OPeNDAP usage in Ferret
Note that starting with V6.6 of Ferret, Ferret is linked with NetCDF-4 libraries (from http://www.unidata.ucar.edu/software/netcdf) instead of OPeNDAP libraries (from http://www.opendap.org),and V6.6 has new qualifiers on commands to allow handling NetCDF-4files. Future releases of Ferret will all be linked with NetCDFlibraries. NetCDF-4.1 and higher includes classic NetCDF dataset access and also netcdf 4 fileswith HDF5 capabilities for compression and chunking. It also includes remote data access with build-inOPeNDAP client. The Ferret manual, Chapter 2, has more details particular to Netcdf-4. To see which netCDF version the Ferret executable is linked with, the command
yes? show symbol netcdf_version
will query the netCDF library and return the version number.
OPeNDAP is protocol formerly known as DODS, the Distributed Oceanographic Data System. It allows users to access data anywhere from the internet using a variety of client/server methods, including Ferret. Employing technology similar to that used by the World Wide Web, OPeNDAP and Ferret create a powerful tool for the retrieval, sampling, analyzing and displaying of datasets; regardless of size or data format (though there are data format limitations).
Using OPeNDAP datasets in Ferret is just like using any other data set.
Prior to Ferret v7, url must be enclosed in quotation marks. This requirement is lifted in Ferret v7 and higher; quotes are optional.
yes? SET DATA "http://ferret.pmel.noaa.gov/pmel/thredds/dodsC/data/PMEL/levitus_climatology.nc" yes? SHADE temp[z=0] ! Ferret v7 and higher yes? SET DATA http://ferret.pmel.noaa.gov/pmel/thredds/dodsC/data/PMEL/levitus_climatology.nc yes? SHADE temp[z=0]
Documentation on use of OPeNDAP data sets in Ferret is in the Ferret User's Guide Ch2 Sec7: Access to remote data sets with OPeNDAP
Ferret and OPeNDAP demonstrations
dods_demo - html version
dods_winds_demo - html version
dods_demo.jnl - script file
Full OPeNDAP documentation can be found at http://www.opendap.org/
The features of OPeNDAP 3.2 include:
A client-side startup file (.dodsrc)
Client-side caching to store data you have requested on your local computer. The size and location of the cache, and expiration dates can be set using the initialization file .dodsrc
Proxy server support, configured in the .dodsrc file
Availability of a OPeNDAP master data set list
Finding OPeNDAP datasets
Search the OPeNDAP data set list
Search the Global Change Master Directory
Browse OPeNDAP datasets through LAS
PMEL, ESRL, and FSU are sample OPeNDAP servers.
yes? SET DATA "http://ferret.pmel.noaa.gov/pmel/thredds/dodsC/data/PMEL/levitus_climatology.nc"
See also the NVODS homepage, the National Virtual Ocean Data System.
For assistance in setting up an OPeNDAP server at your site, please see the OPeNDAP webpage.