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Geolocating EASE-Grid Data in ENVI

This document describes how to geolocate North, South, and Global EASE-Grid data from the DMSP SSM/I 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. Download map_proj.txt and save it to your ENVI "map_proj" directory, overwriting the existing 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 the "map_proj" directory resides on a Windows PC is D:\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:\RSI\IDL60\products\envi40\save_add\.)

user_proj_cea_convert.pro : 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.

user_proj_cea_define.pro: 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.

3. Download useradd.txt to your ENVI "menu" directory, overwriting the existing file. (An example where the "menu" directory resides on a Windows PC is D:\RSI\IDL60\products\envi40\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 happens, you can copy these files, along with your "envi.cfg" file, to a directory where you have write access. Change your ENVI preferences accordingly.

4. Download the following ENVI header files (.hdr) to your computer.

globl12.hdr
globl25.hdr
north12.hdr
north25.hdr
south12.hdr
south25.hdr

5. Choose a header file that corresponds with the projection (north, south, or global) and spatial resolution (12 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". Be sure the header file and data file are in the same directory.

6. Start ENVI, 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 Tools -->Cursor Location/Value from the display window.

7. 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.