A Web Map Service (WMS) is a standard protocol for serving georeferenced map images over the Internet that are generated by a map server using data from a GIS database. The specification was developed and first published by the Open Geospatial Consortium in 1999.

WMS downloads a raster map image, covering a requested area with a particular projection, on demand.  The user does not have to download and store the data, but must have an internet connection.

Using WMS can be like having Google Earth inside the GIS program, with high quality imagery available anywhere.  You do not get the seamless zoom in and out, and national borders provide a challenge, but for many operations this works well.

Programs communicate with the Map service with two main types of request:

Get capabilities Returns a list of layers, and the geographic coverage of each, delivered in a standard XML format.  This can be turned into a database, which the GIS program can use to see which layers belong on the current map.  It also shows the projections available (SRS field).  This allows a program to access new layers which might not have existed when the program was written, and to allow the program to access data that was not specifically considered when writing the program.  The layer names typically include the geographic area, the resolution of imagery, and the date imagery was acquired.
Get map

Returns just a map image, but the GIS program knows the projection information and the map extent from its request, and thus can perform full GIS work on the layer.

A WMS map request will look like this (this could be cut and pasted into the address bar of any browser, which would display the map image as a "dumb picture" or get the capabilities as an XML file):

Parts of the request, some of which are optional.  While this would be difficult to create manually, it is an easy process for a GIS program to create the request based on the current map area:

Using WMS Maps in MICRODEM.

Last revision 11/9/2015