What are web services?
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 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 an island in the Gulf of Mexico that is 1,125 pixels wide by 889 pixels tall and has an EPSG code of 4326 (WGS 84 Geographic):
