OpenstreetMap: User-Generated street Maps
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Use r - G e n e r ate d Co nte nt
OpenStreetMap:
User-Generated Street Maps
The OpenStreetMap project is a knowledge collective that provides
user-generated street maps.
T
he process of mapping the Earth phone masts or other beacons. However, these
accurately was, until recently, the methods hadn’t gained much market share ow-
preserve of highly skilled, well- ing to their technical complexity and inability
equipped, and organized individ- to provide a universal coverage. In contrast,
uals and groups. For many years, GPS enabled the development of cheap receiv-
it was usually the role of surveyors, cartogra- ers with good positional accuracy, and, by mid-
phers, and geographers to map the world and 2001, it was possible to purchase a receiver unit
transcribe it on paper or, since the 1960s, into for roughly US$100. 3 These receivers helped
the computer. Lewis and Clark’s expedition to more people than ever before collect informa-
map North America’s West, and Lambton and tion about different locations and upload it to
Everest’s Great Arc expedition to measure India, their computers. However, until 2002, when an
are just two famous episodes in the history of interchange standard (GPS eXchange format or
maps and map making. Each country has an GPX) was published, manipulating and sharing
established national mapping agency charged this information was a complicated task that
with keeping the national maps accurate and required computing and data manipulation
current (for example, the US Geological Survey knowledge. Fortunately, most GPS receiver
and the UK Ordnance Survey). developers rapidly adopted the GPX standard,
Less than a decade ago, it was common to as- and, by 2004, it had become commonplace
sume that a person needed a university-level de- (www.topografix.com/gpx.asp).
gree to be able to measure the The wide availability of high-quality location
Earth and transcribe the in- information has enabled mass-market mapping
Mordechai (Muki) Haklay formation on paper or into the based on affordable GPS receivers, home com-
and Patrick Weber computer as well as expensive puters, and the Internet. Although a range of
University College London equipment and infrastructure projects based on user-generated mapping has
to support his or her work. emerged, OpenStreetMap (OSM) is probably
This, however, has changed the most extensive and effective project cur-
dramatically over the past de- rently under development. In this article, we re-
cade. On 1 May 2000, US President Bill Clinton view the project and provide an overview for the
announced the removal of selective availability techniques and methodologies used within it.
of the GPS signal1 and, by so doing, provided
much improved accuracy for simple, low-cost OpenStreetMap Background
GPS receivers. In practical terms, this made it Technological changes over the past 10 years,
possible to acquire the receiver’s position with in combination with increased bandwidth and
an accuracy of 6 to 10 meters in normal condi- the ability to provide better tools for collabo-
tions, in contrast to roughly 100 meters before ration, have led to “crowdsourcing”4 —a term
the “switch off.” Attempts to develop location- developed from the concept of outsourcing in
based services predated this announcement 2 which business operations are transferred to re-
and were based on information from mobile mote, many times cheaper locations. 5 Similarly,
12 P ER VA SI V E computing Published by the IEEE CS n 1536-1268/08/$25.00 © 2008 IEEE
500 50,000
Track points
450 45,000
OpenStreetMap (OSM) registered users
400 40,000
350 35,000
Track point (millions)
300 30,000
Users
250 25,000
200 20,000
150 15,000
100 10,000
50 5,000
0 0
08/08/05
05/09/05
03/10/05
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28/12/05
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23/01/06
20/02/06
20/03/06
17/04/06
15/05/06
12/06/06
10/07/06
07/08/06
04/09/06
02/10/06
30/11/06
27/11/06
25/12/06
22/01/07
19/02/07
19/03/07
16/04/07
14/05/07
11/06/07
09/07/07
06/08/07
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24/12/07
21/01/08
18/02/08
17/03/08
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12/05/08
09/06/08
07/07/08
Date
Figure 1. Graph of user and contributions growth to OSM on a monthly basis. The graph shows the accelerating growth in
number of users and the rapid increase in data entry measured in track points (source: http://wiki.openstreetmap.org).
