Chapter Five | Context and Rationale | Organisational Approach | Implementation Approach
Chapter Five: Geospatial Data Visualisation -- Web Mapping
Contributors: Steve Blake, Australia; Frank Lochter, Germany; Allan Doyle, USA
Introduction
This chapter documents simple web mapping concepts and tools that enable the visualization of geospatial information from various organizations and servers across the World Wide Web. The linkages with Chapter 4 - Geospatial Data Catalogs, are also explored. Discussed are the current best practices related to on-line mapping, and the progress of the OpenGIS Consortium's (OGC) Web Mapping Testbed (WMT) to realize the dream of true inter-operability and disseminating a web mapping specification for the vendors to adopt and promulgate.
Consider these desires:
- Do you want to view your information on a map online? Perhaps either as a simple (one map at a time) view or to overlay views from other sources together to produce a customized map product on your computer screen?
- Do you want to post a map layer from your in-house GIS or image processing system onto the Web for others to see? Do you want to provide views of your metadata so that your clients can picture the data or product you are responsible for?
If the answer to these questions is yes, then you are probably interested in Web Mapping.
Context and Rationale
The rise of the Internet and specifically the World Wide Web (WWW) has created
expectations for ready access to geospatial information on the Web through a
common web browser. Mapping on the Web includes the presentation of general
purpose maps to display locations and geographic backdrops, as well as more
sophisticated interactive and customizable mapping tools. The intention of
online or Web Mapping is to portray spatial information quickly and easily for
most users, requiring only map reading skills. Web mapping services can be
discovered through online directories that serve both spatial data (through
metadata) and services information (see for example the
OGC Catalog Services draft specification). In fact, web mapping services are often used to assist users in geospatial search systems, showing geographic context and extent of relevant data against base map reference data.
Web Mapping implemented as a set of proprietary systems works fine as long as everyone you deal with both internally within your organization and externally utilizes this same proprietary software. Because of this obvious particular limitation the Open GIS Consortium developed a non-proprietary web mapping approach based on the concept of interoperability. The topic of this chapter is not complex on-line GIS, but simple web mapping concepts and tools, i.e. part of a portrayal service to show spatial information on-line when the information originates from several discrete data/ map servers (commonly from different organizations).
Open GIS Web Mapping Activities
The sudden rise of web mapping over the last two years (cf. GIS Online :
Information Retrieval, Mapping, and the Internet by Brandon Plewe - OnWord
Press; ISBN: 1566901375) is demonstrated in the interoperability vision
held by the Open GIS Consortium's Web Mapping Testbed initiative. In the OGC, expert GIS and web mapping technology users work with GIS software vendors, earth imaging vendors, database software vendors, integrators, computer vendors and other technology providers to reach agreement on the technical details of open web mapping interfaces that allow these systems to work together over the Web.
Consensus among vendors in the OGC's Web Mapping Testbed has created ways for vendors to write software that enables users to immediately overlay and operate on views of digital thematic map data from different online sources offered though different vendor software. The Web Mapping Testbed has delivered, among other specifications, a set of common interfaces for communicating a few basic commands/ parameters that enable automatic overlays. This set of interfaces is known as the OpenGIS(r) Web Map Server Interfaces Implementation Specification
(http://www.opengis.org/techno/specs/00-028.pdf)
and was developed by over 20 participating organizations.
The Web Map Server (WMS) specifications offer a way to enable the visual overlay of complex and distributed geographic information (maps) simultaneously, over the Internet. Additionally, other OGC specifications will enable the sharing of geoprocessing services, such as coordinate transformation, over the WWW. Software developers and integrators who develop web mapping software or who seek to integrate these capabilities into general purpose information systems can add these open web mapping interfaces to their software.
"Web Mapping" refers, at a minimum, to the following actions:
- A Client makes requests to one or more Service Registries (based on the OpenGIS Catalog Services Specification) to discover URLs of Web Map Servers containing desired information.
- Service Registries return URLs and also information about methods by which the discovered information at each URL can be accessed.
