WCS Access to netCDF Files

Wrapper software for WCS access to netCDF files

Background

The netCDF file format is a common way of storing gridded meteorological and air quality model results. The netCDF files that are also conformant with the CF convention further enhances the semantics of the netCDF files. Most of the recent model outputs are conformant with netCDF-CF.

The netCDF-CF convention facilitates the storage and transmission of model data sets using standard formats. The low level libraries provided by UNIDATA also provide a clear API for creating and accessing data from netCDF files.

Data access through OGC WCS protocol

The WCS protocol uses a physical coordinate system for accessing data, by explicitaly facilitating space-time queries using a geographic bounding box and a time-range to define the data access request. The purpose of this effort is to create a portable software template for accessing netCDF-formated data using the WCS protocol. Using that protocol will allow accessing the stored data by any WCS complient client software. It is hoped that the standards-based data access service will promote the development and use of distributed data processing and analysis tools.

WCS access to model outputs in the Juelich HTAP archive

The initial effort is focused on developing and applying the WCS wrapper template to the model outputs created for the HTAP global model comparison study. These model outputs are being managed by Martin Schultz's group at Forschungs Zentrum Juelich, Germany.

Wrapper software description and instalation

The WCS protocol consists of three service calls:

1. getCapabilities
2. describeCoverage
3. getCoverage

The purpose of the wrapper software is to respond to these three HTPP GET queries. The main query is getCoverage, which returns the requested data subset. The getCapabilities returns an XML file which lists the datasets offered by the server. The describeCoverage describes each data layer in more detail.

The WCS-netCDF wrapper software was developed for DataFed on a Windows .NET development platform. However, most other servers use the Linux operating system. For this reason, the WCS wrapper developed and tested for both Windows and Linux versions. In fact, test servers have been prepared for both platforms.

• Test server Windows platform (this will link to a live server)
• Test server Linux platform (this will link to a live server)

The main componenets of the wrapper software are shown schematically in the Figure below.

At the lowest level are open source libraries for accessing netCDF, XML files. At the next level are Python scripts for extracting spatially subset slices for specific parameters and times. At the third level, is the WCS interpreter that parses the WCS url. Administrative tools are also provided for creating the Capabilities and the Description documents.

The detailed instructions for the WCS wrapper software are maintained on a separate page, maintained by Kari Hoijarvi (kari@me.wustl.edu), 314 935 5772.