from map to model: the development of an urban information system

From map to model: the development of an urban information system

Alan Day, Centre for Advanced Studies in Architecture, University of

Bath, Bath BA2 7AY, UK

The use of three-dimensional computer models for urban planning and design is discussed with particular reference to a recently completed model

of the City of Bath. Problems in making such models generally available are identified and a solution, which is particularly appropriate for

nonexpert users, is proposed.

Keywords: planning, computer-aided design, urban models

1 Enlot, d The City in Maps: Urban Mapping to 1900, The Brit- ish Library, London (1987)

1 The utility of maps

T he characteristics of the contemporary map are so familiar that

they can easily be taken for granted. The map is a symbolic

representation of the world drawn to an accurate scale, using a set

of uniform symbols. A high degree of abstraction is employed in order to

reduce the amount of information to manageable proportions and this is

then presented in a structured way which allows the map to be read easily.

However, maps, and in particular urban maps, were not always presented

in this way. Early maps, particularly during the medieval period, were

aerial views which showed the geometric form and appearance of the city

as well as its street layout (Figure 1). Accuracy was often sacrificed in

order to ensure that all the finest buildings and monuments were shown

and the artistic and symbolic qualities of the work were at least as

important as its objective status 1. The introduction of improved surveying

techniques in the Renaissance facilitated the production of increasingly

accurate maps and these new maps, which presented the city as a plan

projection, superseded the aerial views. As they were increasingly used

for military purposes and to record land transactions, accuracy became

366 0142-694X/94/030366-19 © 1994 Butterworth-Heinemann Ltd

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Urban information system 367

critical and by 1739 the move to accurate plans of cities was so well

advanced that Louis Bretez's stunning plan of Paris, covering 20 sheets,

(Figure 2) was not particularly well received by the Parisian press 2.

Although aerial views continued to be fairly common in Europe through-

out the 18th century their use was restricted to recording picturesque

towns or presenting new developments, as in Baumeister 's views of

Karlsruhe in 1739.

Accurate surveying, standardized sets of symbols, and, from the late 19th

century, cheap offset lithographic printing resulted in the 2D map

becoming the standard way of representing cities. There is no denying the

usefulness of these maps, be it for navigation, planning new develop-

ments, or mapping spatially related data, but the paradigm of the map is

now so powerful that it tends to blind us to other forms of representation.

The map is a product of the printing press and its cheapness and

portability have been central to its success.

With the advent of computers, new technology has been brought to bear

on the map and the resulting geographic information systems (GIS) have

taken the map and added analytic tools. These systems accept the existing

paradigm and have linked it with database technology in order to give the

map intelligence. Sophisticated though these systems are they have simply

accepted the map as the way of representing urban form without fully

considering its strengths and weaknesses. Accuracy, consistency and

portability are all strengths but consistency has been achieved by reducing

the complexities of the real world through the exclusion of information,

and in particular by eliminating three-dimensional form. Contours have

been developed to overcome this problem when representing landscape,

but there is no equivalent in the urban context.

2 Ibid. p 56

This 18th century trade-off between accuracy and representation is no

longer relevant as it is now possible to have the best of both worlds

through the use of three-dimensional computer modelling. This preserves

the accuracy of an original survey while representing land and building

form realistically. Over the past 30 years computer-aided design (CAD)

systems have been developed which allow the user to create three-

dimensional models of buildings which can be as accurate as necessary

and, once created, can be viewed from any position. As hardware and

software have become increasingly powerful the scale of these 3D models

has increased, from individual buildings to urban blocks and now to whole

cities. There are clearly advantages in bringing GIS and CAD technology

together in order to shift GIS from being map-based to being model-

based. The map was the result of a need married to a particular

368 Design Studies Voi 15 No 3 July 1994

Figure 2 Plan of Paris by Michel Etienne Turgot, 1739 (Copyright - British Library Maps C.11.d.lO)


technology; now that the technology has changed the map can take its

place as just one level of representation within a much richer environ-

ment.

2 Existing urban models Early examples, such as the Skidmore Owings and Merrill model of

Chicago, 3 were just an assembly of simple blocks but increasingly

sophisticated urban models have been built since. The Architecture and

Building Aids Computer Unit at Strathclyde University (ABACUS) have

two, an extensive model of Glasgow and a smaller, more detailed model

of Edinburgh Old Town. This is intended to assist the Old Town Trust

with the conservation and economic regeneration of the historic core of

the city, while the Glasgow model is designed to provide GIS functionality and offer a way of integrating various types of urban information 4.

