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Copyright © 2010 Open Geospatial Consortium, Inc. All Rights Reserved.
the conditions for its rules based on the delineation of the geometry. An example of an existing implementation of this extension to the SE specification is displayed below:
Rule ogc:PropertyIsEqualTo
ogc:Function name
= geometryType
ogc:PropertyName geom
ogc:PropertyName ogc:Function
ogc:Literal Point
ogc:Literal ogc:PropertyIsEqualTo
PointSymbolizer ...
PointSymbolizer Rule
An extension to the SE specification should be considered in order to allow Rules to assess the type of geometry before applying symbolizers. The example above is clearly
proprietary, and yet has cross-domain application. It would be a useful capability to add. Alternatively, to address the simple case above, the SLD itself could be used to
approximate the geometry using the polygons’ centroid or centre of bounding box this choice would need to be controllable from within the SLD. The scales could then be
encoded in the SLD using existing constructs MinScaleDenominator MaxScaleDenominator to control the level of detail trigger ranges.
7.4.2.1 Centre of Line and Polygon geometries - recommendation
When a PointSymbolizer is used with a Polygon or Line geometry, the specification says: “In this case, if a line, polygon, or raster geometry is used with this Symbolizer, then the
semantic is to use the centroid of the geometry, or any similar representative point.” It is recommended that a simple extension is made to the PointSymbolizer to
disambiguate its usage under these circumstances. Furthermore, the choice of “centre-of” algorithm should also be a controllable aspect of the SLD. It should allow for centroid,
centre of bounding box, and for polygons a flag to force the resultant point to fall inside the polygon consider a moon crescent shaped polygon
TBD – example of formal encoding of these options. Consider - are there any further common centre-of algorithms to include?
Copyright © 2010 Open Geospatial Consortium, Inc. All Rights Reserved.
33
Annex A - Aviation Portrayal based on Directional Associations in AIXM
1 Introduction
1.1 Purpose and Scope
This technical note has been produced by Envitia to summarise some of the issues encountered when applying the OGC Symbology Encoding SE to the area of aviation
portrayal. In particular, it concentrates on the issue of portrayal rules that are dependent on associations between feature types in the AIXM conceptual model, but which are not
realised explicitly via GML properties within the AIXM GML application schema.
This report is the result of ongoing research by Envitia within the OGC OWS-8 Aviation Thread [6]. The issue with AIXM associations can be traced to specific requirements in
the ICAO portrayal specifications that were studied in OWS-8 [4].
Section 2 describes the problem space in more detail, while Section 3 proposes some solutions.
1.2 Abbreviations
AIXM Aeronautical Information Exchange Model
Envitia Envitia Ltd
EUROCONTROL European Organisation for the Safety of Air Navigation
FAA Federal Aviation Administration
FPS Feature Portrayal Service
GML Geography Markup Language
ICAO International Civil Aviation Organization
ISO International Organization for Standardization
ISOTC ISO Technical Committee
OGC Open Geospatial Consortium
OWS OGC Web Services
SE Symbology Encoding
SLD Styled Layer Descriptor
