Introduction to the WXXM
What is the WXXM?
The WXXM is a logical data model that combines the concepts from the high-level WXCM packages into a coherent model that takes into account concerns related to data exchange. Similar to the concept of normalizing a database schema to reduce the duplication of information in associated objects, the logical weather model factors common information out of related objects where possible, with a goal of relatively efficient data transfers.
The weather observation object, for example, separates observation-specific information from the result of the observation --via its use of the OGC O&M specification--, allowing for multiple weather properties to be the result of a single observation. The observation information itself is not repeated unnecessarily for each measured property, minimizing the footprint of the data when exchanged.
The WXXM follows the GML object-property model, which requires the properties of objects to be encapsulated by a simple type (domain value). Should a ‘property’ consist of a complex object or feature, the relationship must be represented through the use of an association.
OGC O&M specification -- ISO/DIS 19156
The WXXM has been developed using the OGC schema for Observations and Measurements (O&M). The OGC offers the following definition of an observation: “An Observation is an action with a result which has a value describing some phenomenon. [...] An observation feature binds a result to a feature of interest, upon which the observation was made. The observed property is a property of the feature of interest. An observation uses a procedure to determine the value of the result, which may involve a sensor or observer, analytical procedure, simulation or other numerical process.”
The OGC observation model in a UML diagram:
It should be noted that, in the OGC terminology, a Forecast is considered to be a type of observation, the only difference being the time of the result and the procedure used to determine the result.