GIS Web Services

The convergence of high bandwidth and emerging open standards is reconfiguring the traditional GIS model around web services.

Crucial to these changes is the ongoing work at the Open Geospatial Consortium, OGC, producing a series of GIS Web Service specifications. These OGC Web Services include Web Map Service (WMS), Web Feature Service (WFS), and Web Coverage Service (WCS). Alongside these service specifications are a raft of supporting standards like Geography Markup Language, GML, Styled Layer Descriptors, SLD, or OpenGIS Filter Encoding.  Since 1994 OGC has been successively producing standards moving toward a goal of network interoperability for GIS. These standards are maturing toward a complete network enabled GIS producer model.

In parallel, over at the World Wide Web Consortium, W3C, additional standards have been released for a larger audience as consumer or rendering models. These include the ongoing oversight for HTML as well as new standards like SVG, XHTML, and XSLT. Namespaces abound, but arising from these open standards is a new kind of GIS based on interoperability across the Internet.

Web GIS is based on open standards

OGC Web service specifications have established building blocks for exposing geospatial data sources in an open standards approach. These provide a core of producer standards needed for interoperability.

Modular services

The suite of Web services specifications from OGC makes modular service aggregation possible. Any number of WFS, WMS, WCS sites can be combined to create custom aggregations specific to user client requirements. Data sources no longer need to be replicated at each server with all the subsequent problems of data maintenance and updating. The service producers control the maintenance, as they should, while data consumers can concentrate on the specifics of their interface. For example Terraserver WMS DOQ or USGS NED relief imagery can be combined with WFS road features from a state DOT and overlaid with a local county assessor’s WFS of parcel polygons for an appraisal portal. None of the data needs to be maintained by the portal which can focus on the appropriate interface for the user needs.

Web Service standards are modular

Open source options

The open source community is providing tools for OGC compliant producers and clients. One important open source project is GeoServer. Leveraging work from PostGIS, Java Topology Suite, and Geotools communities, GeoServer provides OGC compliant WMS and WFS access to several popular data store types. The list of data stores includes ArcSDE, GML, MySQL, PostGIS, Oracle, VPF, and Shapefile. Work is continuing on WCS capabilities. GeoServer is available under the GPL 2.0 license.

PostGIS (GNU GPL license) => GeoServer (GNU GPL license) => W3C SVG provides a complete stack for OGC compliant modular GIS web services.

The use of w3c SVG to view web services from the browser provides access to the full range of OGC web services. SVG can use WMS generated images behind vectors or merge multiple WFS sources into a client view. The addition of event listeners creates a dynamic interface viewable through an ordinary browser. SVG can also be customized to take advantage of WFS transactions, which allow dispersed clients to update and maintain geospatial data stores from the field.

Spatial Intelligence in the Enterprise

Decoupling geospatial data stores from the render view makes sharing across diverse organizations much less costly. In a wide area intranet, typical of most enterprises, data stores are not monolithic but dispersed across the enterprise. Often it is difficult to find and share spatial intelligence stored in a wide range of data formats and services specific to organizational branches. In an open standards approach each organizational branch continues to develop and maintain their own data in whatever database or format is right for their own processes. However, adding an OGC compliant web service, OWS, publishes a subset of their data valuable to the enterprise as a whole. Since OWS is a published open standard, it is not subject to the vagaries of any single vendor. Other parts of the organization can view data resources combined from many departments increasing the value of spatial data resources.

National Spatial Data Infrastructure

Public web services are becoming a national resource. Web services from NASA, NOAA, JPL, USGS, FEMA, and the Census Bureau are providing access to the vast data resources compiled by the federal government. A new kind of geospatial infrastructure envisioned by the National Spatial Data Infrastructure, NSDI, is evolving from web based services. Homeland security needs as well as natural disasters like hurricane Katrina highlight the need for policy incentives to close the gap between local, state, and federal data resources as well as industry. Critical infrastructure is a necessary part of the interoperability equation. As these policy level decisions are implemented many industries will be affected by federally mandated data publishing requirements. It is important to develop open standards based services ready to meet public mandates as well as internal organizational needs.