Hi there, I'm trying to define GML 3.2.1. based application schema for FM-related software.
The concept is to create some intermediate base-level schema common to basic software tools (e.g. universal XSLT processing), completed with specific application-level schemas presenting actual application structure. >From "GML Best Practices Guide" (http://www.ogcnetwork.net/system/files/GML_Best_Practices_Guide.pdf) it is advised not to use "type" attribute: quote starts If you wish to denote distinct semantic types such as river and road, it is INVALID GML to use a type attribute as shown in the following examples. Example of Invalid GML Using a Type Attribute <app:GenericFeature gml:id = "p2" type = "Canal"> <app:material>water</app:material> <app:age>90</app:age> <gml:centerLineOf> … </app:centerLineOf> </app:GenericFeature> You can "narrow" the type denoted by the feature type name element (<app: GenericFeature>) by using qualifying properties, as shown in the following example. <app:GenericFeature gml:id = "p2"> <app:type>Canal</app:type> <app:material>water</app:material> <app:age>90</app:age> <gml:centerLineOf> ... </app:centerLineOf> </app:GenericFeature> quote ends But from the other hand, from "XML Schema W3C Recommendation" (http://www.w3.org/TR/xmlschema-0/#abstract) it results that using abstract types makes constructs with "type" attribute perfectly legal: quote starts Declaring a type as abstract simply requires the use of a type derived from it (and identified by the xsi:type attribute) in the instance document. <schema xmlns="http://www.w3.org/2001/XMLSchema"; targetNamespace="http://cars.example.com/schema"; xmlns:target="http://cars.example.com/schema";> <complexType name="Vehicle" abstract="true"/> <complexType name="Car"> <complexContent> <extension base="target:Vehicle"/> </complexContent> </complexType> <element name="transport" type="target:Vehicle"/> </schema> The transport element is not abstract, therefore it can appear in instance documents. However, because its type definition is abstract, it may never appear in an instance document without an xsi:type attribute that refers to a derived type. That means the following is not schema-valid: <transport xmlns="http://cars.example.com/schema"/> because the transport element's type is abstract. However, the following is schema-valid: <transport xmlns="http://cars.example.com/schema"; xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; xsi:type="Car"/> because it uses a non-abstract type that is substitutable for Vehicle. quote ends It results that using Galdos recommendation with <app:type> element is somehow artificial and naive, disallowing for application-specific specialization of concrete GenericFeatures. On the other hand, W3C recommended xsi:type attribute is perfectly legal, and allows using in the same time generic schema to describe specific features. Of course it requires further generalization by defining generic properties for features in base-level schema with their concrete definitions in application-level schema. I wonder what is opinion of GML-gurus about proper constructs for layering of GML-application schemas. Regards, Krzysztof -- View this message in context: http://n2.nabble.com/Searching-proper-schema-constructs-in-layered-GML-application-schema-tp3992089p3992089.html Sent from the OSGeo Discuss mailing list archive at Nabble.com. _______________________________________________ Discuss mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/discuss
