Where Is It Exactly?

Computerworld - 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.

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.