Speaking a global map language
- By Brian Robinson
- Dec 02, 2001
The success of Extensible Markup Language, which is emerging as the universal
language for e-government and other online transactions, has produced an
offshoot — Geography Markup Language (GML) - that has the potential to
change how state and local agencies share mapping information.
Like XML, GML is designed to serve as a common way of exchanging data
between systems, whatever proprietary geographic information system (GIS)
software the systems use. Typically, a mapping application can interpret
data only if it was created with its own software. GML would remove that
barrier, because any application that understands GML would be able to work
with GML data.
It's not unlike translating at an international conference. Rather than
providing a translation service for every participant, conference planners
figure that most people will know one or two common languages. Just as English
or French may make it possible for people from around the globe to exchange
ideas, GML proponents believe this evolving technology could give GIS data
"Interoperability is the bottom line of this GML activity," said Roger
Harwell, mapping and GIS solutions product manager for GeoMedia WebMap products
at Intergraph Corp. "It will allow interoperation among various vendors
so users can grab data from anywhere they want in order to accomplish an
application. It just makes a whole lot more data available to people."
XML allows digital data to be identified and "tagged" so that Web servers
can talk to each other and pull similar kinds of data together from a number
of sources and then display it in different forms on Web pages. It supports
links that point to multiple documents, as opposed to the links provided
through HTML, the current standard for building Web pages, which reference
just one destination on the World Wide Web.
GML similarly tags geographic data so it can be collected from different
sources and displayed as maps or in other forms using a standard Web browser.
That's hard to do with current GIS tools, which use vendor-specific, proprietary
Many government agencies have avoided incompatibility problems by using
a single vendor solution. Even there, however, GML could help by making
GIS data available to many other people in government without the need to
install and maintain software separately on desktop computers.
For instance, a person in a city or county planning department could
access GIS data generated in another department as long as that data had
been converted into GML format. The data could be pulled from the Web server
and displayed using that person's desktop browser.
GML also could be used to splice data that doesn't already have an obvious
connection or a simple way of being displayed together.
"I think GML has a big role to play in government, particularly with
location-based services," said Joe Sewash, GIS analyst in the Tennessee
Department of Finance and Administration's Office for Information Resources.
"If you have an agency with a lot of field work, for example, it would tend
to have a lot of different databases with explicit GIS links to them. GML
can be used to link a lot of traditional nonspatial data with this spatial
Ultimately, GML could change not just how agencies share data but how
they store it, making it possible to develop large databases of geographic
information that could be used to support multiple applications.
Government agencies usually generate and store geographic data with
only their own applications in mind. A city or county in Tennessee, for
example, would have data on rivers and streams that would likely be generated
and used differently than data in Oregon.
However, there are increasing numbers of applications that need access
to this kind of data on a global basis for such things as large-scale analysis
of geographic features, or for specific applications in areas such as environmental
By storing locally generated data in GML format and using XML-based
functions such as XLink and XPointer to provide links to these local databases,
GML could be used to create globally distributed databases of geographic
But some GIS vendors do not expect GML to have such a broad impact.
For one thing, existing GIS software already can store data in various
formats, said Dave Danko, senior consultant for Environmental Systems Research
Institute Inc. Data sharing is GML's "main reason for being," he said.
There's also the power of familiarity. Users have become adept at manipulating
and storing GIS data in its native format, said Bill Shelley, manager of
GIS and remote sensing software for ERDAS Inc. "The real potential of GML
is in Web-enabling geographic applications," he said. The development of
the basic GML standard is being overseen by the Open GIS Consortium, which
draws members from government agencies, research groups, oil companies and
However, despite the initial interest in GML, it's still unclear just
how much demand for GML-based Web services will develop, or how soon. Even
those vendors working within the Open GIS Consortium who are familiar with
the promise of GML are cautious about committing themselves too far in advance.
"ERDAS is not doing anything with it today because we want to see which
direction the market takes," Shelley said. "We'll certainly start using
it when we have customers that want us to support GML, but we haven't had
a large contingent even ask us about it so far."
From what he's heard, Neil MacGaffey, assistant director of MassGIS,
Massachusetts' Office of Geographic and Environmental Information, believes
it's far too early to predict how things will go."This standard is still
in development," he said, "and it is not clear that all GIS vendors are
necessarily going to adopt it."
Tennessee's Sewash also thinks it's too early to tell because "it will
take time for vendors to actually provide systems that take advantage of
[GML]." Nevertheless, given that XML will probably have a big impact on
traditional IT applications, the obvious synergy between GML and IT makes
it likely that GML will be used, he said.
Even though Shelley is cautious about the future demand for GML, he
thinks it stands a good chance of being widely adopted in industry. And,
since it typically is a "background" technology that enables applications,
it may eventually be employed without users being aware of it.
"I'm very optimistic about GML in the long run," Shelley said. "I suspect
it will be only a couple of years at the most until we start supporting
it in our products."
Robinson is a freelance journalist based in Portland, Ore. He can be
reached at firstname.lastname@example.org.
Brian Robinson is a freelance writer based in Portland, Ore.