Network Protocols

The conventional way to distribute geospatial data over a network is simply to use file transfer, e.g. HTTP and FTP.  This works well for some tasks, but by using a server and network protocol that are geospatially aware, much more powerful data requests can be performed, such as requesting exact geographic extents and having datasets created dynamically in response.

The organization producing standards in this area is the OpenGIS Consortium (OGC).  The official specifications for each of the following protocols are on the OGC Specifications page.

WMS

• Web Map Server (WMS) is a protocol for requesting and serving raster geodata over HTTP
• a WMS server responds to WMS requests by returning images, usually in common image formats like GeoTIFF, JPEG or PNG
• a WMS request can either be simply encoded as a URL, or sent via HTTP POST
• here is an example of a WMS request URL, to get an idea of what they look like:
  • http://terraservice.net/ogcmap.ashx?version=1.1.1&REQUEST=getmap
&LAYERS=DRG&SRS=EPSG:4326&BBOX=227476,2221563,232979,2228442
&WIDTH=512&HEIGHT=512&FORMAT=JPEG
• the WMS Cookbook is a guide to implementing an OGC Web Map Server
• some other useful resources on the OGC Network site
• there are numerous free WMS server implementations available, including MapServer and Deegree (LGPL)
• WMS is very general-purpose and hence not particularly efficient or optimized for specific clients.  WMS-C is a wiki for ideas about how WMS could be extended (or subset) with Tile Caching.

WFS

• Web Feature Server (WFS) is a protocol for requesting and serving vector geodata over HTTP
• a WFS server responds to requests by returning data in the Geography Markup Language (GML)
• a WFS server can even support transactions which allow a client to modify data on the server
• there are numerous free WMS server implementations available, including GeoServer (GPL) and Deegree (LGPL)

Other Protocols

• Web Coverage Service (WCS) is similar to WMS but can serve types of coverage besides imagery, such as elevation data.
• Web Terrain Service (WTS) is similar to (and builds on) WMS but serves a static 3D rendered image of a dataset instead of the data itself.
• OGDI is an open raster/vector reading network protocol.  It is an API which implements a client/server architecture to transfer geospatial data products over any TCP/IP network.
• OPeNDAP (DODS) is an open raster/vector reading network protocol, mostly used for oceanographic data.

Ad-hoc Protocols

• There are many servers with their own format for making geo requests, for example the USGS Elevation Query Web Service which defines its own XML request format for querying the elevation in feet or meters for a specific latitude/longitude (WGS 1984) point from the USGS Seamless Elevation datasets.

Servers

• As of 2012, there are surprisingly few servers in existence actually using WMS or WFS to serve anything relevant to terrain visualization.
• For example, SearchOGC lists a number of WMS/WFS servers hosting geological data, primarily "lithography", faultlines, and oil and gas fields, which are barely related to anything visible.