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GIS Web Services

Geographic information system (GIS) Web services represent an evolution of traditional standalone GIS applications available in desktop environments with database back-ends for storing data, and they provide the components for a spatial data infrastructure (SDI). GIS Web services are self-contained application components that are published and accessed over the World Wide Web using standard protocols such as HTTP, SOAP, and XML. Each service performs a specific GIS function as part of a loosely coupled and distributed geographic application (also called a distributed GIS).

The functionalities provided by GIS Web services can be classified into four basic categories—geographic data discovery, access, computation, and visualization—which are described in the following sections. Additional functionality is required to orchestrate the interaction among several services in order to provide specific applications. GIS Web services categories and orchestration are illustrated in Figure 1.

The main challenge in using GIS Web services is that they require an agreement on a common architecture and a portfolio of standard Web services to support reusability and create applications that share the use of services. Emerging interoperability specifications, in particular from the Open Geospatial Consortium (OGC), have addressed this challenge.

Discovery

Discovery services enable the ability to publish, query, and retrieve descriptive metadata for resources, which can be data and services. A discovery service is significant for the establishment of an SDI, as it enables users to discover which data and services exist and could potentially be used in a given application. Metadata offered by such a service should include descriptions of the content and functionality, quality information, and access details and/or restrictions that enable a user to evaluate fit for the purpose of the resource.

Figure 1 GIS Web services categories

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Source: Author.

A prominent specification of discovery services is the OGC's Catalogue Services Specification, which offers a range of service interfaces that support the discovery, access, maintenance, and organization of catalogs of geospatial resources. Various profiles of the catalog specification offer different bindings for protocols—among others, HTTP and SOAP.

Current catalog implementations mainly rely on matching keywords with metadata entries, leading to low recall and/or precision during search. They do not yet make use of semantic Web technologies.

Access

A prerequisite for a distributed GIS application is the possibility to access geographic data through services. This type of service provides access to any kind of geographic data (represented both as vector and as raster) and enables queries to provide subsets of data, for example, based on types, attribute values, or spatial or temporal extent. In addition, it can provide functionality for inserting and updating.

OGC provides several specifications of services for geodata access. The Web Feature Service (WFS) and Web Coverage Service (WCS) provide access to vector data and coverages, respectively. These services provide access (read and write) and enable querying of geographic data using a standardized interface. To support exchange of data between a client and a service, the Geography Markup Language (GML) has been proposed by OGC as an encoding of geographic data. GML is an XML (EXtensible Markup Language) grammar that can be used to represent both discrete features (vector) and continuous-coverage surfaces (raster). Keyhole Markup Language (KML) is an alternative XML-based encoding, now also an OGC standard. WFS uses GML as data exchange encoding, and WCS can use GML but also supports traditional binary formats such as JPEG, (geo)TIFF, and PNG.

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