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 in Web Mapping Service - Cached) as a constrained profile of OGC WMS that permits servers to optimize their image generation, and allows tiles to be cached at intermediate points. The basic idea is that, unlike with WMS, two different requests for a given WMS-C tile should form the exact same HTTP GET request. This invites several constraints on WMS GetMap requests:

1. Minimal query string arguments (i.e. no optional arguments permitted, versus §7.2)
2. Fixed query string argument ordering and case (versus §6.4.1; q.v. also §7.2.2, Table 8)
3. Fixed range of possible bounding boxes, computed from the WMS-C profile parameters
4. Fixed precision on bounding box values (clarifying §6.5.6)
5. Fixed tile size in pixels
6. Fixed layer name and/or layer name ordering
7. Fixed styling
8. Fixed output format

Some means of identifying these constraints programmatically on a per-server basis, a la GetCapabilities might be desirable. From the table below, it appears that minimum/maximum scale (and scale quantization factor, which should probably never be other than 2) are the only suggested tiling regime parameters that would be difficult to directly express in a WMS GetCapabilities document. Is there some existing way to add custom parameters to a GetCapabilities declaration?

Note that the LAYERS, STYLES, SRS, HEIGHT, WIDTH, and FORMAT arguments to a GetMap request would become fixed for a particular WMS-C tiled layer, but the WMS specification would still require the inclusion of these arguments in every WMS-C request (§ 7.2).

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

Possible Tile Scheme Parameters

Parameter	Default value	Specifiable in WMS GetCapabilities?
Projection	EPSG:4326	<SRS>
Maximal extent	(-180,-90,180,90)	<BoundingBox> (§7.1.4.5.7)
Number of horizontal and vertical tiles at the maximal extent	none	Could be implied by <ScaleHint> (§7.1.4.5.8, but note that the format is weakly specified)
Tile size in pixels	some power of 2	<Layer> attribute fixedWidth, fixedHeight (§7.1.4.6)
Minimum scale	none	<ScaleHint> (see note, above)
Scale quantization factor	2	?

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Existing Tiling Schemes

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

Related Projects

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

Interested Parties

• Schuyler Erle
• Allan Doyle
• Norman Vine
• add yourself