1.5.2 OGC Web Mapping Service WMS

From Geostandards

Contents 1 OGC versions WMS 2 Format 3 WMS specifications 4 Profiling

OGC versions WMS

The OGC has its specifications neatly in order. Every specification has a separate webpage. The specification for the Web Mapping Service (WMS) can be found at: [1].

It is remarkable that there is more than one version of the WMS specifications. This is why it is important to be familiar with the software market, because the software market has to apply the version in its software. Some versions have never been applied or will no longer be supported by all the software, such as WMS 1.3.0 (= equal to ISO 19128).

It is, therefore, important for the application of the OGC versions, to make the right choice for each region or country. In 2006, the Netherlands have, for example chosen to use WMS 1.1.1. In 2008, INSPIRE has chosen version 1.3.0. The choice made by INSPIRE ensures that software will be made which is applicable to that version. The Netherlands will adapt its profile, as soon as INSPIRE makes it mandatory, or even earlier.

Format

The format of an OGC specification is more or less equal to that of ISO. Parts one to five apply to the WMS specification. (This may be different for other OGC specifications). The details of the Submitting Organizations are always mentioned. The reason for this is a commercial one, but it is, in general, highly valued. Because these details are mentioned, if there are any questions, some of the organisations can be contacted; without this specific mention, information of this nature would be hard to find for someone who is not part of ISO/TC 211.

Another difference with ISO is that there is no UML notation in WMS specifications. If necessary, it is given for other specifications. Instead of UML notations, normative verbs are adopted: these form an important part of the specifications.

In the sections labelled normative, the key words "required", "shall", "shall not", "should", "should not", "recommended", "may", and "optional" are to be interpreted in the same way as they are described in IETF RFC 2119. The verb "deprecate" indicates that the referenced portion of the specification is being retained for backwards compatibility with earlier versions but may be removed from any future versions of the specification without notice.

WMS specifications

AWMS specifications consist of GetCapabilities (what the service offers) and a GetMap. When implemented, GetFeatureInfo information of the geo-objects themselves can be retrieved.

A GetMap has the following mandatory and optional parameters:

From this table one can see that these are global agreements which can be implemented or made more precise (profiling) in order to increase interoperability. For example, making the PNG format mandatory, in addition to the optional formats such as JPG and BMP, increases interoperability, because other formats might make mistakes during an exchange.

Profiling

Here a number of characteristics are listed that have been profiled in more detail (specified) for WMS:

• Paragraph 4.1.1, for example, indicates that the number of the version should be filled in and that it consists of 1.1.1.
• Paragraph 4.1.2 states that it is mandatory to support PNG.
• Paragraph 4.1.3 indicates which SRS codes should be present, at minimum. Worldwide these are in their thousands.

In order to check whether WMS has those characteristics, validators have been developed. The WMS validator for the Dutch profile for WMS can be found at: [2].

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