Working%20with%20GML%20Application%20Schemas

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Working with GML Application Schemas
Milan Trnini, Galdos Systems, Inc.mtrninic_at_galdo
sinc.com
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Introduction

• Purpose of the experiment Analyze whether
  working with GML application schemas is
  hard/easy/feasible.
• Application schema abstraction what does it
  mean, what is its purpose?
• Different uses of GML application schema
• Information captured in GML application schemas.
• API for working with GML application schemas.

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GML Application Schemas

• Intention of GML is to be base for creating
  applications in different domains.
• This is achieved by GML and XMLSchema
  extensibility mechanisms.
• Example of GML extension mechanism type
  information in tag name.
• GML occupies one level of abstraction.
• Application schema is built on top of it as
  another level.

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GML Application Schemas
GML Application Schemas
Logistical
Transportation
Utilities
Location Service
Network
GML
Feature
Geometries
Topology
Other types
W3C
Other types
Strings
Numbers
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Multiple Application Layers
Transportation Services
Route finder
Route
Transportation Primitives and operations
Other
Intersection
Event
Road Segment
GML
Feature
Geometries
Topology
Temporal
W3C
Other types
Strings
Numbers
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Class Hierarchy

• Objects from one layer are used to compose or
  create objects in a higher layer.
• Use XML mechanisms type extension substitution,
  sequencing, choices, etc.
• Use GML mechanisms collections, associations,
  feature-property model, etc.
• Objects in different layers have different
  scope/meaning/semantics.

Feature
Road Segment
Intersection
Event
Accident
Route
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XML and GML Types
GML instance
ltapproadNetworkMembergt ltappRoadSegment/gt
ltgmlcenterLineOfgt ltgmlLineStringgt
ltgmlcoordinatesgt300.003,1234.232 306.234,
1235.090lt/gmlcoordinatesgt
lt/gmlLineStringgt lt/gmlcenterLineOfgt
ltappotherPropertygtlt/appotherPropertygt
lt/appRoadSegmentgt lt/approadNetworkMembergt
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XML and GML Types
GML application schema
ltxsdelement nameRoadSegment
typeappRoadSegmentType substitutionGroup_Fea
ture/gt ltxsdcomplexType nameRoadSegmentTypegt
ltxsdcomplexContentgt ltxsdextension
basegmlAbstractFeatureType/gt
lt/xsdcomplexContentgt lt/xsdcomplexTypegt

• We dont care about XMLSchema types.
• Element declarations are our GML types.

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GML Types Names
GML instance
ltappqwegt ltappXYZ/gt ltappabcgt
ltappASDgt ltgmlcoordinatesgt300.003,1234.23
2 306.234, 1235.090lt/gmlcoordinatesgt
lt/appASDgt lt/appabcgt ltappotherPropertygt
lt/appotherPropertygt lt/appXYZgt lt/appqwegt
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GML Types Names
GML application schema
ltxsdelement nameXYZ typeappXYZType
substitutionGroupgml_Feature/gt ltxsdcomplexTy
pe nameXYZTypegt ltxsdcomplexContentgt
ltxsdextension basegmlAbstractFeatureType/gt
lt/xsdcomplexContentgt lt/xsdcomplexTypegt

• GML types names in themselves do not mean
  anything!!

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Applications and Schemas

• What do applications need from the schemas?
• A piece of semantics is needed in order to know
  what to do with the data. Not all of it.
• What is semantics? How do we find it? Set of base
  primitives used to compare artifacts and draw
  conclusions about them.
• Semantics in schemas type hierarchy, model
  (feature property), metadata.

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Captured knowledge

• The knowledge exists in the schemas.
• Different uses and types of the knowledge
• Information about content or structure of types
• Information about types semantics type
  hierarchy.
• Information from the GML model we care about
  this
• Information from XMLSchema model - concerning
  schema validation

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Semantics

• Again what do we need?
• Understanding of arbitrary schema?
• Understanding and processing of arbitrary
  features?
• Understanding always comes down to very simple
  comparisons of strings (or other types).
• Semantics by metadata same thing.

