WMS Tile Caching

Goal

The goal of a WMS Tile Caching proposal, perhaps WMS-C for short, is to find a way to optimize the generation of map imagery for transmission across the Internet. The proposal needs to offer ideally a single means by which map clients can fetch tiles from existing servers, in such a way that map tiles can be cached on the server, or at an intermediate location, or even be completely pre-generated, if desired. The proposal should leverage the existing investment in OGC-compliant WMS servers on the 'Net.

Proposal

WMS-C as WMS Profile

One approach to take might be to define WMS-C as a constrained profile of WMS that permits servers to optimize their image generation, and allows tiles to be cached at intermediate points. This invites several constraints on WMS:

1. Minimal query string arguments
2. Fixed query string argument ordering
3. Fixed range of possible bounding boxes, computed from the WMS-C profile parameters
4. Fixed precision on bounding box values
5. Fixed tile size in pixels
6. Fixed layer name and/or layer name ordering

Some means of identifying these constraints programmatically on a per-server basis, a la GetCapabilities might be desirable.

A WMS-C proxy or server should be free to return an error or a redirect, if it receives a WMS request that is not WMS-C compliant.

Existing Tiling Schemes

• Mikel Maron's description of OnEarth's LandSat tiling scheme
• World Wind's tiling scheme: PDF, wiki

Possible Tile Scheme Parameters

• Maximal extent (typically, the entire world)
• Projection (typically, EPSG 4326)
• Number of horizontal and vertical tiles at the maximal extent
• Tile size in pixels
• Scale quantization factor (typically, powers of 2)
• Minimum scale (i.e., the number of zoom levels)

Related Projects

• osgPlanet
• NASA WorldWind (wiki)
• Community MapBuilder
• other AJAX map clients, presumably

Interested Parties

• Schuyler Erle
• add yourself