Geolocating EASE-Grid Data in ENVI

This document describes how to geolocate North, South, and Global EASE-Grid data from the DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures product using ENVI software. These procedures were tested with ENVI 4.0 on a Windows 2000 platform; other versions of ENVI may have slightly different menu options and steps. ENVI header files for other NSIDC EASE-Grid data sets will be added here in the future.

  1. Open map_proj.txt and copy the last four lines of the file. Find the ENVI map_proj directory that contains ENVI's copy of map_proj.txt. Paste the four lines you copied earlier to the bottom of this file. NSIDC revised this ENVI map projection definition file to include EASE-Grid North and South (azimuthal equal-area) and EASE-Grid Global (cylindrical equal-area) projections. (An example where this directory resides on a Windows PC is D:\Program Files\RSI\IDL60\products\envi40\map_proj\.)
  2. Download the following files to your ENVI save_add directory. (An example where this directory resides on a Windows PC is D:\Program Files\RSI\IDL60\products\envi40\save_add\.) Computes forward and inverse transformations for both the spherical and ellipsoidal forms of the cylindrical equal-area projection to serve as a User Projection in ENVI. Provides a user interface to obtain the latitude of true scale from the user for the cylindrical equal-area projection provided as ENVI "User Proj Cylind Equal Area" additional parameter #1.

    Note: For the above files to work as .pro files, save as ‘All files’ from the dropdown in the download dialog, then add .pro to the end of the filename.  Without this, Windows XP will not save them properly and will not recognize them as .pro files.
  3. Download useradd.txt to your ENVI menu directory, overwriting the existing file. (An example where this directory resides on a Windows PC is C:\Program Files\ITT\IDL71\products\envi47\menu\.)

    Unix and Linux users may not have write access to the default save_add or menu directories and the useradd.txt and map_proj.txt files. If this is the case, you can copy these files, along with your envi.cfg file that resides in the ENVI menu directory, to a directory where you have write access. Change your ENVI preferences accordingly.
  4. Download the appropriate ENVI header file (.hdr) from the following list to your computer. Choose a header file that corresponds with the projection (north, south, or global) and spatial resolution (12.5 km or 25 km) of your data file, and rename it to match the data file (keeping the .hdr extention). For example, if your North 25 km EASE-Grid data file is named L152A19V, then rename north25.hdr to L152A19V.hdr. Note: Be sure the header file and data file are in the same directory.

  5. Start ENVI, from the main ENVI menu bar, select File --> Open Image File, and select your data file. The Available Bands List window appears and lists the data file. Select Band 1 and click Load Band. The data file displays, and you can confirm the latitude and longitude of each pixel by selecting WINDOW -->Cursor Location/Value from the display window.
  6. Now that you have opened a data file using one of the header files from Step 4, you can open any data file with the same spatial resolution and hemisphere using this same header file, as long as the original data file remains open in ENVI. To see how this works, try the following steps.

    From the main ENVI menu, select File --> Open Image File and select a new data file. When the Header Info window appears, click Input Header Info From --> Other File... and select the original data file you opened in Step 6. ENVI uses the same header information to open your new file.