Using GrADS via OPeNDAP

This page explains the use of Grid Analysis and Display System ( GrADS ) 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 for illustration. You can 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 GrADS Applications (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.

• 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.

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:

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 to display the Radiative Cloud Fraction variable. It should look like: 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:

Last modified: 11/11/2020