NOAA/NASA Pathfinder SSM/I Level 3 EASE-Grid Brightness Temperatures Frequently Asked Questions (FAQ)

This document is intended as a quick question and answer reference guide, and, rather than including some lengthy information in two places, often refers to the location of complete information in the User's Guide for this product.

The Questions

  1. What does the acronym EASE in EASE-Grid stand for?
  2. What do the data filenames refer to? What do subdirectory names like 'D213_217' refer to?
  3. What types of data are in different files? What are the units?
  4. What are the dates of data available on a specific CD?
  5. What grid resolutions are used?
  6. What are the dimensions of the grid arrays?
  7. How do I uncompress data from the CD? How big will uncompressed files be? (See also Question 20.)
  8. What are the parameters used to define the EASE-Grids used on the CD?
  9. Do I need IDL to read and display this data?
  10. In the latitude/longitude files, to which location in the pixel does the latitude/longitude refer?
  11. What are the *.doc files that appear in the pentad directories on early CD volumes and not later ones?
  12. What other passive microwave Pathfinder data are available and for what time periods?
  13. How have the tools changed between different CDs in the series?
  14. I can't find the latitude/longitude files in /yyyy/proj/. Where are they?
  15. How should I cite these data in my publications?
  16. Why are there no 85 GHz data files for the date I am extracting?
  17. Why don't my images look like the ones on the cover of the CD-ROM?
  18. The link to the EOSDIS Acronyms appears broken in the data documentation. What is the correct link?

Tool Questions

  1. The IDL EASE-Tools scroll window doesn't work right on PCs with Windows. What can I do to fix this?
  2. The IDL EASE-Tools show_print function doesn't work right. How can I make it work?
  3. The IDL EASE-Tools show_set_grid procedure always prompts me for the grid number, even when I enter the grid-name. Shouldn't this occur only when using it without the argument?
  4. Do I have to uncompress the CD data to read it into IDL?
  5. On a DEC workstation, the extraction script getidl.csh didn't maintain any of the .hdf or .pro file extensions. What is wrong?
  6. On a DEC workstation, the IDL EASE-Tools ssmi_read procedure says the file doesn't exist, but I know it does. What is wrong?
  7. The function HDF_ISHDF is not working with IDL. What is wrong?
  8. On a Windows PC, the displays are just shades of gray, instead of color coded output from the show_* routines. What is wrong?
  9. Why did the link to the EASE-Grid tools (maps.tar.gz) change? Where can I access them?

The Answers

  1. EASE-Grid stands for Equal-Area Scalable Earth Grid, and refers to the set of three equal-area projections (North and South azimuthal, and global cylindrical), combined with an infinite number of possible grid definitions. (Originally, the 'S' in EASE stood for 'SSM/I' but because there is nothing SSM/I-specific about the 25 and 12.5 km grid definitions, NSIDC changed the acronym connotation.) The grids chosen for this SSM/I data set include 25 and 12.5 km resolutions.

  2. The complete data filenames (including pathname) determine the projection, resolution, year, day of year, direction of satellite passes (i.e. ascending or descending) and channel/time of the contents. The subdirectories named 'Dddd_ddd' contain daily temperature and time data for (at most) 5-day pentads. These subdirectory names include the day of year numerals that indicate the dates spanned in a given subdirectory. For more detailed information, refer to the data format section in the User's Guide.

  3. These answers depend on the kind of file, which can be determined from the directory locations and filenames. For detailed information, refer to the data format section in the User's Guide.

  4. Each CD contains data collected from the SSM/I on DMSP-F8, -Fll or -F13. The CD-level metadata file (i.e. the file with a .TXT extension in the root directory) indicates from which platform the data were collected, and the dates of data provided on a given CD. Each file name consists of the CD projection series letter, N, S, or M, and the volume number, e.g. NVOL38.TXT on the Northern Hemisphere CD series, volume 38.0.

  5. Both high and low resolutions grid are available. L indicates 25 km (i.e. 'low') resolution, and H indicates 12.5 km (i.e. 'high') resolution, which is only used for the 85 GHz channels. No 85 GHz data exist from 1 February 1989 through 31 December 1991, please see the 85 GHz data temporal coverage section of the Users Guide for more information.

