WMS Tiling Client Recommendation

2010-09-13T05:42:12Z

Wiki-Demgeorge5: /* Miscellany */

== Introduction ==

This document details extensions to and constraints on the OGC WMS 1.1.1
specification for the purpose of (a) improving the quality and scalability of
tiled maps, particularly on the Web, and (b) making it possible to cache the
results of tiled WMS requests.

This recommendation relies on two basic concepts to support this purpose:
First, cachability of map imagery can be improved by
using image tiles of fixed width and height, referenced to some fixed
geographic grid at fixed scales. A "valid" tile request is one that conforms to
the specification of fixed image parameters and geographic grid(s) for a given
layer. By analogy, an "invalid" tile request is one that does not.

Second, caching of HTTP GET requests is further made possible by
constraining the URL parameters used in the request. This recommendation
identifies the WMS GetMap parameters minimally needed for a client to
request a valid tile.

For more information and further background, please refer to the [[TilingStandard]]
and [[Tile Map Service Specification]] documents on this wiki.

== Tiling Profiles ==

WMS servers that support tiling should publish a profile as part of their
GetCapabilities advertisement, which specifies all of the information needed to
request valid tiles from the tile server. The required information comes in two
varieties: (1) WMS request parameters which must be supplied to the server
verbatim, and (2) a tile grid definition, which a client can use to calculate
the extents of valid tiles.

=== Verbatim Request Parameters ===

Each tiling profile needs to establish verbatim values for the following
WMS request parameters:

* ''srs''
* ''height''
* ''width''
* ''format''

Additionally, each tiled layer is defined in terms of verbatim values for:

* ''layers''
* ''styles''

=== Tile Grid Definition ===

Rather than rely on the much-overloaded concept of "scale," this recommendation
instead will describe scales or zoom levels in terms of "resolution," defined
in terms of map units (typically degrees or meters) per pixel.

In order to be cacheable, requested tiles must have bounding boxes aligned to a
set of consistent grids at successively larger map resolutions in geographic
space. Tile grids originate in the lower left corner of the tiled layer's
''BoundingBox''.

A tiling profile must supply a list of supported resolutions, corresponding to
the different "zoom levels" it may be used to serve.

The bounding box for a valid tile request must align to the grid given by the
layer's tiling profile. In practical terms, this means the ''bbox'' coordinates
must be equal to the grid origin, plus some non-negative integer multiple of
the tile size in pixels, multiplied by one of the supported resolutions listed
in the layer's tiling profile.

=== Global Profiles ===

The following pre-defined global profiles use resolution values that are related by
powers of two. Servers may offer tiles at as many resolution levels as are
appropriate to the data represented.

==== Unprojected Profile ====

All tiling WMS servers should be able to deliver tiles in a generic "unprojected" global
profile, with the following tiling parameters:

* Width: 256 px
* Height: 256 px
* Format: image/png
* SRS: EPSG:4326
* BoundingBox: -180 -90, 180 90
* Resolutions: 0.703125, 0.3515625 ...

There are two tiles at the highest resolution of the Unprojected Profile.

==== Mercator Profile ====

* Width: 256 px
* Height: 256 px
* Format: image/png
* SRS: OSGEO:41001
* BoundingBox: -20037508.34 -20037508.34 20037508.34 20037508.34
* Resolutions: 156543.03390625 78271.516953125 ...

There is one tile at the highest resolution of the basic Mercator profile.

=== GetCapabilities Responses ===

==== Internal DTD ====

Tiling WMS servers should include the following internal DTD to advertise the
presence of tiling metadata, as recommended by the OGC WMS 1.1.1 specification:

<nowiki><!DOCTYPE WMT_MS_Capabilities SYSTEM
"http://schemas.opengeospatial.net/wms/1.1.1/WMS_MS_Capabilities.dtd" [
<!ELEMENT VendorSpecificCapabilities (TileSet*) >
<!ELEMENT TileSet (SRS, BoundingBox?, Resolutions, Width, Height, Format, Layers*, Styles*) >
<!ELEMENT Resolutions (#PCDATA) >
<!ELEMENT Width (#PCDATA) >
<!ELEMENT Height (#PCDATA) >
<!ELEMENT Layers (#PCDATA) >
<!ELEMENT Styles (#PCDATA) >
]></nowiki>

==== Example VendorSpecificCapabilities ====

A suitable tiling WMS GetCapabilities response might look like this:

<nowiki>
<WMT_MS_Capabilities>
...
<VendorSpecificCapabilities>
<TileSet>
<SRS>EPSG:4326</SRS>
<BoundingBox srs="EPSG:4326" minx="-180" miny="-90" maxx="180" maxy="90" />
<Resolutions>0.703125 0.3515625 0.17578125 0.087890625 0.04394531250</Resolutions>
<Width>256</Width>
<Height>256</Height>
<Format>image/png</Format>
<Layers>coastline</Layers>
<Styles></Styles>
</TileSet>
<TileSet>
<SRS>OSGEO:41001</SRS>
<BoundingBox srs="OSGEO:41001" minx="-20037508.34" miny="-20037508.34"
maxx="20037508.34" maxy="20037508.34" />
<Resolutions>
156543.03 78271.52 39135.76 19567.88 9783.94 4891.97
</Resolutions>
<Width>256</Width>
<Height>256</Height>
<Format>image/png</Format>
<Layers>coastline</Layers>
<Styles></Styles>
</TileSet>
</VendorSpecificCapabilities>
...
</WMT_MS_Capabilities></nowiki>

This declaration asserts that our hypothetical tiling WMS server supports tiling in the two basic global profiles for the <code>coastline</code> layer defined elsewhere in the capabilities response. Note that the ''Resolutions'' list is whitespace separated.

== Tile Requests ==

Requests for map tiles from a WMS server should include the vendor-specific
parameter <code>tiled=true</code>.

Verbatim request parameters must be given in a tiled WMS request exactly as
specified in the tiling profile / layer definition.

Valid tile requests must include all mandatory WMS GetMap parameters, e.g. ''service'', ''version'', ''request'', ''layers'', ''styles'', ''srs'', ''bbox'', ''width'', ''height'', ''format''. Hopefully, standard Web caches will be smart enough not to care about parameter ordering. Values for the ''request'' parameter should follow the mixed-caps case used in the WMS spec, e.g. "GetMap", "GetCapabilties", and so on.

URL parameter names should be given in lowercase (constraining §6.4.1 of the WMS 1.1.1 spec). Any characters in URL parameter values other than alphanumerics and the punctuation characters '''-''', '''_''', '''.''', ''':''', and ''',''' should be URL-escaped. Spaces in URL parameter values should be escaped using ''+''.

Valid tile requests must omit any optional WMS request parameters, except those required in practice by the server in question.

All decimal values given in a tiled WMS request (e.g. in the <code>bbox</code>
parameter) should be rounded to six places, with trailing zeros omitted.

By convention, the non-standard SRS codes proposed by the associated [[Tile Map Service Specification]] are recommended for use in tiled WMS requests as well.

=== Invalid Requests ===

If a request is made with <code>tiled=true</code>, a tiling WMS server may
respond with HTTP 404 or 500 to indicate that a request fails to meet the
recommendations given here.

== Miscellany ==

This recommendation is based on discussion at the [[FOSS4G 2006 Tiling BOF|FOSS4G Birds of a Feather Session on Web Map Tiling]] in Lausanne, Switzerland on 12 Sep 2006.

By consensus of that meeting, the recommendation is published under a Creative
Commons Attribution-ShareAlike license.

[[Category:Tiling]]

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OSGeo - User contributions [en]

WMS Tiling Client Recommendation