crowdsourcing is how large groups of and a core group, estimated at approx- the US Census Bureau’s Tiger (Topolog-
users can perform functions that are ei- imately 40 volunteers, dedicate their ically Integrated Geographic Encoding
ther difficult to automate or expensive time to creating and improving OSM’s and Referencing)/Line program, the
to implement. infrastructure, including maintaining details provided are limited to streets
OSM follows the peer production the server, writing the core software and roads only; it doesn’t include green
model that created Wikipedia; its aim that handles the transactions with the space, landmarks, and the like. In ad-
is to create a set of map data that’s free server, and creating cartographical dition, owing to the high cost of map-
to use, editable, and licensed under new outputs. There’s also a growing com- ping, the Tiger system’s update cycles
copyright schemes. The project, born munity of software developers who de- are infrequent and don’t take into ac-
at University College London (UCL) in velop software tools to make OSM data count rapid changes. Commercial geo-
July 2004, was founded by Steve Coast; available for further use across differ- graphical information products from
although Coast moved on to start his ent application domains, software plat- providers such as NAVTEQ are also
own company, UCL still supports and forms, and hardware devices. expensive and aren’t available for indi-
hosts the main server infrastructure. A key motivation for this project is to vidual users in an accessible format.
As of May 2008, OSM had more than enable free access to current geograph- The OSM project’s hub is the main
33,000 registered users (with approxi- ical information where, in European OSM Web site (www.openstreetmap.
mately 3,500 currently active contrib- countries, accurate digital geographical org), which contains four parts. Visitors
utors), and data contribution growth information is considered to be expen- are first greeted with a Google Maps-
continues to rise quickly (see Figure 1). sive and out of the reach of individu- style online mapping interface, which
A considerable number of contributors als, small businesses, and community lets visitors pan, zoom, and search the
edit the world map collaboratively us- organizations. In the US, where basic OSM world map and discover which
ing the OSM technical infrastructure, road information is available through geographical areas are completed. An
October–december 2008 P ER VA SI V E computing 13User-Generated Content
export function allows users to down- weight online Flash-based editor, Pot- can be extended using several indepen-
load portions of the OSM information latch, which lets users add, update, or dently developed plug-ins. Examples
in different raster and vector formats delete geographical features through a of user-contributed plug-ins include
for further use or processing. The edit- relatively easy-to-use interface. The in- custom Web mapping service (WMS)
ing tab allows anyone to contribute to terface is kept deliberately simple, with background imagery and Yahoo aerial
the project by digitizing geographical more advanced functionality provided imagery, live recording of external GPS
features, uploading GPX traces from through keyboard shortcuts; Potlatch data, and a data and tagging scheme
hand-held GPS units, or correcting er- gives extensive guidance to users by validation tool, to name just a few.
rors they might have discovered in their providing predefined tagging schemas Apart from individual user contribu-
local areas. The OSM community wiki, for frequently occurring features (such tions from GPS tracks and the digitizing
open for all registered users, contains as motorways or primary roads). of aerial imagery, OSM has also taken
information about the project and of- Potlatch also lets users upload and advantage of the availability of free geo-
fers guidance on best practices to ca- integrate GPX tracks recorded from graphical information in certain parts
sual and advanced mapping contribu- handheld GPS units. Other important of the world. For example, over the past
tors and an extensive documentation of data sources from which users trace year, US contributors have imported the
the project’s technical infrastructure. road networks and other features is public-domain Tiger information, add-
Noticeably, OSM decided to follow satellite imagery and out-of-copyright ing to OSM’s comprehensive street and
the route of allowing only registered maps, which are integrated into the highway coverage for the entire US.
users to edit the map, not following mapping interface. Notably, at the end In July 2007, commercial navigation
Wikipedia’s open-for-all approach. of 2006, Yahoo granted OSM the right information provider AND (Automo-
OSM project leads wanted the ability to use its satellite imagery Web service tive Navigation Data) donated the en-
to trace the information source in case to trace roads and other features. For tire street map of the Netherlands to
of copyright disputes and ensure the in- example, Yahoo hosts detailed aerial the project, thus completing street-level
formation was maintained. imagery of Baghdad, Iraq, which let mapping of the first country in OSM.
In the following sections, we describe OSM contributors remotely map the Local and national authorities have
the OSM GeoStack — the set of tools city in great detail, resulting in the most contributed information to the project
that lets users capture, produce, com- detailed online map of Baghdad to date. as well—for example, the Isle of Man’s
municate, aggregate, and consume the However, this map is based solely on Department for Local Government and
geographical information produced in amateur image interpretation, and, de- the Environment donated geographical
the project (see other work6 for further spite the original effort, very little fur- information to OSM, enabling the cre-
ation of an excellent map of the island.
