How does map services work?

18.03 2019

Viewing services allow geographical data to be displayed in the form of online maps.

Mapping services are produced by a map server, which is specialized software that facilitates communication (based on client/server architecture) between a conventional web server and a database containing spatial data. The steps of a user calling up a specific map is described as follows:

  • The user defines the location of interest (site) in a browser (Edge, Mozilla, Google Chrome, Safari, etc.) by specifying its extent, required layers (data sets), potential dimensions, resolution and format of desired map output.
  • The browser (client) sends request to the web server (e.g. MS IIS, Apache) using HTTP protocol (HyperText Transfer Protocol).
  • The request is subsequently forwarded to the map server, which queries the data server (file or database service) and sends the received data back to the web server, which then displays the results in the client application. The results may include a generated bitmap image, text or geographical data.

Communication between the client and the server takes place each time a user moves on the map (changing extent) or calls up any of the available interactive tools (such as searching an address or route, displaying the properties of a selected building or site, etc.). The entire process may be more complex if the client queries data from several map servers simultaneously—an important role is then played by compliance to the OGC (Open Geospatial Consortium) standards. Several map servers (i.e. underlying technologies) are available that facilitate publishing map services. For example, the commercial ArcGIS Server, GeoMedia WebMap, T-MapServer, TopoL Internet Server, or free-of-charge services such as UMN MapServer, OSGeo MapServer and GeoServer.

Service Standards for Spatial Data according to OGC

There are several types of web services depending on the type of service offered to users. Simple publication of data (WMS, WFS, WCS services) may be accompanied by offering users a choice of using analytical tools for processing (WPS). All services mentioned below are subject to the specifications of the OGC consortium.

WMS – Web Map Service is a basic protocol providing users with a map composition in bitmap form. Users may request several layers simultaneously (vector or bitmap), though the resulting presentation will always be produced as a bitmap, and the data cannot in any manner be edited or used in subsequent analyses. This service is suitable for providing users with an option to view the data but not to process or work with it further. If high-volume layers are published (such as orthophotographs), WMTS service (Web Map Tile Service) can be used, which speeds up loading the view in the browser by accessing pre-defined map tiles on the server for defined scales. WMS can also provide information to users about sites or buildings at a location if the selected layers allow such information to be provided.

WFS – In contrast to WMS, Web Feature Service does not provide a simple bitmap image but allows subsequent data processing. Data is forwarded to users in the form of objects. WFS specifies four basic operations with geographical features that the map server can provide.

WCS – Web Coverage Service supports the acquisition of spatial data describing objects that may be variable or change in time or space. These multi-dimensional layers are called coverage and include, for example, satellite imagery of land cover, data relating to weather or climate and digital terrain models. WCS service allows complex analyses and inclusion of data into complex calculation models.

WPS – Web Processing Service provides pre-defined analytical tools via the map server, from simple calculations with the attributes of selected objects up to complex calculation models. Users in these cases can define the data to be used, which operations should be completed and how the result should be displayed. In practical terms, this is a partial transfer of geographical information system functionality to the web.

Besides mapping services, catalogue services for the web (CS-W) are also available. These provide searching of metadata records related to geographical data, mapping services and applications. Searches in metadata  sections are especially used with CS-W services.

The specifications  of all the above services are available at the OGC website: http://www.opengeospatial.org/standards.

Data provided by IPR is administered using an ArcGIS Server that provides clients with information in a simple output format, i.e. bitmap or text. Standard viewing services are published in two formats:

  • AGS (ArcGIS Server service) – native format for map services published via ArcGIS Server
  • KML (Keyhole Markup Language) – standard format for map services based on XML

 

Contact person: Ing. Filip Drda, drda@ipr.praha.eu