“Two roads diverged in a yellow wood”
A Brief History of WMS
Open Geospatial Consortium, OGC, has been an influential geospatial standards body since the late 90’s. OGC standards are at the root of SQL spatial databases, commercial GIS tools, as well as most open source mapping projects found at OSGeo. OGC standards have been adopted at all levels from Microsoft SQL Server, Oracle Spatial, ESRI, and AutoCAD to PostGIS, GeoServer, and Quantum GIS. OGC standards are part of the DNA of modern GIS and internet mapping.
One of the more popular OGC standards, first published at the turn of the millennium, is Web Mapping Service. WMS is a map distribution standard for showing arbitrary BBox map areas in common internet browsers.
One of the early significant WMS services was a partnership between Microsoft Research and the USGS called Microsoft TerraServer, which was in continuous operation from 1998 until today, 5/1/2012. More recently the name was changed to MSR Maps. Microsoft TerraServer pioneered free availability of geospatial data in an OGC compliant service. At the same time, TerraServer research established the viability of quad tile storage, but then merged tiles into arbitrary BBox chunks for WMS GetMap requests.
requiescat in pace
How WMS works
WMS defines a standard method of publishing capabilities to the web and furnishing images of requested areas. A WMS client first asks what layers are available, in what formats, styles, and projections. Some decisions are made depending on need and availability. These decisions along with a specific bounding box are sent back to the WMS server. The WMS server builds the requested image and returns it as a rectangular png, jpeg, tiff etc. Behind the WMS server is some type of data repository, either vector data or imagery, but the WMS server handles all requests for specific parts of this data resource.
This was an ambitious scheme in 2000, attempting to smooth the distribution of a multitude of proprietary formats and display methods. The concept was simply to make access to spatial data a “write once, use anywhere architecture.” Momentum built slowly at first, but by this time, May of 2012, there are many choices for WMS servers, and WMS clients are built into a multitude of mapping tools and websites.
KaBoom! Two roads
In 2005 this whole approach was upended with the introduction of web maps and a new tile serving slippy map architecture. The problem WMS ran into was the limit of CPU resources attempting to build individual map requests for millions of users. Servers eventually bogged down trying to fulfill requests for arbitrary WMS viewports. The new slippy map architecture doesn’t build anything. Tiles flow to clients simply as requested, directly from storage or cache. The browser client is responsible for rendering, so web mapping suddenly became massively parallel. Map pan and zoom went from request, pause, display, to continuous uninterrupted motion, quite magical at its first introduction.
This approach was a recapitulation of TerraServer storage but dropped the WMS at the end of the server pipe. Tile pyramids became just static image stores streaming into a map client as needed by pan and zoom.
WMS was fine for the technical folks and enterprise focus on proprietary assets, but it just didn’t scale into the internet consumer realm. Web mapping forked at this point. WMS continued to grow as a basic function of every desktop map tool and internet client, while web mapping adopted vast tile pyramids prebuilt from vector features and aerial imagery. Microsoft Bing Maps, MapQuest, and Google Maps are the glamorous handmaidens of search, serving up map tiles to the masses. OGC services haul water behind the scenes, with the pocket protector crowd.
Between Two Roads
The two architectures coexist in the internet realm. OGC services are work horses in engineering cubicles, but the mobile generation is all about slippy tiles. Many web apps merge one of the world scale base map tile pyramids with layers of OGC services on top. This approach marshals world level extent together with locally maintained assets. Example Fig 3
Against the Stream
WMS layers can live on top of popular web maps like Bing, but what about the opposite direction? Can WMS clients make use of worldwide tile pyramids?
“A dead thing can go with the stream, but only a living thing can go against it.”
Far down stream from Jim Gray and Tom Barclay, OnTerra created a simple WMS service to bridge the fork in the road between OGC and Bing Maps tiles, but in the opposite direction. MapSavvy is a WMS service that simply exposes Bing Maps Aerial and Road imagery to existing users of WMS clients. Subscribe to the service and imagery for the entire world is available inside your mapping tool of choice.
There are thousands of man years of development tied up in tools that perform very specific business functions. These tools are not especially public facing. They perform the required task, but the task is of no particular interest to the rest of us. Companies have to pay people to use tools like this. In cases of business focused WMS tools, wouldn’t it be nice to access the vast resources of tile imagery abounding on the internet?
For example, Microsoft is just finishing a program to capture most of the US and Western Europe at 30cm pixel resolution and serving this in Bing Maps Aerial. This kind of resolution at national scales is a very useful resource. Designers, engineers, defense agencies, and GIS analysts can always use higher resolution imagery available for large parts of the planet. Lower resolution imagery fills in context for the remainder of the world. Ref: Bing Maps resolution chart
Bing Maps 30cm Aerial: “The project is scheduled for completion by June 2012 at which point a refresh cycle will begin to update much of what will already have been collected, with priority placed on locations that have been more subject to change (as opposed to, say, the Mojave Dessert).” See Fig 4
Here is a short list from the multitude of WMS clients:
• AutoCAD Map 3D – Fig 1
• Bentley MicroStation V8i
• Global Mapper – Fig 4
• Smallworld GeoSpatial
• gvSig – Fig 5
• Quantum GIS – Fig 6
• TatukGIS Viewer
Bing Maps, like all web mapping tile services, leverages the symmetry of a Mercator world projection, which is technically defined as epsg:3857. In order to support the widest range of WMS clients MapSavvy publishes both epsg:3857 and epsg:4326. The second projection has less value, as it distorts the imagery, but, in cases where a WMS client cannot make use of the more accurate epsg:3857, epsg:4326 is helpful. This is especially useful, at higher resolutions where the distortion is reduced. MapSavvy helps resolve some of the distortion issues of epsg:4326 by using transformed images for zoom levels at the top of the tile pyramid.
Unlike Google Maps, which explicitly excludes use of Google tiles in a WMS service, Bing Maps wants customers to use their tile servers. OnTerra licenses Microsoft Bing Map tiles for use in a WMS. MapSavvy subscriptions include a restricted Microsoft sub-license for use of Bing road and imagery tiles in the client, which simplifies life for small business users.
Some TerraServer Alternatives
TerraServer has historical interest but in the past five years there hasn’t been much demand for DOQ, Urban Ortho, or DRG Topo maps. The demise of TerraServer may go largely unnoticed. For those left orphaned, National Atlas publishes several WMS endpoints. However, Bing Maps Aerial is a much improved replacement for USGS DOQ and USGS Urban Ortho. MapSavvy WMS can replace TerraServer DOQ and Urban Ortho layers directly in existing WMS clients.
If you’re looking for alternative USGS Topo contours, Ahlzen’s TopOSM is a good place to start.
Microsoft TerraServer was the starting point for a lot of what we take for granted in 2012. Its passing is understandable but leaves a twinge of regret.
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
- Robert Frost
“When you arrive at a fork in the road, take it.”
- Yogi Berra