OpenStreetMap’s aim is to create a set of map Technical Infrastructure
The next element of the GeoStack fo-
data that’s free to use, editable, and licensed cuses on the way in which geographi-
cal information is stored and orga-
under new copyright schemes. nized. Here, at the core of OSM data
management, it’s easy to see how open
discussion of the GeoStack). As already ther information (such as street names) source philosophy permeates the proj-
noted, the Web site provides a mecha- has been added to the map. ect’s technical infrastructure. OSM is
nism to consume OSM data — but in More experienced OSM contributors built iteratively using the principle that
the next section we move to the starting also use the Java OpenStreetMap Editor the simplest approach to any problem
point of the stack, where data is cap- (JOSM), an editing suite with an inter- is the best way to ensure the success of
tured and produced. face more akin to traditional geographi- the project as a whole. OSM’s develop-
cal information systems (GIS) packages. ers deliberately steered away from us-
Editing Tools The application lets users import, edit, ing existing standards for geographical
User-contributed geographical informa- and tag OSM data offline and allows information from standard bodies such
tion is obviously a core part of OSM, bulk uploads of OSM updates through as the Open Geospatial Consortium
and the OSM developer community has the OSM application programming in- (OGC)—for example, its WMS stan-
made a considerable effort to implement terface (API). JOSM offers advanced dard. They felt that most such tools and
tools to facilitate user contributions to functionalities such as linking OSM standards are hard to use and maintain,
the database. For most casual contribu- features to photos and audio notes, citing performance issues with, for in-
tors, the OSM Web site offers a light- supports data conflict resolutions, and stance, MapServer (a popular open
14 P ER VA SI V E computing www.computer.org/pervasiveFigure 2. Main OpenStreetMap Web site Slippy Map with detailed coverage of central London. Notice the ability to edit, export, and upload data by using the top tabs (source: www. openstreetmap.org). source WMS) and a lack of adaptability of OGC-compliant software packages to support wiki-style behavior. At the heart of OSM’s technical infra- structure lies the central database hold- ing the live data, which is implemented in MySQL. The database schema is de- signed to support wiki behaviors, such as versioning and rollbacks, and keeps cop- ies of modified or deleted features indefi- nitely. All geographical entities are re- corded as points (nodes), which contain the latitude and longitude coordinates along with user name and timestamp information. Linear and area features are defined by reference to a list of or- dered nodes, called ways. Area features aren’t explicitly defined in the database schema—rather, they’re defined im- is voluntary. Guidance on best prac- butes OSM contributors have collected, plicitly by the condition of a way that’s tices for tagging is given at the editing the main cartographic output from the closed (the first node of a way is the same software level for casual contributors. OSM information is presented on the as the last one) and explicit tagging con- The lack of restrictions also benefits us- OSM Web site as a Google Maps-like ventions (using the tag area=yes). ers because they can freely implement interface, coined “Slippy Map,” which Along with the geographical coor- custom tagging schemas for specific uses the open source AJAX library dinates of features in the OSM data- applications. OpenLayers to dynamically update the base, attributes of features are recorded Access to the core OSM database is map display and allow interaction with for each node and way as semicolon- provided by a dedicated RESTful API, users. As users drag the map, the vis- separated key=value pairs (for exam- which is implemented in Ruby on Rails ible extent is updated and new map tiles ple, type=pub;name=The Bull). This tagging and supports authentication, enabling are requested in the background with- schema, which is increasingly being users to add, update, and delete geo- out reloading the entire HTML page. developed into a complex taxonomy of graphical features. The API accepts A search function (implemented as an real-world feature classes and objects, and outputs data in OSM XML, a dedi- external Web service) lets users quickly is a core part of the OSM initiative and cated data transport format developed find cities, villages, or other points of in- is community-driven. Any member of for the project that replicates the data- terest (POIs) in the database. Recently, the community can contribute to and bases’ specific entity model. All editing OSM developers added an export tab, update the schema by proposing new tools use this API for accessing and up- which lets users quickly generate map key=value pairs. Discussion on the OSM dating the main database. As a result, images, PDF files, and raw data down- wiki leads the community to vote on editing and presentation tools can be loads of custom bounding boxes. tag propositions and reach a consensus developed independently from the data- The default set of tiles on the