Geospatial web services allow users to perform data discovery, data visualization, and data access across the web. Web services are usually accessed by applications based on user input, such as zooming in to an online map or searching a data catalog. The most common protocols are the Web Map Service (WMS), which returns a rendered map image, and Web Feature Service (WFS), which typically returns GML, which was mentioned in this chapter's introduction.
Many WFS services can also return KML, JSON, zipped shapefiles, and other formats. These services are called through HTTP GET requests. The following URL is an example of a WMS GET request, which returns a map image of the world that is 640 pixels wide and 400 pixels tall and has an EPSG code of 900913: http://ows.mundialis.de/services/service?SERVICE=wms&VERSION=1.1.1&REQUEST=GetMap&FORMAT...