The most direct use for urban models is as part of the planning process.

Hall 5 reports on tests carried out on the use of computer visualization in

development control where three projects were modelled and the results

used for discussions between the applicant and planners. He comments on

the advantages to lay people of this kind of visualization and on its

usefulness to professionals engaged in the design process. The objective

status of the computer material was thought to be particularly advan-

tageous. As Hall puts it, 'It is suggested that a central component of the

argument for the use of computer visualisation in planning control is that

it can be seen to be independent of the positions of all the negotiating parties '6.

3 Mitchell, W J Computer-Aided Architectural Design Van Nos- trand Reinhold, New York (1977) pp 264-265 4 Grant, M 'Urban GIS - The application of the information technologies to urban management', in J A Pov~ll and R Day (ads) Informing Technologies for Construction, Civil engineering and Transport Brunel/SERC, Ux- bridge, UK, (1993) 10P 195-199 5 Hall, A C 'The use of computer visualisetion in planning control' Town Planning Review, Vo164 No 2 (1993) 193-211 6 Ibid. p 206 7 Merchant, A N 'Expert system: a design methodology' Computers, Environment and Urban Systems Vo116 No 1 (1992) 21-41

By combining computer modelling with expert systems new kinds of

design tools can be created. Merchant 7 discusses the development of an

Urban Design Expert System which was created to provide advice on the

design of a public space. The system initially offers the designer assistance

by offering a range of appropriate visual precedents and, when the scheme

is complete, the system can be used for evaluation with evaluation criteria

themselves being varied by the designer. The scheme design is carried out within a CAD package and the geometrical information it contains

provides much of the data .for analysis by the expert system. At the moment such systems are fairly primitive and work best with constrained

problems, but they indicate a way in which intelligence can be added to

urban models which is complementary to the kind of analytical tools that GIS systems provide for maps.

The issues involved in the creation and use of large 3D representations of

370 Design Studies Vol 15 No 3 July 1994

urban areas have been investigated over the past two years in Bath where

a model of the entire Georgian city has recently been completed.

Experience has been gained on the effectiveness of various ways of

constructing such models and on the opportunities and constraints

inherent in their use.

3 Development and conservation in Bath In most historic towns, including Bath, there is particular concern about

how new should relate to old. The strongest feelings often have to do with

the appearance of modern buildings and, in particular, how they relate to

existing buildings within a historic setting. When new buildings are

proposed there are often difficulties in understanding what is intended

and in assessing the impact of the proposal on its surroundings. There is a

desire for public debate, but that debate can very easily be stifled by

difficulties in understanding what is proposed because of problems in

reading traditional architectural drawings. Bitter experience has shown

that slick draughtsmanship and a carefully contrived perspective view can

all too easily fool a planning committee into thinking that a proposal is

appropriate only to find, when the building is complete, that it intrudes on

its surroundings in ways that were never anticipated. In order to address

this problem two things are required, an accurate and easily understood

representation of the proposal, and a presentation technique that is

objective and not completely under the control of the applicant.

4 Visualizing change In order to address this problem, some 25 years ago Bath Preservation

Trust commissioned a wooden model of the city as a vehicle for

considering the implications of new developments. The intention was to

alter the model in order to show change, but the expense and difficulty of

doing this meant that it was never used in this way. However, had it been

used, its scale of 1:500 would have meant that it would only have been

useful for considering change at an urban scale and would not have been

particularly appropriate for detailed consideration of the small-scale

changes that constitute the bulk of planning applications.

A computer model, on the other hand, is inherently flexible and so change

can be accommodated and detail added in order to visualize even very

minor alterations. As computers are being used increasingly for

architectural design it is not unusual for a planning application to be accompanied by drawings derived from a CAD model. However, such

presentations can be as deceptive as their hand-drawn counterparts as the

views are carefully selected and the surroundings may not have been modelled particularly accurately. The Bath computer model is designed to


address this problem. When planning permission for a new building is

sought the design can be included in the urban model thus providing an

accurate context and allowing views to be set up from any position, and by

any party. Also, parts of the model can be supplied to architects who are

working on sites in the city, thus giving them immediate access to an

accurate representation of their buiiding's immediate context.

