@XmlSchema(URL="http://schemas.opengis.net/wms/1.3.0/capabilities_1_3_0.xsd", specification=ISO_19128)

## Package org.opengis.layer

Basic unit of geographic information that may be requested as a map from a server.

See:
Description

Interface Summary
AbstractSizedURL: Extends the `AbstractURL` by adding a width and height for the data retrievable from the linkage.
AbstractURL: A simple starting point for most of the various URL elements used by `Layer`s and `Style`s.
Attribution: Provides a way to identify the source of geographic information used in a `Layer` or collection of `Layer`s.
AuthorityURL: Encloses an `OnLineResource` which states the linkage of a document defining the meaning of the `Identifier` values which reference this `AuthorityURL`.
DataURL: Offers a link to the underlying data represented by a particular `Layer`.
FeatureListURL: Points to a list of the features represented in a given `Layer`.
Layer: Organizes the basic GO-1 constructs that may be added to `FeatureCanvas` or `Canvas`.
LegendURL: Contains the linkage to an image of a map legend appropriate to the enclosing `Style`.
LogoURL: Provides the linkage to the logo image of the data provider defined by the enclosing `Attribution`.
MetadataURL: Offers detailed, standardized metadata about the data corresponding to a particular `Layer`.
Style: Encapsulates the information necessary to style the renderable data for a given `Layer`.
StyleSheetURL: Provides symbology information for the enclosing `Style` element.
StyleURL: Offers more information about the data or symbology underlying a particular `Style`.

Exception Summary
LayerException: Throws when a layer source failed to create a layer.

## Package org.opengis.layer Description

Basic unit of geographic information that may be requested as a map from a server. The following is adapted from Web Map Service:

Each available map is advertised by a `Layer` element in the service metadata. Conceptually, each Layer is a distinct entity. However, as a means of classifying and organizing layers, and as a means of reducing the size of the service metadata, a single parent Layer may enclose any number of additional layers, which may be hierarchically nested as desired. Some properties defined in a parent layer are inherited by the children it encloses. These inherited properties may be either redefined or added to by the child.

A Map Server shall include at least one `Layer` element for each map layer offered. If desired, layers may be repeated in different categories (i.e., enclosed in more than one parent `Layer`) when relevant.

A list of keywords or keyword phrases describing each layer should be included to help catalogue searching. Each keyword may be accompanied by a "vocabulary" attribute to indicate the defining authority for the keyword. No controlled vocabulary of Layer and Style Names or Titles is defined by the International Standard, so they may be chosen at the discretion of the service provider organization or information community.

### Scale denominators

The minimum scale denominator and maximum scale denominator elements define the range of scales for which it is appropriate to generate a map of a layer. Because maps from a Web Map Service may be viewed on arbitrary displays rather than on paper, the values used are actually the scale denominators relative to a common display pixel size. The intent of scale denominators is not the translation between "actual" and "standard" scales always be completely accurate. Rather, the intent is to reduce the amount of clutter or crowding of features portrayed on the map. The scale denominator values are guidelines for clients, not firm limits. Upon receiving a request for a map that is not within the scale denominator range, the server may return a blank map, or may return a portrayal of the layer that is crowded with features or otherwise poorly suited for display; the server shall not respond with a service exception.

For the purposes of the ISO 19128 International Standard, the common pixel size is defined to be 0.28mm × 0.28mm (millimetres). Because arbitrary clients can request maps from a server, the true pixel size of the final rendering device is unknown to the server.

Because it is possible to integrate the output of multiple Web Map Servers into a single displayed result, it is important that different servers have consistent behaviour with respect to processing scales, so that all of the independent servers will select or deselect layers at the same scales. To ensure consistent behaviour, scales relative to coordinate spaces must be handled consistently between map servers. For geographic coordinate systems, which use angular units, the angular coverage of a map shall be converted to linear units for computation of scale by using the circumference of the Earth at the equator and by assuming perfectly square linear units. For linear coordinate systems, the size of the coordinate space should be used directly without compensating for distortions in it with respect to the shape of the real Earth.

Floating-point roundoff-error control should also be applied to these calculations, to ensure consistency between systems. Because the scale denominators used in Layer metadata will often be "round" numbers, such as 250000, if a calculation of the current scale results in a value of 249999.99999999, it should be considered to match 250000.

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