Geography Markup Language (GML) 2.0 – Enabling the Geospatial Web

Ron Lake is the President of Galdos Systems Inc., a company that provides advanced software tools to enable the delivery of services over the Internet, in particular GML and Application Service Providers (ASP). Ron Lake is a well known innovator in the field of geo-spatial information systems and the primary author for Geography Markup Language (GML). Ron Lake has also written several articles on GML for a variety of online publications. His company won a contract with the Census Bureau to provide GML-based translations for TIGER files.

1. Introduction

On February 20, 2001, the OpenGIS Consortium (OGC) published Version 2.0 of the Geography Markup Language (GML), thus laying the foundations for the development of a Geo-spatial world wide web. Since the publication of GML 1.0 in May 2000, interest in GML 2.0 has developed rapidly. Organizations and individuals in every corner of the globe are now pursuing GML technology development.

This article is intended to help you understand the potential impact of GML 2.0 on both the existing world of geo-spatial technology as well as on the emerging world of Location-based Services. While we will take a few peeks “under the hood”, this article is focused more on the implications of GML 2.0 then on its internal workings.

2. Building on XML

Like its predecessor, GML 1.0, Geography Markup Language 2.0 builds on the evolving world of XML technology, a technology that has impacted almost every area of information processing.XML is a means of encoding data in text. A GML 2.0 encoding of a road segment looks something like the following.

<uka:Road  fid ="highway11">
<uka:numLanes>3</uka:numLanes>
<uka:surfaceType>gravel</uka:surfaceType>
<gml:centerLineOf>
<gml:LineString srsName = "epsg4361">
<gml:coordinates> …. </gml:coordinates>
</gml:LineString>
</gml:centerLineOf>
</uka:Road>

XML technology today is extremely widespread. It is embedded in the browser on your desktop. It is the “lingua franca” of emerging e-business frameworks, and it powers the generation of thousands of web sites. Why has it become so successful? How did we move so rapidly from merely marking up documents for publication to using XML as a general tool for data description?In part the explanation lies in the evolution of the Internet itself from an environment of distributed pages of information to one of distributed business services. While this evolution is only in its infancy, the new Internet demands tools with greater expressive power and ones that integrate together many kinds of information. The Internet of distributed information relied mainly on text and imagery. The new Internet demands the ability to express the elements of automated business interaction, from invoices and purchase orders to currency and other types of financial transactions. Moreover the Internet has moved into every type of business and with that has come the need to express not only the financial aspects of business interaction but also the specialized contents of differing business domains.The world of the Internet is also a world of information collision, and yearned for integration and fusion. Tap a search engine and you see information of often bewildering diversity. Information collides and we think of new ways to integrate and extend it. This in turn demands technologies that thrive on information integration rather than isolation.In political terms the Internet has been a great leveller. E-mail reaches across the spaces of the globe and knows nothing of the boundaries between states, nor that between individuals. It fuses us with one another.

There is also a world of reality outside the Internet that especially in geo-spatial terms drives us toward data integration. Events in the world do not take place in isolation. Neither do they align themselves with the boundaries of government departments or provinces or states of national governments. A flood in El Salvador or an earthquake in India ripples around the world. The flood does not care that there is a Ministry of Forests, nor a Ministry of the Environment, or that the administration of one is not integrated with the other. The flood tears through the fabric of the country merging the trees and the soil and the water, mingling agriculture and industry, homes and social infrastructure. To respond to the inherent integration of the world we must integrate our information resources.

The explosion of the Internet has also demanded that our technologies be extensible and comprehensible. This was one of the lessons of HTML. A simple text based language that has come to dominate the world. Visibility and comprehensibility are increasingly demanded in a world that is already too complex.

The character of XML has in many respects been shaped by responses to these issues. XML like HTML is text based. It can easily be read and understood by human beings. Since it is text, XML can readily combine together a wide variety of data types including text, finance, graphics, audio, voice and more. This means that geographic data can readily be integrated with a wide range of non-geographic data types thus greatly enhancing the value and accessibility of spatial information.

XML technology has also evolved in response to the limitations of HTML. While enormously successful, HTML and the World Wide Web are not without shortcomings. All of us are familiar with the “404” message displayed when a broken hypertext link points us to a non-existent page, or to the different appearances that can derive from viewing a single web page in different web browsers.

Where HTML mixes content and presentation together, XML strictly separates the two. XML, the encoding standard deals only with data structure. This simple fact liberates it from mere document description to become a general tool for data description. GML continues this fundamental idea.

GML is concerned with the description of geographic content. GML must be styled for presentation. Presentation may mean being styled to a graphical form such as a map, but equally it could mean being styled to text or even to a sequence of voice instructions.

HTML provides a simple form of linking one web page to another. This linking mechanism is one of the key foundations of the web. The link is established through an anchor or bookmark embedded in the target page and a link reference embedded in the source page. Note that such a link associates only two resources (the source and target pages) and it does so in a unidirectional manner (source to target). Note further that the HTML link is a coarse grained mechanism. It only allows one to point to complete web pages and only to single points in those web pages.

XML goes much further. XML provides a mechanism for linking multiple resources into a complex association. XML links also can be traversed in both directions. XML further enables fine-grained associations to be constructed. Where HTML linking only supports the linking or association of web pages, XML linking can associate single XML elements or even element fragments. As we shall see, this has profound implications for GML’s ability to build associations between spatial features.

Since XML separates presentation and content, XML technologies have developed for style transformation. These are now available for a wide variety of devices from the desktop to hand held and wireless PDA’s.

The ubiquity of XML has other implications for GML. With more and more types of data being expressed each day in XML, the ability to combine and associate geo-spatial data with hundreds of other data types, one of the long objectives of the geo-spatial community, moves closer to reality.