Accessing the NCTR Forecast Propagation Database
The NCTR Propagation Database can be accessed for Hazard Assessment studies through the OpenDAP interface for netCDF files. This allows accessing the large files (totaling tens of terabytes of data) without having to download them. The OpenDAP interface is a programming interface allows analysis programs like Matlab, Ferret, or simple viewing programs like ncBrowse and IDV, to access just the prortions of these netCDF files they need. The beauty of this interface is that the user sees no difference between a netCDF file sitting on their desktop, and on sitting on an OpenDAP server.
The NCTR Propagation Database can be accessed through this OpenDAP URL: http://sift.pmel.noaa.gov/thredds/catalog.html .
Accessing the Propagation Database with a web browser interface
Figure 1 shows a web-browser interface to the NCTR OpenDAP server, showing the Data URL, the Global Attributes including all information about the MOST model run, such as timestep, grid used, seismic parameters, such as Rake, Slipe, Dip, Strike and Depth, as well as all the Variables in the file, and the axes the variables are defined on.
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| Figure 1. Access to the Propagation Database through a browser, showing the simple form interface, global attributes, and variable a and axes data. |
Accessing the Propagation Database with popular analysis packages
Access to the database using an analysis package can be as easy as replacing the filename of a netCDF file with a URL. Figure 2 shows a plot of a MOST propagation run plot using the Ferret analysis package. The “use” statement would normally reference a local file by filename, but in this case it has been replaced by the URL for the file referenced in Figure 1. The data access speed depends upon available bandwidth, but most packages like Ferret only request the data range needed, instead of copying the whole file over.
Other packages like Matlab and IDL have plug-ins for OpenDAP access, too, and there is a large community of users of on-line OpenDAP data access. Unidata, the NSF-funded multi-institution program that brought us the netCDF file format and the OpenDAP interface, also provides a free, open-source data visualization program called the Integrated Data Viewer, or IDV. Figure 3 shows the maximum amplitude field of on of the Aleutian Island propagation model runs using IDV.
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| Figure 2. Using the OpenDAP interface within Ferret by simply refering to the URL instead of a filename. | Figure 3. Maximum amplitude plot of Propagation Database file ac_018_a_ha.nc from OpenDAP connection plotted using Unidata's Integrated Data Viewer (IDV). |
Accessing the Propagation Database with the OpenDAP API
The OpenDAP connection can also be used as an Application Programming Interface, or API, as well. This means that any program (e.g., a numerical model), that accesses netcdf files, can access on over the internet using OpenDAP with a very few changes in file reference. NCTR takes advantage of this feature to allow Inundation models to run, forced by Propagation output over OpenDAP. The only code changes that were necessary was to change the filename to a URL (as before), and to recompile against the OpenDAP libraries instead of netCDF libraries.
More information about the NCTR Propagation Database can be found at https://nctr.pmel.noaa.gov/propagation-database.html.