main on the definition of and best practices base, with the lightweight communica- OSM Web site (see Figure 2) is rendered for feature tagging. Interestingly, and tion protocol acting as a glue between using Mapnik, an open source library sometimes controversially, the OSM the elements of OSM’s GeoStack. for generating high-quality map images. database doesn’t pose any limitations It uses a weekly database dump as the on the tags users can attach to features, Mapping Outputs source for the rendering of map tiles, and thus adhering to the core tagging Given the database, which contains all given that live rendering of tiles on client schema the community agreed upon the geographical information and attri- request would be too computationally October–december 2008 P ER VA SI V E computing 15
User-Generated Content
expensive to be practical; map tiles are or Gosmore to generate maps quickly. Europe spent two days driving, cycling,
therefore rendered for all zoom levels Other members of the OSM community and wandering around the island with
and saved on the server so that they can are also developing import and conver- GPS receivers to collect a complete cov-
be served rapidly as static images. sion tools to support OSM data in GIS erage of roads and footpaths. After col-
Owing to the open source nature of packages such as Manifold GIS (www. lecting the individual contributions,
all the tools needed for map render- manifold.net) and ArcGIS (www.esri. processing them, and uploading the
ing, several OSM contributors have com/software/arcgis/). Significantly, data, a practically complete map of the
developed custom map tile sets that vendors of commercial GIS packages, island emerged.
cater to specific needs and target au- such as CadCorp SIS and Global Map- Although these community events
diences. One example is a tile set that per, have recently included OSM XML positively contribute to the overall proj-
highlights cycle-path networks and data support out of the box. ect by generating new data and street
other features relevant to cyclists. Users have converted OSM informa- labeling, they aren’t only meant as data
Because of the high demands on com- tion for use on a multitude of devices, collection exercises. Mapping parties
puting during rendering, a community including mobile phones, PDAs, and play an essential part in creating and fos-
grid element of OSM has developed. GPS units. A community-maintained tering local OSM user groups and creat-
Tiles@Home (T@H) is a distributed software package lets users trans- ing a vibrant social community around
map-tile-rendering system that com- late OSM data into the Garmin IMG the project (see Figure 3). User groups
prises a central coordinating server and GPS map format, despite this for- have now formed all over the UK, as
approximately 100 active software cli- mat’s proprietary nature and lack of well as in continental Europe and the
ents (as of May 2008), which members of documentation. rest of the world. A detailed discussion
the OSM community contributed. The of a mapping party appears elsewhere.7
main server distributes rendering jobs Social Collaboration
between clients, which collect the rele- Unlike Wikipedia, on which individu- Motivations and Challenges
vant data from the OSM API and render als create the majority of content at dis- OSM’s success should be attributed to
a set of map tiles that are then uploaded parate locations, the OSM community its founder’s vision of the project as a
back to the server for distribution. The organizes a series of local workshops combined social and technical chal-
main benefit of T@H is that it distributes (called “mapping parties”), which aim lenge. In many interviews and presen-
the computational load between multi- to create and annotate content for local- tations, Coast emphasizes the social
tudes of clients, enabling the system to ized geographical areas. These events side’s importance: “A big aspect of
quickly render large numbers of up-to- are designed to introduce new users and getting OSM off the ground was the
date map tiles. T@H is especially use- contributors to the community with the mapping parties: getting drunk and
ful for OSM mapping contributors that hands-on experience of collecting, pro- arguing with people.”8 Nick Black,
want to quickly see the results of data cessing, and uploading data to the OSM another OSM core contributor, noted
changes, as they can request specific ar- project. The meetings might take the that people have a range of reasons for
getting involved in the project—from
certain ideological views such as a be-
OpenStreetMap is built iteratively using lief in the provision of free information
to improve the world, to anti-national
the principle that the simplest approach mapping agency views, to those who
enjoy going out and mapping or sitting
to any problem is the best way to ensure at home and writing computer code,
to those who enjoy feeling like part of
the success of the project as a whole. a community.9 Coast also adds that
there’s an “addictive” aspect of getting
eas to be added to the T@H rendering shape of informal and small gatherings involved in the project, which adds to
queue. Typically, such requests for up- for a few hours to complete missing fea- the participants’ commitment.