  6. Table 1 lists the dimensions of the grid arrays. For more detailed information, refer to the data format section in the User's Guide.
    Table 1. Dimensions of the Grid Arrays
    grid name file name width height
    Nl /yyyy/NORTH/pentad/L* or /yyyy/NORTH/NL* 721 721
    Nh /yyyy/NORTH/pentad/H* or /yyyy/NORTH/NH* 1441 1441
    Sl /yyyy/SOUTH/pentad/L* or /yyyy/SOUTH/SL* 721 721
    Sh /yyyy/SOUTH/pentad/H* or /yyyy/SOUTH/SH* 1441 1441
    Ml /yyyy/GLOBL/pentad/L* or /yyyy/GLOBL/ML* 1383 586
    Mh /yyyy/GLOBL/pentad/H* or /yyyy/GLOBL/MH* 2766 1171

  7. The CD contains compressed binary data files, with *.GZ extensions. To uncompress any of these files and write the uncompressed binary file to your machine, use:
    gunzip -vc /cdrom_path/DATAFILE.GZ > /your_machine_path/DATAFILE
    If you do not already have gunzip, refer to information on GZIP provided in the software section of the User's Guide.
    Uncompressed files will have the following sizes:
    Table 2. Northern and Southern Hemisphere Projections
    File Name Size (bytes)
    Ldddpccc   1,039,682
    LdddpTIM      519,841
    Hdddpccc  4,152,962
    {N,S}L (LAT/LON)  2,079,364
    {N,S}H (LAT/LON) 8,305,924
    Table 3. Global (Cylindrical) Projection
    File Name Size (bytes)
    Ldddpccc    1,620,876
    LdddpTIM       810,438
    Hdddpccc    6,477,972
    ML (LAT/LON)    3,241,752
    MH (LAT/LON)  12,955,944

  8. Please see EASE-Grid: A Versatile Set of Equal-Area Projections and Grids.

  9. We have included some IDL* tools for those users who already have IDL, but if you do not, any image processing software can be used to display the flat binary data files. *(IDL is a display package produced by Exelis Visual Information Solutions, for more information see their Web site at

  10. The center of the pixel. Refer to the discussion of grid coordinates in the Summary of EASE-Grid Map Projection Parameters.

  11. There are two *.doc files in each pentad directory only on CDs with data through December 1991. These are ASCII files containing metadata information for the data files contained in that directory. These files were discontinued for CDs beginning with Northern and Southern Hemisphere Volumes 16.0 and Global Volume 29.0.

  12. For complete information on temporal coverage and dates of missing data, see the SSM/I-SSMIS Data Availability page. NSIDC also distributes Nimbus-7 SMMR Pathfinder Daily EASE-Grid Brightness Temperatures from 25 October 1978 through 20 August 1987.

  13. IDL tools: Beginning with Northern Hemisphere Volume 79.0, Southern Hemisphere Volume 38.0, and Full Global Volume 159.0, the files and replace the versions included in earlier CD-ROMs. The new versions allow:
    • Windows users to display images using a partitioned color table instead of shades of grey
    • Linux users to take advantage of gunziping data on-the-fly directly from the CD-ROMs.

    Please refer to /TOOLS/IDL/README.IDL on Northern Hemisphere Volume 79.0 or later, Southern Hemisphere Volume 38.0 or later, or Global Volume 159.0 or later for details of these changes.
    Beginning with Northern and Southern Hemisphere Volume 38.0 and Full Global Volume 70.0, the following files replace the versions included on earlier CDs:,,, and Beginning with the same volumes, the file is obsolete and is no longer included. The file released on Northern Hemisphere Volume 4.0 or later, all Southern Hemisphere volumes and Global Volume 2.0 or later replaces from earlier CDs. Major features in the revised tools allow Unix users to:
    • read compressed data directly from the CD into IDL
    • include a new keyword, ARRTYPE, to the ssmi_read procedure that allows users to read 4-byte geolocation files
    • perform coordinate conversions between lat/lon and col/row for all EASE-Grids (AVHRR, TOVS, AARI, as well as SSM/I) currently defined as of the release of these tools
    • use the EASE_FILENAME function (in to build a full CD filename (including pentad pathname) for a given combination of date, channel, pass direction and grid
    • use a revised version of SSMI_AVERAGE (in to average data from a set of filenames, even when the files are in different pentad directories.

    Please refer to /TOOLS/IDL/README.IDL on Northern and Southern Hemisphere Volumes 38.0 or later or Full Global Volumes 70.0 or later for details of these changes.
    Documentation: The documentation files for the CDs with data through December 1991 are ASCII format. Documentation for CDs with data after December 1991 has been completely revised and edited, and is included in html format. Users requiring the documentation in another format please contact NSIDC User Services.
    Data: The data pentad directories contain data specific to the time period, sensor platform and projection of the CD. Beginning with Northern and Southern Hemisphere Volumes 16.0 and Global Volume 29.0, higher precision, 4-byte geolocation files are included that replace the 2-byte geolocation files included on earlier CDs
    Other Tools: Beginning with Northern Hemisphere Volume 79.0, Southern Hemisphere Volume 38.0, and Global Volume 159.0, the freeform and makeHDF utilities are no longer included on the CD-ROMs. Users who want copies of these should contact NSIDC User Services. For the same CD-ROM volumes, NSIDC replaced the GETBENCH.CSH script with GET_EASE_DATA.CSH. The new script can gunzip data for either SMMR or SSM/I data. Unless you have plans to use SMMR data, you should not need to replace GETBENCH.CSH with GET_EASE_DATA.CSH.
    Metadata: Pentad metadata (*.DOC) files have only been included on CDs containing data through December 1991.

