2007-02-26: DataFed WCS Characteristics
This page is currently a workspace for describing the DataFed interpretation of the OGC Web Coverage Service (WCS) specification, in particular the unique characteristics of using WCS for point and grid coverage data. The goal is to develop a set of interoperability guidelines for others who wish to serve data using the same approach.
Current documentation available on the DataFed WCS:
 WCS Definition
The Web Coverage Service (WCS) supports electronic retrieval of "coverages" – that is, digital geospatial data representing space-varying phenomena. The Web Coverage Service provides three operations: GetCapabilities, DescribeCoverage, and GetCoverage.
The GetCapabilities operation returns an XML document describing the service and brief descriptions of the coverages that clients may request.
The DescribeCoverage operation lets clients request a full description of one or more coverages served by a particular WCS server.
The GetCoverage operation returns a coverage (that is, values or properties of a set of geographic locations), encoded in a well-known coverage format. Its syntax and semantics bear some resemblance to the OGC Web Map Service (WMS) GetMap and Web Feature Service (WFS) GetFeature requests, but several extensions support the retrieval of coverages rather than static maps or discrete features.
See the OGC WCS specification page for details.
This discussion will focus on aspects of the WCS GetCoverage for serving and retrieving spatial and temporal data.
 What is a coverage?
The current version of WCS only supports grid and image data types (point data are not explicitly covered by the specification). However, the OGC Coverage Abstract Specification provides a broader definition of a coverage:
"A coverage can be designed to represent a single feature, or a set of features. For example, a coverage may have a spatial domain containing a single parcel, or an entire platt. In the latter case, the coverage may be treated as modeling a single feature (the platt), or as a collection of features (the collection of parcels)."
Therefore a coverage can also include a collection of points (e.g. weather monitoring stations measuring spatially continous parameters such as temperature and wind speed). In our example, the air quality monitoring stations themselves are considered features as they represent discrete objects with an associated set of attributes (station type, start date, operator,…). The observations from those monitors are considered coverages as, taken as a whole, represent the state (in space and time) of a spatially continuous phenemona.
Expanding WCS beyond grids is a item of future work for the WCS revision working group.
Because point data are a core data type in DataFed, it has implemented the WCS to include point data along with gridded data as coverages.
 DataFed Implementation
 Filtered Data Request
The current OGC WCS specification for GetCoverage requests defines ways to request data based on:
- Spatial bounding box
- Time instant or time range
The DataFed implementation extends the GetCoverage request to allow requests for:
- Restricting request to specific sites (location code filter)
- filtering based on specific months (e.g., only request data for the months June, July and August over a multi-year time range)
 Type of data access needed for an interoperable point/grid WCS
Here are some examples of implementing WCS interfaces to point and grid data.