Where Is It Exactly?

Everybody's got to be somewhere. And the emerging Geography Markup Language (GML) provides a standard, text-based way to describe that somewhere.

Text-based geographic-data description languages have existed for years among small pockets of users in different disciplines. But the description efforts never crossed disciplines and formats.

Starting With XML

GML makes that leap to a universal standard by taking as its starting point the emerging industry standard—Extensible Markup Language (XML)—says GML's principal author, Ronald Lake, president of Galdos Systems Inc., a geographic information system consultancy in Vancouver, British Columbia.


Just the Facts

What GML Offers
  • Using different map-styler software, two users could create entirely different-looking maps from the same GML data, because GML describes geographical data, not how it should be displayed.
  • Users don't have to buy expensive mapping software. All they need is a Web browser, such as Microsoft's Internet Explorer, which supports vector graphics (geometry represented as a picture rather than bits).
  • Panning and zooming are immediate. Anyone who has tried to pan or zoom a GIF or JPEG map online has had to wait while the server generates a new view. A GML-based map already contains all the data and, effectively, all the views.
  • One format works for many uses. GML data can be displayed on any XML-enabled device, such as a personal digital assistant or cell phone as well as on desktop PCs.
  • GML facilitates embedding of links in features. Click on the opera house on a map and you're automatically taken to the opera house's Web page.
Note: For more information, go to www.galdosinc.com/reasons_for_gml.htm Source: Ronald Lake, Galdos Systems Inc.

XML schema are definitions written in a specific XML syntax of tags, which describe a particular kind of content, such as a person's name or the products on an e-commerce Web site.

GML schema define how you put geographic information into XML, said Kurt Buehler, chief technical officer at industry group OpenGIS Consortium Inc. (OGC) in Wayland, Mass.

A GML tag describes content, such as the geographic coordinates and properties of a coastline. How the information appears—as a solid line, colored blue, for example—would be determined through a style sheet.

Whereas XML is based on a document, GML is based on geographic features, such as buildings, roads, rivers and mountainsides. OGC defines a geographic feature as "an abstraction of a real-world phenomenon; it's a geographic feature if it's associated with a location relative to the Earth."

Each feature has certain properties, such as a name or height above sea level. It also has geometries. A geometrically complex geographic feature may comprise multiple geometry types, such as points and polygons.

Features may be grouped together as a FeatureCollection: the entire mountain. And FeatureCollections can be grouped together as a single, larger FeatureCollection: a mountain chain.

But it's important to note what GML is not, says Lake. Geographic data isn't synonymous with the graphic interpretation of that data, such as a map. GML can be used to encode, store and transport geographic data. It can let applications accept spatial data from multiformat sources, manipulate it and combine it with nonspatial data and multiple formats of spatial data.

Map-styling software can locate GML elements and display them. By applying map-styler style sheets to GML data, users can represent combined data graphically as a map showing, for example, roads, school districts and population density.

"Many kinds of information have a geographic component," Buehler said. "If we can standardize on how that geographic component is presented, then we can integrate other data such as utilities information by using the GML schema to encode the geographic element."

Data, Not Pictures

OGC published GML 1.0 in May and includes it in its Web Mapping Specification (WMS) 1.0. WMS describes an interface that, via a Web browser, lets users build a map that merges data from multiple sources and formats. But the resulting JPEG or graphics interchange format (GIF) file is an image rather than manipulable geographic data.

OGC in WMS provides for GML as an output format, which lets users request the data on a geographic feature, such as a river or a road, rather than a map.

"The difference is that in the first instance, what you get is a picture of the data; with GML, you get the data," Lake said.

GML will also be an integral part of OGC's wireless Open Location Services (OpenLS) initiative, announced Oct. 30, to develop specifications for location-aware applications such as vehicle navigation, route determination and display, driver assistance and accident response, as well as map and feature display and interaction.

Bumps on the Road to GML Acceptance

Although GML is gaining support globally, there are still barriers to its adoption.

The Central IT unit of the U.K. government, which has standardized on document-tagging language XML, has also adopted GML 1.0 for exchanging geospatial data in its e-Government Interoperability Framework (e-GIF).

The U.S. Census Bureau has commissioned GML author Ronald Lake's company, Galdos Systems, to write a translator that will let GML users read the agency's extensive TIGER GIS database.

But some in the geographical information industry say they think GML's creators and supporters may be too ambitious and that they doubt the efficacy of one evolving standard built on another evolving standard.

Another sticking point is that there are actually two GMLs. One industry consortium, led by Tokyo-based NTT Data Corp., has developed Geography XML (GXML) to support location-aware services available via cell phones in Japan.

GML was created by Lake to support a broader range of applications and now involves other developers around the world.

Work on both standards has continued for two years concurrently but along diverging paths, and it was only at the Oct. 6 meeting of the OGC that the two groups agreed to work to merge the two during the next six to 12 months.

Twenty years ago, the issue would have been of interest almost solely to government agencies, by far the main users of GIS. Not so today.

By next October, U.S. phone carriers will be required by law to be able to identify the location of cell phone callers. Similar requirements are being considered for Europe.

GML is being built into other OGC standards that will facilitate such location-aware applications.

According to a report by San Jose-based Dataquest last year, "increased corporate use of GIS technology to improve customer service and cut costs is expected to help propel the overall market from $862 million [in 1998] to $1.7 billion in 2000."

Copyright © 2000 IDG Communications, Inc.

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