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SSM/I Brightness Temperature and Sea Ice Concentration Grids

Frequently Asked Questions (FAQ)

This document is continually updated. If you have questions, comments, or suggestions regarding the use of these data or this document, please contact NSIDC User Services.


Questions

    Data Description and Formats

  1. What is the difference between the DMSP SSM/I Brightness Temperature and Sea Ice Concentration Grids, and the NOAA/NASA Pathfinder Program EASE-Grid Brightness Temperatures?

  2. Why are there 0%, 5%, 10%, and 15% cut-off files for the monthly ice edge?

  3. How are latitude and longitude values expressed; that is, are there negative longitude values?

  4. If the polar stereographic projection causes some grid cells to be distorted, how can I determine the area of each cell?

  5. Data Display, Manipulation and Analysis

  6. What is HDF and how can I display HDF data?

  7. How can I extract just the region I am interested in from the whole grid?

    Systems, Software and Procedures

  8. What does the Fortran program 'locate' do?

  9. The program 'locate.for' gives different latitude and longitude values for the four corners of the grid cells than what is shown on the polar stereographic map of the User's Guide. What's the problem?

  10. How should NSIDC's data be cited in publications?


Answers

1. What is the difference between the DMSP SSM/I-SSMIS Brightness Temperature and Sea Ice Concentration Grids, and the NOAA/NASA Pathfinder Program EASE-Grid Brightness Temperatures?

The differences between these data sets are summarized in the table below. The DMSP Brightness Temperature and Sea Ice data are in a polar azimuthal stereographic projection only; the NOAA/NASA Pathfinder Program EASE-Grid Brightness Temperatures are mapped to polar azimuthal equal-area grids (Northern Hemisphere and Southern Hemisphere) and also to cylindrical equal-area grids (global).

SSM/I-SSMIS Polar Stereographic Grids EASE-Grids
stereographic projection equal-area projection
coverage limited to polar regions,
oriented towards sea ice applications
global coverage
individual files contain all passes averaged daily individual files contain separate daily ascending and descending passes (no averaging)
binning based on "drop-in-the-bucket" method binning based on weighting functions derived from actual antenna pattern
sea ice data is in Hierarchical Data Format (HDF), brightness temperature data is binary format data in binary format
no compression compressed using GNU gzip

For more information on map projections, see the Map Projection Overview site by Peter Dana, and NSIDC's Points, Pixels, Grids, and Cells: A Mapping and Gridding Primer document.

2. Why are there 0%, 5%, 10%, and 15% cut-off files for the monthly ice edge?

These four minima (or thresholds) accommodate as many potential data applications as possible. For example, to eliminate any possible contamination of a product by the presence of ice, a user might select the 0% threshold. Applications pertaining to the general location of the marginal ice zone might require the 15% threshold. Products created using intermediate thresholds can be examined to study the nature of ice margins or to highlight any residual weather or land contamination.

Monthly averaged data files contain ice concentrations in percent ranging from 0% to 100%. Ice concentrations are then delineated using 5%, 10%, and 15% as the minimum concentrations included in the averages (e.g. a 5% cut-off sets all data values less than 5% to 0%).

3. How are latitude and longitude values expressed; that is, are there negative longitude values?

Longitude values are positive, calculated to the east of the Greenwich (prime) meridian; there are no negative values (range is 0 degrees to 360 degrees). Latitude values are positive in the Northern Hemisphere, negative in the Southern (range is -90 degrees to 90 degrees). See the Spatial Coverage section of the on-line User's Guide for more information about grid coverage.

4. If the polar stereographic projection causes some grid cells to be distorted, how can I determine the area of each cell?

NSIDC has created data arrays which contain the areal values for individual cells in the 25 km SSM/I polar stereographic grids. These arrays (psn25area.dat and pss25area.dat) are available at the NSIDC anonymous FTP site. Area values for the 12.5 km grids are being prepared by NSIDC. Contact NSIDC User Services for more information.

