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Implementation Approach

By way of introduction to implementations of Web Map Servers, the following is excerpted from the WMS 1.0 specification:

A Map Server can do three things. It can:

  1. Produce a map (as a picture, as a series of graphical elements, or as a packaged set of geographic feature data),
  2. Answer basic queries about the content of the map, and
  3. Tell other programs what maps it can produce and which of those can be queried further.

To first order, a standard web browser can ask a Map Server to do these things just by submitting requests in the form of Uniform Resource Locators (URLs). The content of such URLs depends on which of the three tasks is requested. All URLs include a Web Mapping Technology specification version number and a request type parameter. In addition,

  1. To produce a map, the URL parameters indicate which portion of the Earth is to be mapped, the coordinate system to be used, the type(s) of information to be shown, the desired output format, and perhaps the output size, rendering style, or other parameters.
  2. To query the content of the map, the URL parameters indicate what map is being queried and which location on the map is of interest.
  3. To ask a Map Server about its holdings, the URL parameters includes the "capabilities" request type.

Each of these will be described in further detail later. We first provide some sample URLs and their resulting maps:

This requests a US National Oceanographic and Atmospheric Administration AVHRR image:

http://a-map-co.com/mapserver.cgi?WMTVER=1.0.0&REQUEST=map&
SRS=EPSG%3A4326&BBOX=-97.105,24.913,78.794,36.358&
WIDTH=560&HEIGHT=350&LAYERS=AVHRR-09-27%3AMIT-mbay&STYLES=default&
FORMAT=PNG&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&
EXCEPTIONS=INIMAGE&QUALITY=MEDIUM

Figure 5.4 NOAA AVHRR Image of the Gulf of Mexico

Figure 5.4 NOAA AVHRR Image of the Gulf of Mexico

This requests three layers, "built up areas", political boundaries, and coastlines:

http://b-maps.com/map.cgi?WMTVER=1.0.0&REQUEST=map&
SRS=EPSG%3A4326&BBOX=-97.105,24.913,78.794,36.358&
WIDTH=560&HEIGHT=350&LAYERS=BUILTUPA_1M%3ACubeWerx,
COASTL_1M%3ACubeWerx,POLBNDL_1M%3ACubeWerx &STYLES=0XFF8080,0X101040,BLACK&FORMAT=PNG&BGCOLOR=0xFFFFFF&
TRANSPARENT=FALSE&EXCEPTIONS=INIMAGE&QUALITY=MEDIUM

Figure 5.5 Political, Coastline, and Populated Areas, Southeastern United States

Figure 5.5 Political, Coastline, and Populated Areas, Southeastern United States

Notice that in both of these URLs the spatial information is identical:

Because both maps were produced with the same bounding box, spatial reference system, and output size, the results can actually be overlaid by placing the latter map on top of the former. By enabling the use of image formats that provide for transparency information, maps that are meant to be overlaid over other maps can be produced by Map Servers. In this example, background areas of the second map are transparent (because the URL parameter "TRANSPARENT=TRUE" was supplied). Figure 5.6 shows the result of overlaying Figure 5.5 on top of Figure 5.4 to produce a map from the result of two separate Map requests. Finally, note that in this example the two maps were requested from different Map Servers. By standardizing the way in which maps are requested, clients of Map Servers can tailor which layers to request from which servers, thus building up maps that would not have been practical to assemble without the Web Mapping Interface Specification.

Figure 5.6 Combined AVHRR Image and Political/Cultural Map

Figure 5.6 Combined AVHRR Image and Political/Cultural Map

If either of these maps were queryable, a client could request information about a feature on the map by adding to the map URL two additional parameters specifying a location (as an X, Y offset from the upper left corner).

Because each Map Server is likely to have different kinds of information for which it can produce maps, each Map Server must be able to provide a machine-parseable list of its capabilities. That enables the construction of searchable catalogs that can direct clients to particular Map Servers.

It is expected that as a result of the OGC Web Mapping Testbed Phase 2, additional specifications will allow for the query of raster information by pixel location. For instance, if a client application offers the ability to select a position on a map for query, then by adding some additional parameters to the URLs above, a Map Server can return information about the feature value(s) at that pixel. Since each Map Server is likely to have different kinds of information from which it produces map information, each must be able to provide a machine-parseable list of the properties via web mapping requests. This enables the construction of searchable catalogs that can point back to the map servers.

Since the completion of the first phase of the Web Mapping Testbed in August 1999, web mapping is fast becoming a reality. In the first phase of the Testbed participants developed the web mapping interface specifications discussed above. Web Mapping Testbed Phase 2, underway in 2000, will evaluate and prototype additional interface specifications for symbology, extend query and update, enhance vector feature retrieval, and to further demonstrate interaction with the Catalog Services specification. Additionally, future phases of the Web Mapping Testbed may address e-commerce and security issues among other topics considered essential for robust implementation of web mapping.

Available Software

As a result of the Web Mapping Testbed, a number of GIS integrators and vendors have developed prototype versions of web mapping servers and compatible interfaces. The NASA-coordinated Digital Earth project includes software support for mapping NASA data using the specification (http://digitalearth.gsfc.nasa.gov/). OGC Web Mapping Service-compatible interfaces for ESRI Map Objects Internet ap Server version 1.0 and the University of Minnesota "mapserver" product (http://mapserver.gis.umn.edu) will be available to the public by May 2000 from participating organizations within the Web Mapping Testbed.

The US Army Corps of Engineers has developed a web site dedicated to Web Mapping at http://www.webmapping.org/

Other URLs of Web Map Servers built to the WMS 1.0 specification (as of August 14, 2000):

CubeWerx, Canada:
http://www.cubewerx.com/wmt/

ESRI, USA
http://arconline.esri.com/arconline/downloads/ims_/WMS_Connector.html

Geodan, The Netherlands
http://opengis.geodan.nl/opengis/wmt/

IONIC software, Belgium
http://www.ionic.be/

SICAD Geomatics, Germany
http://geo2.sicad.com:80/

Social Change Online, Australia
http://homer.socialchange.net.au/webmap/ogcwmt/demo/mapPage.jsp?appspec=applications%2Faus.xml&al_method=load&javaenabled=true

Demis, The Netherlands
http://www2.demis.nl/mapserver/mapper.php

European Commission Joint Research Center
http://hgss.jrc.it/opengis/

Recommendations

The state of Web Mapping is best illustrated by the progress made in the Open GIS Consortium Test Bed Activity. As the result of potentially competing vendors and software producers coming together and identifying a common set of functionality, a non-proprietary specification for rendering geo-referenced graphics has emerged. This allows one to establish a connection to multiple map servers and generate a stack of images that can be used in visual analysis and basic interrogation.

Although further work needs to be done in the discovery, encoding, and exchange of 3-dimensional geospatial information in support of more advanced analysis and visualization, the basic Web Mapping Service functions provide an excellent starting point in the visual combination of distributed spatial data.

References and Links

Digital Earth Reference Model Home Page,
(http://digitalearth.gsfc.nasa.gov/RM/)

International Continental Scientific Drilling Program (ICDP).
(http://icdp.gfz-potsdam.de/html/icdpinfonet.html)

Open GIS Consortium Web Mapping Testbed Public Page
(http://www.opegis.org/wmt/index.htm)

OpenGIS(r) Web Map Server Interfaces Implementation Specification Revision 1.0.0
(http://www.opengis.org/techno/specs/00-028.pdf)

Chapter Five     |     Context and Rationale     |     Organisational Approach     |     Implementation Approach