The role of web services for spatial data delivery
The mechanics of GI web services
A web service is “published” to the network inside or outside the corporate firewall by
providing a document describing it functionality and mode of operation. This document is
created using the core building block of web services XML the eXtensible Markup Language,
an “information rich” description language. For this particular purpose a particular type or
schema of XML is used to create the document. The web service is described using the Web
Services Definition Language (WSDL), of particular note here is that the WSDL document is
machine readable so that the capabilities of the service can be explored by another web
service. In the context of GI web services a WSDL document might describe a service which
when passed a national grid reference responds with the relevant LandRanger map and its
details. The WSDL describes how the service is evoked and what is returned.
To make the rest of the network aware of this web service the WSDL is published to a public
clearinghouse or yellow pages site known as a Universal Discovery, Description, and
Integration or UDDI registry. A developer of web services or a web service itself can therefore
either couple with another known web service or may use UDDI to discover equivalent
services.
The communication between web services is carried out by passing XML messages wrapped
in an interoperable container to allow the messages to cross different networks, use different
application architectures and systems. The wrapping mechanism is known as SOAP the
Simple Object Access Protocol and is best thought of as a standard DL sized envelope which
keeps the contents of the message secure and also guarantees the message will be
deliverable through a standard letterbox.
Potential GI web Services
So how might the concept of Web Services and the technologies of WSDL, UDDI and SOAP
combine to deliver benefit to the GI industry? From the perspective of a National Mapping
agency, the Ordnance Survey can see a number of valuable Web Services which could be
developed in internally and in conjunction with our partners which would aid in the uptake of
Geographic Information largely my removing much of today’s complexity.
A Mapping web service
The Ordnance Survey like many organisations is making increasing use internally and
externally of web based mapping applications which generate on demand maps based on our
existing cartographic products. These applications could be encapsulated as web services so
that a simple SOAP message could evoke them and generate a map. The building blocks for
such applications are already mature and much of the work of the Open GIS consortium has
been directed and developing the protocols for such web mapping services (OpenGIS, 1999).
As with all web services the sophistication of simple web service based application may be
extended by cascading or chaining services together so that a number of web map services
may combine their maps and then be published through a transaction based commerce
server (Wagner,2002)
A “Change Only” update service
One of the most powerful aspects of the new MasterMap Topo data product is the ability to
supply only updated topographic features online rather than complete resupply on update.
However with this advance it must be recognised come higher data management
requirements for the user of change only update data.
A series of web services could be developed which would allow a user organisation to
subscribe to an update service which would message the changed features to a receiving
service which could then automatically insert the changes into the users own local repository.
From the perspective of the local system administration this could offer up to date consistent
data with very little management overhead.
This same web service application could also be repurposed to serve the Ordnance Surveys
value added partner community allowing them to keep their own local databases
synchronised with the National Topographic Database.
A Web Feature service
Building on from the previous two services, a more sophisticated web service could deliver
raw Geospatial feature data to end users or partners for local rendering into maps or
processing. With the appropriate support infrastructure a web feature service could deliver
spatial data directly into a user’s desktop GIS application removing the requirement to
manage any local repository of data. Such as feature service could also be chained through
a Geospatial analysis web service which could allow analysis of the data to be carried out on
the data as it passes through, for example calculating feature centre points or overlaying
features from different geographies.
The chaining of data and Geospatial applications could be vital in the delivery of successful
Location Based Services where the end users terminal has limited processing and display
capabilities. The “Find the nearest Indian restaurant” example could be delivered by chaining
web services publishing Points of Interest, Integrated Transport Networks, providing Routing,
Map rendering, and text to voice conversion. In the competitive mobile communications
marketplace it might be that the operator with the best collection of branded web service
providers becomes the dominant one.
Miscellaneous services
Because the GI web services in most cases will have no direct user interface and will become
embedded deeply within other systems, many of the web service developed to support the GI
industry will be invisible to the end user. These small services however could be some of the
most powerful; a gazetteer or place name service could be developed along with tools for
identifying locations based upon postcode or census geographies. An important service which
could be developed could provide lookups between the property reference number of the
National Land and Property Gazetteer and the OS MasterMap TOID.
Some issues remain
As with an emerging technology there are risks in embracing web services at this point.
Although a new technology with immature tools the development environment for the creation
of web services is relatively straightforward. There are competing technology platforms in the
form of .NET and J2EE from Microsoft and Sun respectively, but there is complete
interoperability between them.
By far the biggest challenges are cultural rather than technological. From the perspective of a
system developer the move to a web services based architecture is a radical change from the
closely coupled largely proprietary world of GIS. A vendor will no longer be able to “own” the
customer by providing a closed system, in fact with web services the system may only come
together at run time for a single instance, and never be formed of those exact components
again.
From the perspective of a data supplier greater interoperability means more focus on issues
of accuracy, completeness currency etc which in a less interoperable world were hidden.
Perhaps the biggest area of concern is the immaturity of business models on the internet in
general but more specifically the rather more complex (from a business workflow perspective)
world of web services. I believe there is still considerable work required to develop a viable
business model robust enough to cope with a service delivered onto a mobile user’s terminal
for the cost of a few pence which is the product of a complex chain of web services involving
multiple data and application service providers.
Conclusion
The GI industry and to a lesser extent the internet are technologies which are not fulfilling
their obvious potential. The potential for change that web services bring to increase the actual
usefulness of the internet is massive. Web sites will become truly interactive, able to respond
to the user’s needs, and the environment using application logic. These same advances offer
the potential to increase the use of Geographic information by removing some of the
complexity and reducing the high cost of entry. Users of GI data be they professional planners
or special interest activists will be able to gain access to the data they need and the tools to
use it when they need and only when they need it.
Joined up geography will become less of a issues in the web services landscape as all data
will appear to be joined up, as behind the scenes web services work 24/7 to manage and
manipulate the data in a user rather than system centric view of information.
The Ordnance Survey is continuing to investigate the potential benefits of web services, to
ourselves, our partners and our customers. Please feel free to contact us with your
needs/issues as no organisation can hope to develop web services in a vacuum.
Seybold, P.A. 2002 “Web Services Guide for Customer-Centric Executives”, Patricia Seybold
Group, Inc. Boston
OpenGIS Consortium 1999 “Open GIS Consortium Web Mapping Testbed”,
www.opengis.org/archive/wmt
Wagner, Roland M. 2002 “GIS Meets E-Business: Web Pricing and Ordering Service
(WPOS)”, Proceedings on the 25th GITA Annual Conference & Exhibition
