associated
ownership data, is added to provide detailed information. Every media record is also

4. classified according to theme, allowing the same FOI to be described by historical,
topographical, or cultural media records.
The terraVX system also provides scope to add temporal constraints to each FOI or
media record. Thus, if a FOI or media record is only active for a limited period this
can be recorded and used by the system to prioritise the delivery of media records to
the user. For example, bird nesting is limited to a few weeks of the year and this FOI
could be made active only during this period. Similarly, a photograph taken during
the winter months could be suppressed during the summer months. A further feature
of the terraVX data model is the ability to group FOI together into a route. It is
important to note that the ordering of FOI within this route is not spatial, and its intent
is not to provide navigation between FOI. Rather, it is designed to allow a user to
follow a story that is made up of a series of FOI. For example, following the events of
a historic battle would involve a chronological ordering of events rather than the
spatial distribution of events. The system also has the flexibility to allow messages
relating to FOI to be broadcast to users. This might include messages to indicate that a
FOI is currently out of bounds.
When a user logs into terraVX they create a user session that tracks their current
location, location history, and information on the media that the user has viewed and
has yet to view. It is within this subsystem that terraVX applies a series of rules to
determine what data to push to the user at any time. The system gives higher priority
to FOI that are visible, close to the user, and close to their direction of travel. In
addition, the system will prioritise the delivery of media records that meet the user
preferences. The user session subsystem can also generate valuable management
reports for content providers in terms of what information is of interest to users, and
where. Finally, users can record feedback on a location for browsing by other tourists
(Brown and Chalmers 2003).
The most novel functionality of the terraVX system is within the intervisibility and
guidance subsystems. It is here that the system differs dramatically from existing
solutions, in that it uses a true 3D model for representing the landscape surrounding
the user. Digital terrain data and digital topographic data on surface features that will
impair visibility, are brought together to create a composite digital landscape model.
Using this model the system is able to rapidly determine the visibility of a feature
from a given viewpoint.
The calculation of the visible areas of the terrain, or viewshed, from the user's
location can be achieved using a number of variant methods (Fisher 1993). However,
the viewshed will vary depending on factors such as the height of the observer, the
attenuation of light in the atmosphere, and the resolution of the underlying digital
terrain model. Despite these uncertainties, the general approach to determining the
viewshed of a point is well established in geographic information science (Worboys
1995:163). By contrast there is a much more comprehensive, and less well explored
problem in determining all locations that are visible from each and every point where

5. a user might locate themselves. It is here that terraVX deploys an innovative solution,
using a modification of the intervisibility algorithm described by Mineter et al (2003).
The intervisibility analysis results in a 'CanSee' table that allows real-time
determination of all FOI that are probably visible from the user's present location and
also a list of features that are probably not visible, but within a specified range of their
location. The three dimensional data model and digital topographic data are also used
to generate a 'GuideRoute' table that terraVX uses to suggest a route to the currently
active FOI from the present user location, based on a set of rules relating to terrain
slope, ground cover, barriers to travel and existing transport routes.
To make terraVX accessible to the widest range of users, it has been built using
standard technologies and protocols. Thus, any location-enabled device that can run
an Internet browser is capable of accessing the system. This will allow the system to
be embedded within Internet travel portals to assist with pre-visit planning, and also
for post-visit enjoyment from the home (Brown and Chalmers 2003). For real-time
access at tourist sites, a user will need a location-enabled mobile device, preferably
with integrated GPS, connected to a mobile network. When it is not possible or
desirable to access the terraVX server across a network, the key database elements
required to determine visibility and descriptive content about features can be
downloaded to a GPS-enabled PDA.
The use of a 3D landscape representation, from which visibility of features and
navigation routes can be determined allows the terraVX system to provide enhanced
functionality to the tourist. As well as resolving queries typically made to existing
LBS, the design of the system is particularly suited to resolving queries made by
tourists. Tourists entering unfamiliar surroundings have a demand for information that
they can use to orient themselves; this is usually accomplished by identifying
prominent features that are visible from their present position. When entering a site
for the first time, initial requests for information can be very broad ranging,
equivalent to browsing the site to determine what kinds of features are available for a
potential visit. The large volume of information available for supply requires some
means of prioritisation so that the most pertinent information is most accessible to the
user. terraVX prioritises features using sophisticated criteria involving visibility,
direction of travel and accessibility, as well as more conventional criteria used by
other LBS of proximity and themes of interest. The system also needs to respond to
event-driven queries. For example, when a feature of interest comes into view for the
first time, this often generates a request to identify it. terraVX frequently refreshes the
priority that features are given, as a tourist explores a site. Because the system design
matches closely to these types of requests for information, based on the way that
tourists explore areas, we believe that terraVX has the potential to revolutionise the
delivery of location related information for tourists.
During the design phase of terraVX steps have been taken to ensure that the system
can operate in a site specific mode, or in a more pervasive mode. In particular the