- The client locates one or more servers containing the desired information, and invokes them simultaneously.
- As directed by the Client, each Map Server accesses the information requested from it, and renders it suitable for displaying as one or more layers in a map composed of many layers.
- Map Servers provide the display-ready information to the Client (or Clients), which then display it. Clients may display information from many sources in a single window.
The OpenGIS Web Mapping Specifications address basic Web computing, image access, display, and manipulation capabilities. That is, they specify the request and response protocols for open Web-based client / map server interactions. The first of these specifications, described below, are the product of OGC's successful Web Mapping Testbed (Phase 1). They complement the already-available OpenGIS Specifications such as Simple Features and Catalog Services, as well as ISO metadata standards to provide the foundation on which pending OpenGIS Specifications will build an increasingly robust open environment for Web mapping. A second interoperability initiative, WMT-2, is currently engaged in this process, defining Web Feature Servers, Web Coverage Servers, and extensions to the Web Map Servers that will allow a higher degree of control over the symbolization.
Today, the WMS 1.0 defines three main interfaces that support Web Mapping:
GetMap, GetCapabilities and GetFeatureInfo; these
have been demonstrated at the conclusion of Phase 1 (May - September 1999) of
the Web Mapping Testbed and have been released to the public in April 2000.
GetMap specifies map request parameters that allow multiple
servers to produce different map layers for a single client.
GetCapabilities explains what a map server can do (so integrators
know what to ask for). GetFeatureInfo specifies how to ask for more information about web map features.
These interfaces provide a high level of abstraction that hides the "heavy lifting" in the Web Mapping scenario. The heavy lifting includes finding remote data store servers, requesting data from them in specifically defined structures, attaching symbols intelligently, changing coordinate systems, and returning information ready to be displayed at the client - all in a matter of seconds.
Servers conforming to OpenGIS WMS 1.0 will geo-enable Web sites and mobile devices for many new applications of geospatial technology. Consider any of the application domains listed below. Wherever the purchasers of the technology have chosen not to limit their users to a solution based on single vendor client/server pairs, these uses of geospatial data will depend on interfaces that conform to the OpenGIS Web Map Interface Specification
- Business siting, market research, and other business geographic applications
- Cable, microwave, and cellular transmission installation planning
- Civil Engineering
- Education/training, distance learning, multi-disciplinary research collaboration
- Electronic libraries, electronic museums and galleries
- Emergency road services and 911 emergency response systems
- Environmental monitoring, global and local
- Facilities management
- Global disaster/emergency/crisis management
- Health care: telemedicine, better/faster care for rural trauma victims, patient monitoring, etc.
- Intelligent vehicle highway systems (IVHS)
- Maintenance of one's information context and connection (personal logical network) as one moves through space, bridging media and modality; mapping electronic locations of addresses to their physical locations; using concepts of reach space, co-location, and near-by.
- Military applications: surveillance, planning, training, command/control, logistics, targeting
- Municipal public works maintenance and administration
- Natural resource discovery, exploitation, and management
- Navigation
- Precision farming (GPS-guided controlled delivery of nutrients and chemicals based on Earth imagery or automated GPS-located soil or crop sampling)
- Product distribution/warehousing optimization
- Public safety - fire and police departments
- Recreation: hiking, boating, etc.
- Science: climate research, agronomy, biology, ecology, geology, and others
- Security monitoring and intrusion response
- Special wayfinding for elderly and disabled
- Telecommunications network planning -- mobile communications
- Transportation planning
- Urban and regional planning
- Water resource management
There is a productive recent trend within the OGC to use Interoperability Initiatives like the Web Mapping Testbed to rapidly produce OpenGIS Specifications, as opposed to creating all of them through a traditional committee process. Phase 2 of the Web Mapping Testbed, scheduled to be completed in late 2000, is focusing on map authoring and publication, integrating graphical data and data elements (legends, symbolization, etc.), clients that can exploit XML-encoded information, further work on catalog and discovery services, and work on transporting XML encoded data over the Internet.