OGC's GML discussion forum is probably the best place to ask this.
http://feature.opengeospatial.org/forumbb/viewforum.php?f=9
Staff monitors the board and tries to make sure posted questions get
looked at by topic experts within the membership.
---
Raj
On Nov 12, at 6:21 AM, Krzysztof FF wrote:
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
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