Introduction Import GML dictionary schema components

238 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. Authors of application schemas may create their own dictionary types if GML lacks the desired construct. To do this, authors shall ensure that the object elements of these definition types are in the substitution group either directly or indirectly of the corresponding GML object element: gml:Dictionary. All rules specified in Clauses 7 and 15 shall be followed.

21.11 Schemas defining values

21.11.1 Introduction

GML allows for user defined value types. Such value types may be used to express the property types of feature and other types of GML object. The basic root types for user-defined values are defined in 7.2.2.1. An alternative form for the expression of values is contained in 16.4. This is used mainly to provide values for the gml:resultOf parameter for an observation.

21.11.2 Import GML value objects schema components

The application schema shall import the GML schema as described in 21.2.3. Any GML profile referenced from the application schema shall include at least the gml:Value group and all schema components used by this element.

21.11.3 Construction of new value types

New value types may be created by derivation typically by restriction from any of the root types shown in Table 9. Table 9 — Construction of new value types Content model Description gml:MeasureType A numerical quantity with a unit of measure uom gml:CategoryType A classification gml:CountType A count of occurrences, incidences etc. Some standard value types can be found in the schema components specified in 16.3.

21.12 GML profiles of the GML schema

Typically a GML application schema will only require a limited subset of the schema components of the GML schema. It is recommended to identify and document the GML profile, see Clause 20, required by a GML application schema. Subclauses 21.3 to 21.11 contain some guidelines which schema components may be required depending on the type of the GML application schema. NOTE 1 Annex G contains a method to automatically create a GML profile based on a list of schema components explicitly required by a GML application schema. As a starting point, consider the following guidelines: a In an application schema modelling geographic features, gml:AbstractFeature and all schema components required by this element are required. See Clause 9.