SWE Common Data Model OGC 08-094r1
Examples
Satellite location data can be defined as a vector of 3 components, expressed in the J2000 ECI Cartesian frame, the 1
st
component being associated to the X axis, the 2
nd
to the Y axis and the 3
rd
to the Z axis. Angular velocity data from an Inertial Measurement Unit can be defined as a vector of 3 components, expressed in
the plane reference frame for instance ENU defined by local East, North, Up directions, the Euler components being mapped to X, Y, Z respectively.
Relative time data can be given with respect to an arbitrary epoch itself positioned in a well defined reference frame such as TAI from the French “Temps Atomique International” = International Atomic Time.
Requirement
http:www.opengis.netspecSWE2.0reqcoretemporal-frame-defined
Req 9. A temporal quantity shall be expressed with respect to a well defined
temporal reference frame and this frame shall be specified.
Requirement
http:www.opengis.netspecSWE2.0reqcorespatial-frame-defined
Req 10. A spatial quantity shall be expressed with respect to the axes of a well
defined spatial reference frame and this frame shall be specified.
The “Time” class detailed in clause 7.2.9 is designed for carrying a temporal reference frame or a time of reference in the case of relative time data.
The “Vector” class detailed in clause 7.3.2 is a special type of record used to assign a reference frame to all its child-components.
The “Matrix” class defined in clause 7.5.2 allows the definition of higher order tensor quantities.
This standard does not impose requirements on the type of reference frames that a standardization target shall support. Standards that are dependent on this specification can
and often should however define a minimum set of reference frames that shall be supported by all implementations.
6.4 Data Quality
Quality information can be essential to the data consumer and the SWE Common Data Model provides simple and flexible ways to associate qualitative information with each
component of a dataset.
Copyright © 2011 Open Geospatial Consortium
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OGC 08-094r1 SWE Common Data Model
6.4.1 Simple quality information
Simple quality information can be associated with any scalar data component, in the form of another scalar or range value. The quality information defined here applies solely to
the value of the associated data component i.e. the measurement value and, depending on its data type, quality can be represented by a numerical, categorical or textual value, or
by a range of values.
This quality information can be static, i.e. constant over the whole dataset, or dynamic and provided with the data itself. In this case, the quality value is in fact carried by
another component of the dataset and described in SWE Common as such.
The exact type of quality information provided should be specified via semantic tagging just like with any other property in SWE Common.
Examples
Examples of quality measures are “absolute accuracy”, “relative accuracy”, “absolute precision”, “tolerance”, and “confidence level”.
Quality related comments can also describe operating conditions, such as “sensor contained blockage and was removed” or “engineer on site, values may be affected”. This information can inform the user of potential
inaccuracy in the data across certain periods.
6.4.2 Nil Values
The concept of NIL value is used to indicate that the actual value of a property cannot be given in the data stream, and that a special code i.e. reserved value is used instead. It is
thus a kind of quality information. The reason for which the value is not included is essential for a good interpretation of the data, so each reserved value is associated to a
well-defined reason. In that sense, a NIL value definition is essentially a mapping between a reserved value and a reason.
Requirement
http:www.opengis.netspecSWE2.0reqcorenil-reasons-defined
Req 11. A model of a NIL value shall always include a mapping between the selected
reserved value and a well‐defined reason.
Each component of a dataset can define one or several NIL values corresponding to one or more reasons.
Examples
In low level satellite imagery with, for instance, 8-bits per channel, the imagery metadata often defines: - A reserved value to indicate that a pixel value was “Below Detection Limit” usually set to ‘0’
- A reserved value to indicate that a pixel value was “Above Detection Limit” usually set to ‘255’
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Copyright © 2011 Open Geospatial Consortium
