This chapter focuses on the data, specifically on the spatial information discovery, display, and exchange over the Web. The development of GIS metadata standards, the basics of Extensible Markup Language (XML) and Geography Markup Language (GML) will be introduced and discussed. . This chapter start with a brief discussion of HyperText Markup Language (HTML) and its shortcomings, and follows with a discussion of the basic elements of XML and GML. HTML is a web-based language which can embed texts, images, and multimedia information in a single, integrated format. HTML documents can be interpreted and displayed in a Web browser. The original design of HTML focuses on the display of information rather than information search, discovery, and exchanges. The lack of information query and search functions is the major limitation of HTML documents, especially for geospatial data and maps.
GML is a markup language that is based on the XML standard to construct structured spatial and non-spatial information to enable data sharing and interchange over the Web.

Back to Top

Chapter 7 Geographic Markup Language

7.1 Introduction: Problems of Spatial Information Discovery, Display, and Exchange over the Web

      7.1.1 Problems

      7.1.2 Limitations of HTML

      7.1.3 Current and Potential Solutions

7.2 What Is Geography Markup Language (GML)

      7.2.1 GML Is A Means to Encode Geospatial Data

      7.2.2 GML Is a Means to Transport Geospatial Data Over the Web

      7.2.3 GML Is a Means to Store Geospatial Data

      7.2.4 GML Is NOT a Presentation Language for Data Display

      7.2.5 GML Is NOT A Programming Language for Data Processing

7.3 Characteristics of GML

      7.3.1 Text-Based, Open and Non-proprietary

      7.3.2 Extensible Elements

      7.3.3 Strict, Expandable and Enforceable Hierarchy Structure

      7.3.4 Separation of Structure from Presentation

      7.3.5 Separation of Content from Relationships

      7.3.6 Interoperable GML-Based Services

7.4 Markup Basics of GML

      7.4.1 Basic Building Blocks: Elements, Attributes and Values

      7.4.2 XML Declaration and Comments

7.5 Simple Feature Data Model Used in GML
7.6 Document Type Declarations in GML

      7.6.1 Element Declarations

      7.6.2 Attribute Declarations

      7.6.3 Entity Declarations

      7.6.4 Internal parsed entities

      7.6.5 External parsed entities

      7.6.6 Unparsed entities

      7.6.7 Parameter entities

      7.6.8 Shortcomings of DTD

7.7 GML Schemata in GML 2.0

      7.7.1 Element and Attribute Declarations in XML Schema

      7.7.2 Simple Element Types

      7.7.3 Complex Element Types

      7.7.4 Namespaces

      7.7.5 Referencing to different XML Schemata Using Namespace

      7.7.6 Encoding features without geometry in GML 2.0

7.8 Geometry Schema in GML 2.0

      7.8.1 Encoding Coordinates

      7.8.2 Geometry Elements

      7.8.3 Primitive Geometry Elements

      7.8.4 Geometry Collections

      7.8.5 Encoding Features with Geometry

7.9 Xlink Schema in GML 2.0

      7.9.1 Basic Concepts in XLink

      7.9.2 XLink Element Types and Attributes

      7.9.3 Simple Links

      7.9.4 Extended Links

      7.9.5 Use XLink to Encode Feature Collections

      7.9.6 Use XLink to Encode Feature Associations

7.10 Making Use of GML Data on the Web

      7.10.1 Making Vector Maps on the Web with GML Data

      7.10.2 GML Web Applications

      7.10.3 Extracting Features from GML Documents

7.11 Summary: GML and Internet GIS


Back to Top

  • The FGDC Metadata Standards (
    Metadata, which is normally be recognized as "data about data", describe the content, quality, condition, and other characteristics of data. In this website of Federal Geographic Data Committee who coordinates the development of the National Spatial Data Infrastructure (NSDI), you can follow the links to get detailed information on geospatial metadata standards, to get yourself familiar with metadata through the metadata tutorial and to learn the multiplemetadata tools available. FGDC approved the Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998) in June 1998. You can also visit the FGDC Clearinghouse to learn more about how metadata is used in Internet-based clearinghouses to search for specific data sets.

  • XML Specifications (
    Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). Due to its characteristics that XML can not only be used to describe the contents of the information through tags but also the structure of the dataset, it has been playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere.This page describes the work being done at W3C within the XML Activity, and how it is structured. W3C is abbreviated for World Wide Web Consortium. It is aimed to develop interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential. Work at W3C takes place in Working Groups. You can hyperlink to XML core working group public page for XML1.1 specifications and other document, such as XML namespace, etc.

  • SGML Resource (
    SGML, Standard Generalized Markup Language, is an enabling technology used in applications such as HTML. As a markup language, SGML defines a specific vocabulary (tags for elements and attributes) and a declared syntax (grammar defining the hierarchy and other features). This page provides useful links to SGML resources such as specifications, draft reports and publications, you may also join some of the discussion forums to post your questions or opinions.

  • The metadata model of the Alexandria Project (
    Alexandria Project was started in 1995 to develop the Alexandria Digital Library, a working digital library with collections of geographically referenced materials and services for accessing those collections. This page devotes specifically to illustrate the metadata model applied in the Alexandria Project, a digital library. Papers focusing on the metadata for spatial data in digital form as well as the documents and document of metadata standards and specifically of Alexandia can be explored. In addition, the metadata tools section bring you to those hyperlinks where you can use those tools to create metadata for your own purposes.

  • GML 2.0 ( )
    The Geography Markup Language (GML) is “an XML encoding for the transport and storage of geographic information, including both the spatial and non-spatial properties of geographic features (GML 2.0, OGC Recommendation Paper, Feb 2001).” It is a recommended standard to encode or markup spatial and non-spatial information in XML format by the OpenGIS Consortium (OGC). This pages provides detailed introduction on the GML 2.0 specifications. Besides, it also provides workable examples of application schema for reference. It is a good site for beginners to start their tour of GML.

  • Vector Markup Language (VML). W3C. (
    VML is an application of XML. This page first gives an overview of the way VML is organized and how it interacts with both XML and HTML as defined by the HTML 4.0 Specification and then presents detailed technical definition of the behavior of every VML element and the permitted and recommended behaviors for all applications.

Back to Top







This Website is hosted by


For comments about the website, please contact



Copyright © 2002 Department of Geography, San Diego State University
All Rights Reserved