WMS Tile Caching

2006-03-29T23:20:19Z

Wiki-Joshli:

== 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 [http://www.opengeospatial.org/docs/01-068r2.pdf 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:

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

{| cellpadding="5" cellspacing="1" border="1"
| '''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 || ?
|}

=== Calculating Valid Tile Extents for a Given WMS-C Request ===

----

== Existing Tiling Schemes ==

* Mikel Maron's description of [http://lists.eogeo.org/pipermail/tiling/2006-March/000017.html OnEarth's LandSat tiling scheme]
* World Wind's tiling scheme: [http://www.ceteranet.com/nww-tile-struct.pdf PDF], [http://www.worldwindcentral.com/wiki/Making_Layers wiki]

== Related Projects ==

* [http://www.ossim.org/tiki-read_article.php?articleId=3 osgPlanet]
* [http://worldwind.arc.nasa.gov/ NASA WorldWind] ([http://www.worldwindcentral.com/wiki/Main_page wiki])
* [http://mapbuilder.sf.net/ Community MapBuilder]
* other AJAX map clients, presumably

== Interested Parties ==
* [[User:SchuylerErle|Schuyler Erle]]
* [[User"Adoyle"|Allan Doyle]]
* [[user"Nhv"|Norman Vine]]
* [[user"Joshli"|Josh Lieberman]]
* ''add yourself''

OSGeo - User contributions [en]

WMS Tile Caching