4.1 The construction of the Bath computer model The 3D computer model includes the whole of the Georgian city, most

commercial and business activities and a considerable portion of the city's

residential fabric (Figure 3). The model was constructed using aerial

photogrammetry, a technique which uses pairs of photographs which are

viewed in stereo, allowing the operator to see the subject in three

dimensions. A photogrammetric plotter connected to a personal compu-

ter is used, and when the operator digitizes a point on a building its x, y

and z co-ordinates are entered directly into a CAD package running on

the PC. In Bath, survey photographs taken by Ordnance Survey in 1991

were used with any buildings constructed since that date being entered

manually from the architects' drawings. One of the great benefits of

employing aerial photogrammetry is that fieldwork can be kept to a

minimum thus allowing the construction of the model to proceed rapidly.

Also, as the photographs are taken looking vertically downwards, build-

ings that are difficult to get to, or are on private land, can easily be

surveyed. The external form of any building can be determined accurately

and the roof geometry and details, such as parapets and chimney stacks,

fully represented. The result is a model which is particularly convincing

from above and which accurately represents the grain of the city (Figure

4). The weakness of the system is the capital cost of the equipment,

upwards of £30 000 for a basic stereo plotter, and the paucity of

elevational information, something that is inevitable when the photo-

graphs are taken from above. However, this deficiency can easily be

rectified by taking additional photographs where more detail is required,

either as single images, or as stereo pairs to facilitate the modelling of a

facade in depth.

In order to introduce a precise scale when constructing a photogrammetric

model actual dimensions between known points on the photograph are

required. These control points can be surveyed conventionally but, as this

is expensive and time-consuming, spot heights on 1:1250 Ordnance

Survey maps were used. Each city block was treated as a separate CAD

model and these were located relative to one another using their National

Grid co-ordinates. Depending on size and complexity, each block took

between three and ten days for a skilled operator to construct and no


, . .', . .~

Figure 3 Plan showing the extent o f the Bath computer model

significant inaccuracies were experienced when the blocks were assembled

into the complete urban model. The whole model comprises some 150

urban blocks, occupies 60 Mb of disk space in Au toCAD .dwg format.

and cost about £100 000 to produce over an 18 month period. A 3D

terrain model was also constructed to show the countryside for 10 km

around Bath to ensure that any views taken from the city centre include

the rural skyline which is so important to Bath's setting. As it was

important for architects' CAD models to be incorporated into the urban


~t


model it was decided to use industry-standard CAD software with well

developed data exchange procedures and so Au toCAD Release 12

running on 486-based personal computers was used to construct the urban

blocks with the renderings and animation sequences being carded out in

Autodesk 3D Studio running on a PC, or in Wavefront Advanced

Visualiser running on a Silicon Graphics workstation.

4 . 2 Using the model To date, the main use of the model has been for development control. In

one example a school wanted to build a new sports hall on a sensitive

sloping site in front of one of Bath's most elegant crescents. Planning

permission had been refused previously and the city council planners

recommended that the model be used by newly appointed architects to

test a number of different strategies on the site. Three different schemes

were modelled and a series of views and animations set up for each. In

Bath the planning committee is especially concerned about the impact of

new developments on long distance views and so views of the site were set

up from precise locations on the other side of the valley for each of the

proposed schemes.

Two meetings, each lasting about 90 minutes, were held with the planners

and client's architects to consider the results of the computer visualiza-

tion. All parties agreed how helpful the technique was and it emerged that

the long distance views were deemed to be of little importance when

compared with close-up views and the impact of the new building on its

immediate neighbours. The result of the meetings was that all those

present agreed that one of the solutions showed particular promise and

this was then developed to a full planning application.

4.3 Issues in use At the moment the Bath model comprises some 150 submodels which can

each be accessed separately or combined as required. This process is

carried out via a reference drawing which shows the outline of each block

as a single polyline with the Au toCAD drawing name indicated. Indi-

vidual urban blocks can be loaded by clicking on the area within the

appropriate polyline, or the whole model assembled automatically.

Currently, when the model is being used for development control and a

view or animation sequence is required, the planners have to contact the

university and ask for the proposed building to be modelled and the

images created. Although this system works well enough it constrains the

use of the model by placing it at one remove from the end-user.

