WCS General Intro
This screencast is an illustration of standard-based data access services in the data system DataFed. The data sets are distributed geographically and maintained by their respective custodians. The purpose of the federation is to provide a uniform data access interface to all data regardless of their location, operating environment, and native data access protocols. The homogenization of the heterogeneous, distributed data is accomplished through the DataFed Middleware. At this time the focus of DataFed is on facilitating access to data sets relevant to air quality science and management.
The federated data sets are accessed through the catalog where the user can identify desired data sets. In this illustration, we will use the TOMS Aerosol Index data set, which is available from the TOMS website in form of images as well as in form of gridded numerical data. The TOMS images will illustrate data delivery through the OGC web map service, WMS, while the gridded data will show the use of the coverage service, WCS.
The TOMS daily global aerosol images are available since 1996. For each day the user can browse any particular day by entering the date in the text box.
The data access options are shown in the web service program that creates this image. The WMS access to the above image is shown by the “GetMap” service. The URL service call is shown in that address box of the web browser.
The associated WMS OGC compliant service description is accessible through ‘GetCapabilities’ button.
With these two services, any OGC compliant thin client can access, display, zoom, etc the daily TOMS data set. All spatial data sets in DataFed are accessible through OGC WMS interface.
The TOMS images are convenient for use since the data rendering and portrayal are already completed at the server. However, such image data do not lend themselves to numerical analysis, filtering, aggregation, fusion, etc or for alternative user defined rendering. The TOMS data are also available as numerical, gridded data values which allows all the above operations to be performed either through the Middleware or by the client.
Standard-based access to spatially distributed observational data is facilitated through the Web Coverage Service (WCS). In principal, the coverage can be a gridded array, irregular grids or spatially dispersed set of points’. The gridded TOMS data for a particular day are shown in the MapView. However, it is now possible to change the rendering by setting the scale to a different value.
The gridded data can be accessed through the Web Map Service (WMS) and used in any OGC WMS client. However, it should be noted, that in this case the user is able to save her settings in a ‘view file’, change the rendering, etc.
Equally important is that the gridded data are accessible through the Web Coverage Service. The format of the returned coverage data can be a simple Comma Separated Variable (CSV), GML grid, NetCDF cube, GeoTIFF Image, etc. The WCS query is executed through GetCoverage service. The URL of the WCS query is shown in the address box of the web browser. The GetCapabilities and DescribeCoverage services complete the WCS set and facilitate grid data access by OGC compliant WCS clients.
Another advantage of the WCS data access protocol is that it allows the retrieval of time series data, vertical profiles, multidimensional data cubes, etc.