6. terraVX architecture places the main processing overhead on the server, resulting in a
very small footprint on the client device. Thus, the service can be made available on
any mobile device that is location enabled, and has access to a mobile
telecommunications network.
It is recognised that there are many technical challenges in building a pervasive
terraVX system. Not least of these will be the processing of the 3D data, building and
maintaining pervasive FOI databases, and managing the service for the benefit of
higher user volumes. However, we suggest that the technical issue of scalability,
including concerns about data volume, storage, access and distribution of spatial data
as a web service, is being assisted by the converging technologies of LBS, GPS, high
performance computing and spatial data processing. The conceptual design of the
terraVX system is scalable and not inherently restricted to the scale of the existing
trial project areas.
3 Heritage Tourism Example
To illustrate the concept a prototype terraVX database is being developed in
partnership with the National Trust for Scotland who are responsible for running a
large number of tourist visitor attractions. Two sites were chosen for evaluation: St
Abb's Head National Nature Reserve and the Bannockburn Heritage Centre. St Abbs
is a coastal site with many important historical features and nationally important
populations of seabirds. By comparison Bannockburn is a historic battlefield close to
the city of Stirling.
A visitor enters the site and hires a terraVX device for use during their visit. They
leave the visitor centre, switch on the mobile device and allow the system to initialise
itself. Once satellites have been acquired, and the connection made to the terraVX
server the system is ready for use. The first step is for the user to specify their
preferences in terms of language, thematic interest, media preference, etc. Once this is
complete the visitor receives their first prioritised list of FOI visible from their present
location.
The PDA screen is split to show a map highlighting the visitor's location, and the
relative location of the visible FOI (see Fig. 1). In addition the screen displays those
FOI that are not visible, but within 1km of the visitor. Without any further action the
visitor will be presented with media records relating to the first FOI in the list. For the
presently active FOI the user is also given direction, distance, and height above line of
sight to assist in identifying the FOI in the surrounding landscape. As the visitor
moves through the site the system updates their location every few seconds, and
submits this to the server. In response the server regenerates the prioritised list of FOI
and pushes it to the mobile device.

7. Fig. 1. Illustration of terraVX interface
During the visit the Ranger broadcasts a message indicating that a rare species of bird
has been spotted. The system pushes a message only to those users who are within
1km of the bird location, to give them the opportunity to move to where the bird has
been spotted. Some time afterward the visitors have found a particularly spectacular
viewpoint, from where the list of FOI and media records is very large. However, the
weather is deteriorating so they bookmark the location, allowing them to browse the
media records at a later time. Throughout, the system avoids repetition and potential
boredom by ensuring that the users do not see the same media record again, unless
they so choose.
This scenario illustrates how terraVX might operate in the context of a heritage site.
However, in addition to information for the visitor, the terraVX system can also
generate useful data for the site managers. These management reports might include
what FOI and themes visitors were interested in, where they have been, and how long
they spent within the site. In real time, the system can also broadcast a message to
users that a particular FOI is temporarily closed, or that a temporary FOI is available.
In this way the system can support the proactive management of visitors to the site to
meet strategic goals.

8. The experience of building a system for a heritage tourism site illustrates that terraVX
is becoming a catalyst for change in the way that heritage site interpretation will
develop. Presently, it is typical for all interpretative materials to be located within one
site or building, irrespective of whether they all relate to that specific location. For
example all the information relating to the Bannockburn site is presently held within
the Heritage Centre. Yet, elements of the data relate to locations that are distant from
the site, for example, the route of march of the English Army, or the Stirling Castle
garrison. All are distant from Bannockburn, but terraVX can deliver information to a
subscriber at any location from which any of these FOI are visible. Thus, the user will
be able to access interpretative data according to the spatial location of specific events
rather than by the physical location where that information is actually stored.
4 Conclusions
In this paper we have reviewed how a tourist interacts with their surroundings and
enhances their understanding of unfamiliar spaces primarily driven by their sense of
sight. We argue that the underlying geometric data models of existing location-based
information and mobility services do not adequately match the instinctive means by
which most tourists orient themselves to main features of the landscape, request
information about features and navigate between these. Existing LBS for tourists
provide information according to what is nearby, irrespective of whether or not it can
be seen from the tourist's present position and hence may fail to meet their needs for
the most pertinent information quickly. To overcome this failing we propose a new
LBS system, terraVX, that uses a true 3D geometric data model. This offers the
potential to deliver information considering first whether a feature of interest is
visible to the user, not just whether it is nearby. The design also opens the prospect of
supplying open terrain navigation, which can assist tourists especially in remoter
areas where the road or track network may be limited.
We have argued that this extended design for LBS is particularly suited to the needs
of tourists, who have a typically more diverse range of queries than most other users
of LBS. The visibility paradigm central to the design of the terraVX system enables
an efficient and intuitive method for presenting a prioritised list of features of interest
to the tourist as they explore a site and for responding to ad-hoc queries about features
that emerge into view for the first time. We illustrate how the terraVX system can
enhance the visitor experience using the example of a visit to a heritage tourism site,
and share our long term vision of the challenges involved in scaling up the system to a
pervasive service available across existing mobile networks.
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