The reasons for this are both economic and technical. The quantity of data


contained in an urban model is large and a considerable amount of

computer power is required for its manipulation. This is expensive for an

organization where computer-aided design is a luxury rather than an

essential working tool and where staff lack training and experience in its

use. Even a planner with considerable enthusiasm for computing would

find it difficult to gain sufficient experience in the range of CAD and

visualization software that is used to create views of the Bath model. If

one wants the end-user to have access to the system then their needs have

to be addressed directly and, assuming that the model is going to be

maintained and updated by experts, these needs are relatively simple.

They want to be able to navigate around the model, set up views and

animation sequences, print out the results, and import and export urban

blocks for use by designers. To be economic such a facility should be

capable of being delivered on a standard personal computer using a

graphical user interface, such as Microsoft Windows.

5 Theproblem ofsize With a model of the size being discussed here, such simple requirements

can be problematic. Even navigation around the Bath model is difficult.

In Au toCAD, with the whole model loaded, it takes about 20 minutes to

regenerate the screen after moving the viewing position, and rendering a

single view of the entire city in 3D Studio can take over 50 hours. Even

using Wavefront Advanced Visualiser on a powerful Silicon Graphics

workstation a single view can take several hours to render. There are, of

course, strategies that can be adopted to reduce the amount of data that

has to be manipulated, for example, by excluding all the information that

is outside the view and structuring the data so that the level of detail

present in any part of the model is related to its distance from the viewing

position. For fast rendering it also makes sense to reduce the number of

faces in the model and increase the amount of texture mapping, but such a

strategy results in a model that is unsatisfactory for design purposes. The

Bath model is intended to be used as a design aid and therefore must be

accurate and have sufficient geometric detail to be realistic. A simple

rectangular prism with scanned photographs applied to its faces to

represent a building may be quite satisfactory for an interactive computer

demonstration but is of limited use as a design tool, particularly when

dealing with the intricacies of a historic city.

Although the power of computers is increasing rapidly in relation to cost,

the amount of information that makes up an urban model also tends to

increase as detail is added over time. So, it is unlikely that it will be

practical to ' run' a large urban model with anything approaching interac-

tive navigation on a personal computer within the next five years at least.


8 The terms hypertext and hypermedia were coined by Ted Nelson in the 1970s and refer to a new media form in which material from a variety of sources is linked together in a flexible and open- ended way. One of the first hyperlext applications, was Ap- ple's Hypercard.

If one simplifies the model, for example by having a number of different

versions of each urban block, then one can navigate by using the simplest

level of detail and only switch to the more complex level when the viewing

position is established. In the Bath model there are currently three levels

of detail: the outline polyline, a simple block model and the fully detailed

geometric model. Although such a strategy works well enough, it is a

device to overcome a technical deficiency and can be awkward for the

user. What they want is to have the full model presented on the computer

screen and to be able to move around it interactively. Clearly this is very

difficult to achieve while maintaining realistic modelling and rendering

and, even if one adopts strategies such as automatic scene culling and

level-of-detail selection, interactive navigation will only be possible on the

most powerful computers.

However, by being selective in terms of what one renders one can bring

rendering times down to minutes rather than hours and it is therefore

possible to build up multiple views of the model in a relatively short time.

If sufficient views are rendered, from known positions then one can move

between these views quite quickly and thus simulate a form of interactive

navigation. Although there are constraints to this approach, if sufficient

views are rendered a reasonable degree of interaction can be achieved

without the need to switch between various levels of detail. The disadvan-

tage of using views of the model, in the form of bitmapped pictures, is that

the pictures lose all the intelligence that is available in the CAD model

where attribute data can be assigned to any object. This allows the user to

select an object on the screen and, as the system knows what it is, a link

can be made to an external application, such as a database. This is a core

feature of geographic information systems, and if one can bring this kind

of intelligence back into bitmapped pictures then both navigation and

object selection can be achieved through the pictures without the need to

run the CAD model at all. With the Bath model this has been achieved by

programming Au toCAD to export a data file along with each bitmapped

picture which identifies each object in the view by specifying the area it

occupies in the picture. When this area, or 'button' , is selected on the

picture a link can be made to another picture, or to an external

application, thus re-introducing some of the intelligence that existed in the

original model.