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Schema Validator

• For this purpose all the information from the
  schema is needed, and it has to be interpreted
  correctly according to the XMLSchema
  specification.
• XML Schema specification is fairly complex.
• Validating schemas means implementing all details
  from XML Schema Specification (example
  derivation by restriction)
• Not an easy task.
• Performance issues are complex (depends on the
  application where validator is used).

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GML Application Schema Parser (Content Examiner)

• Easier task than validating.
• Some assumptions can be made
• A client application will get a valid data
• If that is not the case, client can only report
  the error, no need to analyze it according to the
  XMLSchema specification.
• Client is interested only in content, if
  available.
• No need to implement the whole specification it
  is ok to assume and hardcode some things.
• Errors might not be very precise.

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Uses of GML Application Schema Parser

• In all standard applications that use GML data
  such as map portrayal applications, geocoders,
  gazetteers, etc.
• GML data editors that provide content guidance.
• GML application schema editors that provide
  content guidance.
• Applications for mapping of different data
  models.
• Hard to remodel existing schemas.

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Code Example
Vector getFeatures(Document gmlData) Vector
result NodeList nodes gmlData.getDocumentEl
ement().getChildren() for (int index 0
index lt nodes.getLength() index)
Node property nodes.get(index) boolean
isFeatureMember _schemaParser.isFeatureMember(pr
operty) if (isFeatureMember false)
continue Node value property.getFirstCh
ild() boolean isFeature
_schemaParser.isFeature(value) if
(isFeature true) result.add(value)
return result
boolean isFeature(Node node) return
((getSubstitutionGroup(node) _Feature)
(getBaseType AbstractFeatureType))
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Multiple Application Layers
ltapproadNetworkMembergt ltappRoadConstruction/gt
ltgmlcenterLineOfgt ltgmlLineStringgt
ltgmlcoordinatesgt300.003,1234.232 306.234,
1235.090lt/gmlcoordinatesgt
lt/gmlLineStringgt lt/gmlcenterLineOfgt
ltappotherPropertygtlt/appotherPropertygt
lt/appRoadConstructiongt lt/approadNetworkMembergt
ltxsdelement nameRoadConstruction
typeRoadConstructionType substitutionGroupRoa
dSegment/gt ltxsdcomplexType nameappRoadConstru
ctionTypegt ltxsdcomplexContentgt
ltxsdextension baseappRoadSegmentType/gt
lt/xsdcomplexContentgt lt/xsdcomplexTypegt

• This means only little more work for the parser
  complexity is linear!

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GML Application Schema Parser API

• Content retrieval
• getFeatures
• getGeometry
• getProperties
• allowedFeatures
• allowedProperties
• allowedGeometries
• etc.

• Type functions
• isFeature
• isFeatureMember
• isGeometry
• isOfType
• etc .

• Miscellaneous
• getCardinality
• etc.

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GML Level of Abstraction

• Work with features, geometries, feature members
  etc.
• Not concerned about XMLSchema types, elements,
  etc.
• Use the same paradigm and model with different
  data description languages (RDF, ).

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Performance

• Verbosity of XML/GML.
• Use of existing tools.
• Schema parser data model new DOM, tables and
  arrays.
• Solutions overlap with the ones regarding any XML
  processing.
• Not a bigger issue than processing of instance
  documents (example - processing of Simple
  Xlinks).

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Summary

• Extensibility and semantics on the level of the
  application schemas achieved using simple methods
  type extension.
• Validator needs to perform deep and extensive
  analysis of the schema it has to follow the
  specification which can be (and often is)
  complex.
• Content examiner needs to follow type hierarchy,
  metadata information etc., which is simple
  comparable to processing instance documents.
• Physical complexity of schemas is not the same as
  the complexity of XMLSchema specification and
  except for the basics does not mirror it.

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References

• XMLSchema specificationhttp//www.w3c.org/TR/xmls
  chema-0/
• GML 2.1 specificationhttp//www.opengis.net/gml/0
  2-009/GML2-11.html
• Schema Infosethttp//www.research.ibm.com/XML/sch
  ema/WD-XML-Schema-Infoset-API-Req.htm
• GML model (specification, papers)
• GML4Jhttp//gml4j.sourceforge.net