Using GrADS via OPeNDAP

This page explains the use of Grid Analysis and Display System (GrADS external ) OPeNDAP visualization client to access and display HDF-EOS data. The GrADS is an interactive desktop tool for easy access, manipulation, and visualization of earth science data. Follow these simple steps to display the data of interest.

We use a NASA OMI HDF-EOS5 data external for illustration. You can also download it here.

1. Download and install GrADS

GrADS can be downloaded and installed from any of the following web links:

GrADS home page.
OpenGrADS Sourceforge page.

In this example, we used the OpenGrADS version v2.0.a5.oga.5 and any versions higher than 2.0 of GrADS or OpenGrADS will work. GrADS require an X-server. An X server is the software which processes instructions from "X clients" such as GrADS to do things like draw on the screen. However, starting with Version 1.9, an Xming X Server is bundled and integrated with Win32 GrADS.

2. Start GrADS

Several GrADS applications are available. The ones that are useful for our purpose include:

Application	Description
grads	Win32 GrADS with the classic command line interface (ga-> prompt). This version can read all supported GrADS formats including NetCDF-3, NetCDF-4/HDF-5, HDF-4, GRIB-1/2 and on-line access to data on OPeNDAP servers (both gridded and station datasets.)
gradsgui	Win32 GrADS with a Graphical User Interface. This version can read all supported GrADS formats including NetCDF-3, NetCDF-4/HDF-5, HDF-4, GRIB-1/2 and on-line access to data on OPeNDAP servers (both gridded and station datasets.)
gradsdap	This the same as grads above. In the past you needed a separate build of GrADS to access OPeNDAP servers. Starting with v2.0.a5.oga.2 this functionality is available in the main executable Grads.

Table 1 (Source: 'Getting_Started' HTML document in the package.)

To start up GrADS, either double click the 'grads' program file or enter grads at the command line prompt. If grads program file is not in your current directory, or if it is not in your PATH somewhere, you may need to enter the full path name.

Figure 1 GrADS Command Prompt Window

In the above text window (where you started grads from), press enter for the question Landscape mode? ('n' for portrait): and you should now see a prompt: ga>. You will enter GrADS commands at this prompt and see the results displayed in the graphics output window below.

Figure 2 GrADS Graphics Output Window

3. Choose the file to open

In order to choose the data you need to work with, issue the following command:

sdfopen <URL>: For selecting the file/data you need to open.

For example, type exactly like below:

Figure 3 Type an example OPeNDAP URL

sdfopen http://acdisc.sci.gsfc.nasa.gov/opendap/HDF-EOS5/Aura_OMI_Level3/OMTO3d.003/2009/OMI-Aura_L3-OMTO3d_2009m0102_v003-2009m0602t151018.he5

Alternatively, if the above NASA URL doesn't work, you can try The HDF Group OPeNDAP demo server URL that serves the same data:

Figure 4 Type an alternate OPeNDAP URL

sdfopen http://eosdap.hdfgroup.uiuc.edu:8080/opendap/data/NASAFILES/hdf5/OMI-Aura_L3-OMTO3d_2009m0102_v003-2009m0602t151018.he5

4. Display the data

You can issue the following interactive commands for displaying the data:

set gxout shaded: For specifying the levels and colors for filled contour plots.
query file: For querying the file opened above. Also, gives you the list of variables that can be displayed.
d <variable_to_display> : 'd' is the short for display. It is used to display the variable you need to view.

You would be able to view the data in the graphics output window. For example, we gave d radiativecloudf (or d a1radiativeclou for ) to display the Radiative Cloud Fraction variable. It should look like:

Figure 5 GrADS Final Image

The same data variable would display in IDV by selecting Radiative Cloud Fraction -> Color Shaded Plan View -> Create Display in the Field Selector window. It should look like:

Figure 6 IDV Final Image

Last modified: 06/01/2011