The urban model should not be thought of as a single entity but rather a

collection of computer files which need to be browsed and selected

according to the needs of the user. As one has pictures with object buttons

already identified there are advantages in using a hypermedia system 8 to

make the necessary links as the very essence of hypermedia is the ability


9 Davis, H C, Hall, W, Heath, I, Hill, G and Wilklns, R 'Towards an integrated information environment with open hypermedia systems' in D Lucarella, J Nanard, M Nanard, and P Paoli- nl (eds) The Proceedir~gs of the ACM Conference on Hypertext ECHT '92, ACM, Milano (1992) pp 181-190

to move between various kinds of data in a seamless and interactive way.

In order to achieve this, use is being made of some recently developed

software, Microcosm, from the Department of Electronics and Computer

Science at the University of Southampton.

6 Microcosm Microcosm 9 is designed to allow the user to browse through large bodies

of multimedia material by following links from one place to another. It

consists of an open message passing system combined with a number of

viewers and filters. Viewers are programs that can display various formats

of documents, including pictures, sound and video, and they communicate

with the filters to establish what active areas exist in a particular

document. When a selection is made, for example by clicking on a

building in one of the pictures, the user activates a link which may be of a

number of types. Some go directly to another piece of information, such

as a picture or document, while others offers choices, perhaps based on a

keyword search made through a range of documents.

When linked to a palette of simple tools, this allows the user to navigate

around the model by moving quickly from picture to picture and to set up

views or animation sequences for subsequent rendering. As the system

knows the co-ordinates of each object in three-dimensional space one can

select a camera and viewing position simply by clicking on two points in a

picture, or indeed across a range of pictures. These pictures need not be

limited to views of the model as maps and drawings can also be included in

order to aid object selection.

As Microcosm offers a way of linking objects in the model with other

kinds of information it opens up the possibility of using the model as a way

of organizing large amounts of information about the city in order to

create an urban information system in parallel with the visualization

model. This could range from text and photographs recording a building's

history, through social statistics derived from the census, to data gener-

ated by an energy modelling package. Microcosm was originally designed

to handle links between text document but it has now been developed to

handle all digital media, including sound and video. It offers a number of

advantages over other hypermedia systems which may be summarized as

follows:

No mark-up required in documents All the information concerning

links is held in a linkbase and not in the document itself. This means

that documents that are held on read-only media, such as a CD-ROM,

can still be marked up.


• Connectivity to other applications It is possible to make links that

dispatch other programs with a given dataset as a Microcosm viewer.

This means that a user can access a document that was created by

another application, such as a word processor or spreadsheet, and

work on it within that application. For example, one could have a link

that would launch Au toCAD, read in a drawing, and allow the user to

edit it directly.

• Wide range of link types In most hypermedia systems links have

specific source and destination anchors. Microcosm supports such

specific links, but also supports more general links. Links can have a

fixed end point, but may be followed from any point where a given

object (such as a specific text string) occurs within a document or

specified set of documents. This means that it is possible to create a

single link to a piece of reference material, which may then be

followed wherever the specified source selection occurs without

having to re-make the link at every possible source point. This results

in a considerable reduction in authoring effort compared to traditional

hypermedia systems.

• Open architecture Microcosm consists of a number of independent

processes which communicate using an open message format 1°. Mak-

ing a viewer is simply a matter of programming an application to send

these messages to the system, something which can be done with a few

lines of code. Microcosm's architecture and message system are open

so it is possible to distribute Microcosm functionality across different

machines on a network, even where the machines themselves have

differing architectures.

10 Fountain, AM, Hall, W, Heath, I and Davis, HC 'MICROCOSM: an open model for hypermedie with dynamic linking', in A Rlzk, N Streltz and J Andre (eds) Hypertext: Con- cepts, Systems and Applications. The Proceedings of The Euro- pean Conference on Hypertext, Cambridge University Press (1990) 11 u, z, Davis, H C and Hall, W 'Hypermedia links end information retrievaJ' Proceedings of the 14th British Computer Society Research Colloquium on Information Retrieval, Lancaster University (1992)

It is important to stress that Microcosm is not a database, it is a link

service and multimedia navigation system n. Microcosm does not hold

documents, these are simply computer files which remain in their original

form. Microcosm holds attributes for these documents, and links between

documents. Users may navigate by making queries from the document

management system in a similar way that an SQL query is made from a

relational database, but there is a much larger range of navigational

techniques available. Microcosm does not make any requirement for data

to be structured or formatted in any particular way, nor does it dictate the

attributes that must be held for any link or document. These may be user

defined in every instance. The result is a system with much greater

flexibility and greater ease of navigation than is available from most

hypermedia or database systems.

