A web mapping service is any platform that can pull a map from the geographic information system (GIS) database while operating within certain parameters. In 1999, the Open Geospatial Consortium set forth a set of standards for web mapping systems. These protocols include a set of required features and an array of optional map calls. Using these standardized systems, a web mapping service is able to interact with and display maps through an Internet-based interface.
The GIS database contains mapping information of all kinds. Rather than just maintaining topographical or political maps, this database breaks down mapped areas into sets of information. Users are able to find important geographical, geological and political information on any point simply by providing a set of coordinates. For instance, a user could call up a map of a given area that specializes in natural resource usage, historical landmarks or underlying ground structures.
In order for this information to be useful to people, there had to be a method of pulling the information from the database. When the initial standards were set for adding and calling up information in the database, the programs that accessed it often had large and complicated interfaces. By taking a lot of the programming out of the clients and putting it in the database itself, the process of accessing the maps became more streamlined. This caused the clients to shrink until they became small enough to be web-based.
In order to comply with the standards, a web mapping service needs to support only two basic functions. The “GetCapabilities” function tells the GIS database about the web mapping service. Things such as the web mapping service’s supported file formats, what map layers it knows how to display and whether any specific features are required or need to be excluded are all common pieces of information.
The second function required in a web mapping service is “GetMap.” This function tells the GIS database exactly what map it needs. The database will read the request along with all of the associated parameters and then pull a map based on the requirements laid out by GetCapabilities. The database then sends the correctly-formatted map back to the web mapping service.
In addition to these two functions, most web mapping services support a handful of other functions. “GetFeatureInfo” will send along specific information about locations on the map, such as a road name or the height of a location. The “GetLegendGraphic” function returns an image that contains the symbols used on the map to help users read the information. There are several other optional functions that all follow this same idea—small improvements to the functions or readability of the map. Both the required functions and these optional functions support a wide range of switches that allow users to specify exactly what sorts of information they require.