dated map tiles are fulfilled in a matter tures of a small defined neighborhood, Indeed, OSM can boast significant
of hours, in contrast to up to seven days up to more ambitious efforts that take achievement and is currently going
for a Mapnik tile set update. several days and involve several dozens through a period of rapid growth in
Users interested in only a small area of participants. One of the first mapping terms of the number of users who con-
and who don’t need a Web-mapping parties took place on the Isle of Wight, tribute to the map, its visibility as the
server set-up can use local software off the south coast of England, in May leading Open Geodata project, and of
rendering packages such as Kosmos 2006. More than 30 participants from the number of map edits. Its infrastruc-
16 P ER VA SI V E computing www.computer.org/pervasiveFigure 3. A mapping party. In February 2008, Schuyler Erle and Mikel Maron, contributors to OSM, held a series of multiday mapping parties in several universities to create local contributor groups (source: www.flickr.com/photos/ tags/freemapindia2008/). ture is demonstrating how a modern, open source, and light structure can provide a reliable and scalable GIS; the innovation that OSM offers can also teach many established GIS providers valuable lessons. The mainstream GIS literature has already noted and dis- cussed OSM and similar activities.10 Along with these achievements, some open issues still must be taken into ac- mation, such as traffic directions and which the complexities of adding or count, such as the fitness for purpose of restrictions or street names, can be cru- updating data to OSM due to the us- OSM data, the influence of geography cial. Although data capture based on ability of the data entry application and and participation on the project, the aerial imagery sped up road mapping, the need to understand the ontological ability to continue to update the infor- the road name can’t be captured with- schema need to be added. In short, this mation, and licensing. out a survey at the ground level unless is a challenging aspect that the OSM Since early 2007, with the completion the surveyor breaks copyright laws and community must solve. of the mapping of Cambridge, OSM in- uses an existing map to copy the name. Finally, the issue of licensing has be- formation and the cartographic ability Furthermore, mistakes from misinter- deviled OSM from the start. The proj- of Mapnik demonstrated the useful- preting aerial imagery also must be ect started with the Creative Commons ness of the information for several car- rectified through ground survey. Here, framework (CC-By-SA), which has tographic products and presentation there’s no escape from the world’s proved to be unsuitable for geographical of paper maps. However, this cover- physical geography, and OSM is de- information. For example, the “share age isn’t universal. Although, as Coast pendent on willing contributors on the alike” principle means that by mixing notes, “it’s important to let go of the ground in the area under survey. The OSM information with other sources concept of completeness,”8 some idea same is true for updates and changes of information, the resulting map must about information quality is crucial to the data—for example, when a road be shared under the same terms. How- for evaluating how fit OSM data is for changes from two-way to one-way ow- ever, OSM creates the base map, which various applications. Considerations ing to local traffic regulations. in most geographical analysis projects that should be included are how well Although the common argument is the starting point. The map is aug- an area is covered by data, the data’s from OSM advocates is that because of mented either through cartographic accuracy in terms of positional accu- the data’s free nature, people who spot a processing to produce products such as racy and attributes, the consistency mistake are likely to be more motivated tourist maps or combined with other in terms of classification or data-cap- to rectify it, in practice, OSM is exhib- sources of information, such as the cen- ture procedures, and quality control. iting the same participation inequality sus, to create thematic maps. It can also Currently, OSM doesn’t provide any as many other user-generated content be combined with sensitive commercial of these measures and doesn’t have in- projects.11 A few users are responsible information for analyzing store loca- ternal quality assurance procedures. for contributing a significant amount tions, among other things. In all these Preliminary results from an evaluation of the information—this phenomena cases, the CC-By-SA prevents the use of completeness at UCL suggest that is also visible in terms of places, where of the data or might complicate matters OSM has covered about 29 percent many areas are mapped by a single significantly. The OSM community has of roads in England. Even in London, user. It’s reasonable to assume that, been discussing a new license for more where the project started, large areas as on Wikipedia, the number of users than two years and hopes to publish are undermapped. who don’t contribute any improvement one soon, although it’s clear that some In many applications, attribute infor- to the map can reach 99.8 percent, to use issues will remain unresolved. October–december 2008 P ER VA SI V E computing 17
User-Generated Content
the Authors provided early support for the engagement of
OpenStreetMap in 2006.