  14. If you are reading one of the CDs with data from two years, the latitude/longitude files will be located in the projection subdirectory that corresponds to the earlier of the two years. For example, if you are looking at a CD that contains 1987 and 1988 data, these files will be located in /1987/proj/. Obviously, these data are not date-specific, but we have retained this convention to remain consistent with older CD volumes. Note to users of lat/lon files from Northern and Southern CDs through Volume 15.0 and Global CDs through Volume 28.0: higher precision versions of these files have been included on subsequent CD volumes.

  15. In the [insert date of data acquisition] space in the following citation example, you should insert the date you obtained the data from NSIDC, not the temporal coverage of the data.

    Armstrong, R.L., K.W. Knowles, M.J. Brodzik and M.A. Hardman. 1994 - [insert date of data acquisition]. DMSP SSM/I Pathfinder daily EASE-Grid brightness temperatures. Boulder, CO: National Snow and Ice Data Center. Digital media and CD-ROM.

  16. No 85 GHz data are included on the F8 CDs from 1 February 1989 through 31 December 1991. Please refer to the discussion of temporal coverage in the User's Guide.

  17. Data in individual files are daily, ascending or descending pass data. Due to a combination of orbit geometry and sensor characteristics, polar orbiting sensors like SMMR and SSM/I do not acquire full, global coverage in any 24-hour period. The images on the cover of the CD-ROM do not represent data from single files, but are derived from an average of ascending SSM/I brightness temperatures for a sequence of eight days.

  18. The URL for the EOSDIS Acronyms Web site has recently changed. Data documentation on some CD volumes may contain the outdated URL (, which has been replaced by the new URL:

Answers to Tools Questions

  1. It has been brought to our attention that PC users running the IDL EASE tools have problems re-sizing the scrollable windows, where only a portion of the re-sized window is refreshed properly. ITT's technical support staff tells us this is a known bug that will be fixed in a future release. In the meantime, PC Windows users can force a correct refresh action by clicking on one of the scroll bars after re-sizing the window.

  2. The README.IDL file for the Northern Hemisphere Volume 1.0 and 2.0 CDs had an incorrect example for running the show_print procedure. The example should read:
    IDL> show_print, -1,"Whatever label you want"
    instead of
    IDL> show_print, REVERSE=-1,"Whatever label you want"
    Basically, there is no REVERSE keyword, but there is a required argument (1 or -1) that is used by the procedure to reverse the black and white colors, or not. Users should refer to the usage message by typing show_print with no arguments, for complete information.

  3. Yes. Double check the grid name you are using. The grid names are all 2-letters, the first letter is always uppercase, either 'M', 'N' or 'S', that specifies the projection, followed by a lowercase letter, 'l' or 'h', that specifies the resolution (low or high). You may be entering the numeral '1' instead of lowercase 'l'.

  4. No. IDL users on Unix platforms can read the CD data directly into IDL, using the ssmi_read procedure distributed for the first time on this CD.

  5. The getidl.csh does not work as expected on DEC platforms. To extract the tools, perform the following three steps:
    1. Copy all .pro files to your local drive, and make the filenames all lowercase.
    2. Copy all .hdf (that contain map and graticule overlays and land and ocean masks) files to your local drive, and make the filenames all uppercase.
    3. Copy the ease script to your local system and follow the directions in it to set EASE-Tools environment variables.

    These are the steps the getidl.csh script was supposed to perform. We apologize for any inconvenience this may cause.

  6. Users on DEC workstations may get the following error message from ssmi_read:
    % SSMI_READ: File foo does not exist
    This error may be coming from the DEC convention whereby a semi-colon and version number are considered part of the filename. Even if you enter "foo;1" you will get the error message, because the semi-colon will very likely have a special meaning to the operating system. In order for ssmi_read to find the file, use the backslash ("\") character before the semi-colon, thus:
    IDL> ssmi_read,data,"foo\;1"
    and the procedure should then run as expected.
  7. Exelis has identified a problem with HDF_ISHDF for IDL versions 4.x. When called with a non-HDF or a non-existent filename, HDF_ISHDF returns an error condition instead of the expected value 0 (indicating false). For users running IDL 4.x, ssmi_read will return the HDF_ISHDF error. Since the check for HDF files is simply a courtesy in this routine, and does not otherwise affect the correct retrieval of brightness temperatures or times from data files, the user can proceed by commenting out the conditional code that is making the call to HDF_ISHDF.
  8. Users of the IDL tools on Windows machines should add the following line to the end of the STARTUP.PRO file:
    Be sure to restart IDL after making this change to your STARTUP.PRO.) The output from the show_* routines should be displayed in colors other than shades of gray.

  9. NSIDC has a common software tools location, the Data Analysis & Imaging Tools page. You can still access the C version of Mapx (maps.tar.gz) by following the Mapx link on the Data Analysis & Imaging Tools page.