Understanding the structure and usage of an Extensible Markup Language (XML)-based markup language can be a daunting task for someone who is new to the vocabulary and specifications of the language. This book attempts to smooth this path for Geography Markup Language (GML), and succeeds in doing it well.
A critique of the markup language itself would take up too much space, so I will restrict my comments to this book, leaving the language for another time and place. This is actually two short books in one, intended for both managers and developers. It consists of two different sections, one for each target audience. It covers GML 3.0, which is a substantial improvement over earlier versions.
The first section consists of seven chapters, forming an introduction to the fundamental concepts of GML and its uses, intended mainly for the project managerial audience. This introduction and overview section is followed by a technical section discussing the nuts and bolts of the language, written for the potential developer or schema designer.
Section 1 (“Introduction”) starts with a survey of the language’s origins and its contexts, in chapter 1, followed by a very brief outline of relevant XML technology, in chapter 2. The next two chapters introduce the basic structure and key concepts of the language. The last three chapters of this section discuss issues arising in new deployments of GML, and describe the placement of the language in the context of Web services, and in different application areas. Understanding this section should be easy, even for a reader without much technical knowledge about XML and Web technologies: a knowledge of the basic concepts underlying XML and Web services will suffice. Brevity is the governing rule of this section, but there are lists of references at the ends of the chapters (mostly World Wide Web Consortium (W3C) and OpenGIS uniform resource locators (URLs)).
The second section (“Technical Reference Guide”) contains substantially more technical detail than the first. After commencing with another introduction to the fundamental concepts of GML (this one written for the technically oriented reader), it proceeds to describe the structure of the language in terms of its object model, and the schemas constituting its core. This is followed by eight “how-to” chapters, each a mini-tutorial on different capabilities provided by the language, and each with plenty of exemplary schema fragments and illustrations. Following this is another look at GML and Web services, in chapter 19, and a quick look at integrating GML-based information with the relational database storage of geographic information, intended for software developers and analysts who need to deal with these issues during application design and development. Four appendices follow: a tabulation of the GML core schemas, a compendium of resources (refer!ences, tools, and so on), a glossary, and a tutorial on Altova’s popular XMLSpy software tool. An accompanying CD contains examples, and the GML 3.0 specifications and schemas. The “Technical Reference Guide” is more of a tutorial than a reference manual, and as such should be a useful learning resource for the GML modeler or developer. Familiarity with XML and XML schema is essential to get the most out of this section, and at least some exposure to Web services and geographical information systems (GIS) would also help. This section does a competent job of explaining things to the technical user; the coverage and depth are both good, and the exposition is readable and comprehensible.
This book manages to pack a great deal of useful information and numerous hints into less than 400 pages, and succeeds in doing this without wandering into a dense thicket of impenetrability. It would be a useful addition to the library of anybody, developer or manager, who knows XML, and wants to learn about GML.
