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Reference Systems and Record Schemas (General)

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The need for explicit reference systems

Many property values are not scalar, having various degrees of internal structure. Geographic position is the most common case, where the value requires between one and four numbers depending on the dimension of the spatio-temporal space of interest. Each number gives the component of the value on one axis. In order to understand the value, the definitions of the axes must be available in a way that allows each number to be associated with the appropriate axis.

Geographic coordinates

In an XML encoding, the components of a complex value may be indicated by individual tags (elements or XML attributes), e.g.

<position>
   <col>3.4</col>
   <row>9.2</row>
</position>

For a general purpose geographic language, the explicit tag-based approach is not sustainable due to the enormous variety of global, regional, local and private coordinate reference systems that may be used. Instead, the preferred encoding of direct position in GML puts the components of a single coordinate in a whitespace-separated list within a single pos element

<gml:pos srsName="epsg:62836405">-22.48107 132.00713</gml:pos>

and lists of coordinates in a posList element

<gml:posList srsName="epsg:62836405" count="3" srsDimension="2">
    -22.48107 132.00713
    -22.48174 132.00714   
    -22.48240 132.00714  
</gml:posList>

The value of the srsName attribute indicates the coordinate reference system for the value. In this case it is latitude-longitude with the datum GDA94, identified as system 62836405 in the database maintained by the European Petroleum Survey Group. Note that the order of the components is significant, as well as the definition of the individual axes. Standard cartographic practice is for geographic coordinates to be given in latitude-longitude order, while projected coordinates are often given in easting-northing order, though for certain jurisdictions (e.g. South Africa) another order is standard.

GML includes a set of components explicitly to support the description of geographic coordinate reference systems - see CoordinateReferenceSystems.

Other coordinates and tuples

It is sometimes convenient to record other complex values in a list or tuple, instead of in separate elements. This may be done in order to achieve a more compact encoding, though this is usually not a good enough reasons by itself, with modern bandwidth, storage capacity, and taking advantage of the highly compressible characteristics of XML.

A more important requirement is that "soft-typing" of components is frequently needed particularly in observational and scientific applications, i.e. where the exact definitions of the components of a tuple are experiment- or run-specific, so it is not practical to use XML Schema to define specific tags to support this. A common place for this to occur is in observations and in coverages that record systematic observations.

In XMML we follow the GML pattern used for position, described above, for a set of elements and types used for records/tuples and tables, described briefly in SimpleContentTypes. All of the record and table elements carry a RS attribute, whose value indicates the Record Schema for the tuple. At a minimum the description of the reference system should indicate names and units of measure for each component axis.

Since the Record Schema is only loosely coupled to the data, it may be provided externally. For convenience, however, XMML components for the description of record schemas are provided.

Schema documents

XmmlSchemaRepository:trunk/sweCommon/1.0.30/recordSchema.xsd

Phenomenon Definition

Phenomenon definition - for soft-typing properties, tuples, etc

Record Schema Definition

Record Schema definition - to provide tuple structure
Topic attachments
I Attachment Action Size Date Who Comment
phenomenon.PNGPNG phenomenon.PNG manage 29.8 K 04 Jan 2006 - 19:19 SimonCox Property definition - for soft-typing properties, tuples, etc
recordSchema.PNGPNG recordSchema.PNG manage 34.2 K 04 Jan 2006 - 19:19 SimonCox Record Schema - to provide tuple structure
Topic revision: r16 - 15 Oct 2010, UnknownUser
 

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