Mordechai (Muki) Haklay is a senior lecturer in geographical information
science in the Department of Civil, Environmental and Geomatic Engineer-
ing at University College London. His research interests are in public access to
environmental information, human-computer interaction, usability engineer- References
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phy from UCL. He’s a member of the ACM, a fellow of the Royal Geographic 1. B. Clinton, Improving the Civilian Global
Society, and a member of the Association for Geographical Information (AGI). Positioning System (GPS), Office of Sci-
Contact him at m.haklay@ucl.ac.uk. ence and Technology Policy, Executive
Office of the President, 1 May 2000;
ht tp: //clinton4.nara.gov/ W H / EOP/
Patrick Weber is a third-year engineering doctorate student in University
OSTP/html/0053_4.html.
College London’s Department of Computer Science. His research interests lie
in the application of GI science and spatial decision support systems to gain
2. A. Giordano, M. Chan, and H. Habal, “A
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O
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4. J. Howe, “The Rise of Crowdsourcing,”
munities. Using terminology found geographical information, in contrast Wired, June 2006; www.wired.com/
in other work,12 OSM is a knowledge to current practice within the GIS in- wired/archive/14.06/crowds.html.
collective that creates Open Geodata dustry. On the other hand, OSM data
5. T.L. Friedman, The World Is Flat: A Brief
as its main objective. Simultaneously, isn’t complete or consistent across the History of the Twenty-First Century,
it includes a peer production network, world, or even across London, where Updated And Expanded Edition, Farrar,
as different groups within the organi- the project started. The data’s accu- Straus, and Giroux, 2006.
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of the project—digitizing tools, map- no systemic and comprehensive qual- phy, O’Reilly Media, 2006.
rendering software, server software, ity assurance processes integral to the
7. C. Perkins and M. Dodge, “The Potential
and running activities such as mapping data collection. Furthermore, there’s of User-Generated Cartography: A Case
parties. It’s utilizing community com- no intention of universal coverage; as Study of the OpenStreetMap Project and
puting grids in the process of render- Coast remarks: “Nobody wants to do Mapchester Mapping Party,” North West
Geography, vol. 8, no. 1, 2008, pp. 19–32.
ing map tiles through T@H. OSM uses council estates. But apart from those
social mobile computing, especially socioeconomic barriers—for places 8. GISPro, “The GISPro Interview with
during mapping parties when partici- people aren’t that interested in visiting OSM Founder Steve Coast,” GIS Profes-
sional, no. 18, Oct. 2007, pp. 20–23.
pants coordinate their work via mobile anyway—nowhere else gets missed.”9
GPS receivers and mobile phones. The Although the OSM project started 9. N. Black, “OpenStreetMap — Geo-
group-forming network appears on the in 2004, it’s still in its early stages—the data Collection for the 21st Century,”
presented at the AGI annual confer-
main wiki, which contains information area of user-generated geographical in- ence, 2007; www.slideshare.net/nickb/
about the project, and also through an formation will surely grow in the future nick-black- openstreetmap -geodata-
array of active mailing lists, Web fo- with applications in the private, public, collection-for-the-21st-century.
rums, Internet Relay Chats (IRCs), and and voluntary sectors. It has already 10. M.F. Goodchild, “Citizens as Voluntary
other modes of computer-mediated shaken the world of geographical infor- Sensors: Spatial Data Infrastructure in the
communication (CMC). Finally, social mation, and the wider industry is paying World of Web 2.0,” Int’l J. Spatial Data
Infrastructures Research, vol. 2, 2007,
accounting is evident in OSM—for ex- attention to the emerging business mod- pp. 24–32.
ample, in the contributions of various els that OSM volunteers have created.
members of the OSM community high- 11. J. Nielsen, “Participation Inequality:
Encouraging More Users to Contribute,”
lighted on a Web site about the amount Acknowledgments Alertbox, 9 Oct. 2006; www.useit.com/
of computing they’ve contributed or alertbox/participation_inequality.html.
how many edits they’ve performed dur- We thank the anonymous reviewers who pro-
vided useful suggestions on an earlier version 12. A. Saveri, H. Rheingold, and K. Vian,
ing the last week, month, and year. of this article. Thanks to the Royal Geographical Technologies of Cooperation, Inst. for
OSM also demonstrates some impor- Society’s Small Research Grants program, which the Future, 2005.
18 P ER VA SI V E computing www.computer.org/pervasiveYou can also read