7 Developing the Bath method Urban models of the kind discussed here are usually constructed on


powerful computers but have to be delivered on more basic machines to

users who may have limited computing experience. These machines, even

if they are well-specified 486 PCs, will not be able to cope with the full

urban model and therefore a strategy is being devised to use a networked

solution where all the intensive graphics processing is carried out on a

central machine with the results exported to the delivery machines for

viewing.

The software being developed to achieve this is based on Microcosm

running under both Unix and Microsoft Windows. The expensive CAD

and rendering software need only run on one machine with the client

machines using delivery software which can be made available at low cost.

This has the advantage of allowing an interface to be designed specifically

for the unskilled user independently of the CAD software that created the

model. As the model will reside on one machine it can be maintained and

updated by a single skilled operator thus ensuring a high degree of data

integrity. The set of pictures used for navigating the model, along with

their associated Microcosm buttons, can be automatically re-rendered

each week thus ensuring that users are always presented with an up-to-

date representation of the model.

8 The planning information system Currently under development is a computer-based planning information

system which will take the visualization capabilities that have already been

developed and link them to a series of databases. One will give specific

guidance to prospective planning applicants on matters which should be

considered when submitting a planning application, including illustrations

of current best practice selected by the development control officers. A

second database will contain a comprehensive record, in text and pictures,

of previous planning decisions which illustrate why particular decisions

were reached, This will be useful to the public and to development control

officers who aim at achieving consistency in their decision making. Built

into the system will be a feedback mechanism which will allow the public

to browse through current planning applications, see critical ones visual-

ized in the urban model, and offer their comments for entry into the database. This will widen the range of views that are received by the

officers and committee members.

9 Conclusions Large 3D computer models are becoming increasingly popular for urban planning and design. They provide a level of realism that can open up the

planning process to public scrutiny and allow strategies for the future to be


judged more effectively. Also, once created, such models can be used in

other ways as their legibility makes them ideal for accessing a database of

information about a city and for co-ordinating information that is

currently held separately. By linking visualization models to applications

which model other kinds of data, such as social statistics, energy-use

patterns or traffic flows, they can be used to present this information in

ways which make it more easily communicated and hence understood.

There is also the possibility of reconstructing the historical development

of the city as illustrated in the model of medieval Bath shown in Figure 5.

More speculative urban proposals may also be offered for public comment

and discussion, such as the scheme illustrated in Figure 6, which shows a

new mooring basin for the River Avon in the centre of the city.

However, enthusiasm for creating large urban models is currently over-

shadowing consideration of how they will actually be used in practice.

They are expensive to create and strategies must be developed to ensure

that their use is not unduly constrained by the current limitations of

hardware and software. The great advantage of such models is that they

provide a representation of the physical form of the city which everyone

can understand. Maps and plans are two-dimensional projections of

three-dimensional objects and that abstraction makes them much less

accessible than 3D models. For the trained designer the abstraction of

plan and section is helpful as it concentrates the mind on specific issues,

but for those unused to the conventions of orthographic projection the

synthesis of a series of two-dimensional representations into a three-

dimensional form can prove impossibly difficult. Urban models offer

something that has never been available before, a way of analysing the

present and considering the future which is available to all. Just as the

drawing is a tool used by the designer and, as such, becomes a way of

thinking about a problem, so the urban model can become a way for the

whole community to focus and articulate its thoughts on how urban

growth and change might be accommodated.

In a democracy the decisions about how a city should develop must be

made by its residents, and their representatives, on the basis of informed

choice. In the past too many decisions have been made by minorities

simply because there was no effective public debate as little of the

information necessary for making judgements was readily available. Any technique which can make this process more open is to be welcomed. Those described here do this by using new technology to bring informa-

tion together and by using computers to represent change in a way that

makes it intelligible to nonspecialists. Although such techniques will not,


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of themselves, guarantee that the cities of the future will be better than

those we have today they will at least ensure that the information required

to facilitate a more open debate is widely available.


from map to model: the development of an urban information system

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