Internet GIS refers to GIS services that use the Internet as a primary means to access data, conduct spatial analysis, and provide services related to geographic information (Peng 1999; Peng and Tsou 2003). These services are typically GIS applications disseminated through the Internet for interactive use by web users. Thus we use the terms "services" and "applications" interchangeably in this manuscript. The past decade has witnessed passionate interests of researchers, practitioners, and vendors in exploiting various ways to promote the accessibility to geospatial data and services on the Web. This study concerns two important issues that are closely related to Internet mapping services. They are the issues of interoperability and Open source.
Motivated by an emphasis on the democratization of cartography, which aims to empower individuals with electronic tools to map and analyze spatially referenced data (Morrison 1994; Kumar 2000), we are particularly interested in Internet GIS solutions that rely on open source resources, as they are most likely to be developmentally feasible, both financially and technically. The second motivation of the study is the universally acclaimed goal of interoperability.
Generally speaking, interoperability is the ability of a system, or components of a system, to provide information portability and inter-application cooperative process control (Zhang et al. 2003). It boils down to the ability to share data and other resources among different systems. In the literature, the word "interoperability" may confer two types of meaning. The first refers to a data set or other system component being able to be used by different systems. The second refers to the ability of a system to utilize a range of data formats (Laurini 1998). Our study concerns the second type of interoperability. We present a standard-and-open-source-based framework for Internet mapping services that enable direct and interactive display of both vector and raster geographic data of a variety of formats.
Many GIS vendors have developed proprietary Internet GIS software. Examples include Environmental Research Systems Institute (ESRI)'s ArcIMS and ArcGIS Server, Autodesk's MapGuide, Intergraph's Geomedia WebMap Professional, MapInfo's MapXtreme, ER Mapper's Image Web Server, and GE SmallWorld Internet Application Server. However, developing an application with proprietary Internet GIS software packages may be problematic in some situations for any number of reasons. First, it tends to be costly. The associated software cost inhibits many potential Internet GIS services, especially those intended to benefit users in the general public freely. Secondly, proprietary programs use proprietary data formats. Such restriction on data formats, if not linked with standardization, could greatly hamper the interoperability of the Internet GIS service. Lastly, although some of these Internet GIS programs allow the developed Internet GIS service to have sophisticated functions, the geospatial data are either prepared as or converted into raster images when they are delivered to the client sites. Any request that causes a change in the spatial extent of the map calls for a new image to be created, transported via the network, and re-rendered. This actually often causes substantial redundancy in data processing and imposes a heavy traffic load on the Internet. More importantly, it impairs visualization quality due to slower responses and resolution-dependent viewing quality. This situation worsens when users give frequent interactive commands to the system.
In response to the concerns and limitations described above, we seek to investigate alternative approaches to building Internet GIS services. Our study focuses on Internet mapping services, while most discussions are generally applicable to Internet GIS applications. We provide a systematic review of Internet mapping techniques. With a discussion of the issue of interoperability, we also analyze the important role of international standards in developing interoperable Internet mapping services. Finally, we present a general framework of an interoperable Internet mapping service that can take vector and raster data of various formats and render them in their original logical model (i.e., vector or raster) of the geographic world.
This paper is organized as follows. The next section discuses the interoperability of Internet GIS services, focusing in particular on the contribution of standards-based approaches to interoperability. Then we present reviews and critiques of existing Internet GIS architectures and associated techniques. The general framework is introduced in the following section, which is followed by the demonstration of two preliminary, implemented prototypes. In the concluding section, we analyze the potential benefits of such a framework for Internet mapping services and highlight a few research questions for future study.
Interoperability and Standards
Thanks to modern geospatial technologies, huge volumes of geographic data created by a variety of institutions or individuals are increasingly available. It is often difficult to reuse these data or other system components in different GIS applications, however, due to the diversity of geographic data models and data encoding techniques, or due to heterogeneity in semantics and metadata, etc. (Devogele et al. 1998). Following the second commonly accepted general definition of interoperability stated earlier, this study focuses on the interoperability of Internet GIS in the context of the ability of such a system to manipulate, access, exchange, and share geospatial data from various resources for any kind of application over the Internet.
The underlying cause of interoperability in general is heterogeneity. Bishr (1998) identified three distinct facets of heterogeneity in a multi-GIS environment: syntactic, semantic, and schematic. The syntactic aspect puts significant emphasis on the paradigm of database management systems in use, as it is pertinent to the capability of one system to understand the meaning of data from another system. For instance, it is difficult to transfer data between a relational database and an object-oriented database because data are modeled and organized in different ways. Semantic heterogeneity happens when one geographic fact has more than one description in different databases or the same description in different databases has different real-world meanings. Thus, semantic interoperability deals with inconsistencies caused by variations in conceptualizations and interpretations of geographic worlds. Finally, schematic heterogeneity refers to incompatibility among systems concerning conceptual classification and hierarchical structure of the real world.
While all three aspects are important, this study focuses on syntactic interoperability in Internet GIS services as it is directly associated with the second type of meaning of interoperability which is the focus of our discussion. Good syntactic interoperability makes it easier to share and transfer data between different systems. Approaches to improving such interoperability often involve the establishment of standards to be followed by various organizations who issue geographical data. Standardized metadata and data formats, for instance, can facilitate the processing of geospatial data in various environments as it enhances the chance of mutual understanding.
By adopting techniques and specifications that conform to open geospatial standards, an Internet GIS service will be well positioned to achieve better syntactic interoperability. An open standard means the standard is developed in an open, international, participatory process. It includes free, public access to all interface specifications. The "openness" in the "Open Standards" is particularly important, as the goal is to promote the standards as widely as possible, not just among a few organizations, departments, and institutions.
The Open Geospatial Consortium (OGC) is leading the development of such open standards for geospatial data and technologies. The OGC, formerly known as OpenGIS Consortium, is an international industry consortium with hundreds of members, including companies, government agencies, and universities participating in a consensus process to develop publicly available interface standards (specifications). The OGC has initiated several Web mapping interoperability initiatives and implementation specifications including the Geography Markup Language (GML) specification, the Web Map Service (WMS) implementation specification, the Web Feature Service (WFS) implementation specification, and others (OGC 2007a). The focus of these specifications is to define interface standards that allow seamless and vendor neutral access to, and interchange of, GIS data.
Many companies in the GIS industry, such as ESRI, MapInfo, and Galdos Inc. are updating their products to conform to these open standards. For example, the data interoperability extension of ArcGIS, a leading commercial GIS program, is now supporting several OGC specifications, such as GML, WFS, and WMS. We argue that standard-based approaches are viable solutions for interoperable Internet GIS services.
Techniques for Developing Web-based Mapping Services
There are at least two distinct processes involved in providing Internet GIS services, particularly mapping services. The first is transmitting GIS data from the hosting site (server) to the user side (client) through the Internet. The second process is rendering the graphics of spatial data on the client side. Below are brief overviews of techniques involved in each process.
GIS Data Transfer via the Internet
The dominant architecture of Internet GIS is the Client/Server model on which the world wide web and most other Internet services are based. In the early days of Internet GIS, the solutions were implemented on either the...