5. What is HDF and how can I display HDF data?

The SSM/I Sea Ice Concentration grids are in Hierarchical Data Format (HDF). HDF and HDF utilities are public domain software, and are distributed by the National Center for Supercomputer Applications (NCSA). NASA has selected HDF to be the EOS standard data format.

HDF allows self-definitions of data content and easy extensibility for future enhancements or compatibility with other standard formats. HDF includes Fortran and C calling interfaces and utilities to prepare raw images of data files or for use with other NCSA software. The HDF library contains interfaces for storing and retrieving compressed or uncompressed raster images with palettes; an interface for storing and retrieving n-dimensional scientific data sets together with information about the data, such as labels, units, formats, and scales for all dimensions.

HDF-EOS is the data format standard currently being used for EOSDIS science and science-related data. It was adopted by the NASA ESDIS Project for use as the baseline standard for EOS data distribution, production, and archival after an extensive study was conducted of available formats. (Information obtained from the EOSDIS Web pages about HDF.)

For more information on using NSIDC data which are in HDF format, contact NSIDC User Services. For information on HDF, see the NCSA HDF Home Page. There are several resources for finding image display software which will accept data in HDF. NCSA maintains What Tools Use HDF?. The Goddard Space Flight Center DAAC maintains a list of software resources, including freeware imaging utilities available via anonymous FTP.

6. How can I extract just the region I am interested in from the whole grid?

Most data visualization software programs store the image data in a 2 or 3 dimensional data array, and the user can specify a portion of that array to display. NSIDC has created array files containing individual latitude and longitude values for each cell in the 25 km polar stereographic grids. These can be used in conjunction with the image data array to select a portion of the image by latitude and longitude coordinates. Contact NSIDC User Services to obtain these files, or obtain them directly from the NSIDC FTP site.

7. What does the Fortran program locate do?

The program 'locate.for' is used to transform a single (i, j) coordinate of an SSM/I grid cell to latitude and longitude coordinates. The program provides the reverse function as well. The source code for 'locate.for' (with subroutines 'mapll.for' and 'mapxy.for') is available via anonymous FTP from NSIDC.

8. The program locate.for gives different latitude and longitude values for the four corners of the grid cells than what is shown on the polar stereographic map in the User's Guide. What's the problem?

The values output by the program 'locate.for' represent the values at the center of the given cell. The maps shown in the User's Guide show the latitudes/longitudes for the farthest points of the whole grid, and therefore are the values of the outside corners of those cells - not the centers of those cells.

Data files containing arrays of center latitude/longitude values for the 25 km polar stereographic grid cells are available from NSIDC via anonymous FTP. The lat/lon values for the 12.5 km data sets are being prepared. Contact NSIDC User Services for more information.

9. How should NSIDC's data be cited in publications?

In order that we might continue to receive funding and broaden awareness of our services, NSIDC requests that you please acknowledge use of our data sets. The following examples show how to cite the use of our SSM/I data sets in publications. List the principal investigators, year of data set release, data set title, dates of data you used [list ... cited], publishers (NSIDC), and digital media.

Maslanik, J., and J. Stroeve. 1990, updated current year. DMSP SSM/I-SSMIS daily polar gridded brightness temperatures, [list dates of temporal coverage cited]. Boulder, CO: National Snow and Ice Data Center. Digital media.

For the Bootstrap algorithm:
Comiso, J. 1990, updated current year. DMSP SSM/I-SSMIS daily polar gridded sea ice concentrations, [list dates of temporal coverage cited]. Edited by J. Maslanik and J. Stroeve. Boulder, CO: National Snow and Ice Data Center. Digital media.

For the NASA Team algorithm:
Cavalieri, D., P. Gloerson, and J. Zwally. 1990, updated current year. DMSP SSM/I-SSMIS daily polar gridded sea ice concentrations, [list dates of temporal coverage cited]. Edited by J. Maslanik and J. Stroeve. Boulder, CO: National Snow and Ice Data Center. Digital media.


For additional assistance on any of these topics, contact NSIDC User Services:

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org