NSIDC provides software tools to extract and geolocate data in a polar stereographic projection derived from gridded passive microwave data sets at NSIDC, as well as masking tools that limit the influence of weather effects on sea ice concentrations. We also provide software and tools for geolocating and displaying EASE-Grid data sets at NSIDC.

This page provides links to software and tools for geolocating and displaying EASE-Grid and polar stereographic projection data sets available at NSIDC. All tools are available by FTP.

Table of Contents

1. EASE-Grid Data Tools
   1. Geolocation Software
   2. Geolocation Data Files
   3. Additional Tools for EASE-Grid
2. Polar Stereographic Data Tools
   1. Data Extraction Tools
   2. Data Display Tools
   3. Geocoordinate and Pixel-Area Tools
   4. Masks and Overlays

EASE-Grid Data Tools

The software and tools are grouped into the following categories:

• Geolocation Software - Including C, FORTRAN, and IDL (Interactive Data Language) conversion software
• Geolocation Data Files - Look-up files that contain arrays with the latitude and longitude at the center of each grid cell

Geolocation Software

Geolocation Data Files 

For data set users who only have the need of the latitude and longitude of the center of any cell in the grid, the following geolocation data file is available.

This section explains the file naming convention used for the following geolocation data files.

These geolocation files contain flat, binary arrays of 4-byte integers containing the latitude or longitude of the respective grid cell in hundred-thousandth degrees. The user should divide the stored integer value by 100,000. to yield decimal degrees, with 1 meter precision. The user is expected to know which byte-ordering convention is required by their system. Files are provided in PC (little-endian/LSB) byte orders.

Depending on the coverage area, which varies for different grids (such as ML, NH, SL, Sa25 and so on), scaled latitudes range from -90.00000 to 90.00000, and scaled longitudes range from -180.00000 to 180.00000 with missing data (such as corners of grids with hemispheric coverage) indicated by scaled value 14316.55765.

Example file name: MLLATLSB

ggpppooo

Where:

Data Display Tool

Geocoordinate and Pixel-Area Tools

The geocoordinate and pixel-area tools described here provide the same functionality for all polar stereographic passive microwave data sets at NSIDC. They include Fortran routines for determining latitude and longitude from i,j coordinates and a pixel-area grid.

• Geocoordinate Tools
• Pixel-Area Tools

Geocoordinate Tools

The following geocoordinate tools are available:

• locate.for
• mapll.for and mapxy.for
• psn06lats_v3.dat and pss06lats_v3.dat
• psn06lons_v3.dat and pss06lons_v3.dat
• psn12lats_v3.dat and pss12lats_v3.dat
• psn12lats_v3.dat and pss12lons_v3.dat
• psn25lats_v3.dat and pss25lats_v3.dat
• psn25lons_v3.dat and pss25lons_v3.dat

Pixel-Area Grids

The following pixel-area tools are availabe:

• psn06area_v3.dat and pss06area_v3.dat
• psn12area_v3.dat and pss12area_v3.dat
• psn25area_v3.dat and pss25area_v3.dat

Please note that the data ranges given here are latitude and longitude values for the center of each grid cell. The range covered by the full grid extends to the pole (90 degrees latitude) and all longitudes (0 to 360 degrees longitude). Also note that corrected grids for the northern hemisphere were created June 22, 1999 and should replace the three psn25*.dat files that were distributed prior to this date.

To determine the lat/lon values of corresponding (i,j) data grid cells:

1. Read in the array as long (4-byte) integers.
2. Divide these values by 100,000. The resulting array gives the lat/lon values for the data grid cells in decimal degree

¹ Version 3 (V3) pixel area files, developed by the NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC), provide a more accurate mapping of area for the polar stereographic grid. V3 and V2 area files differ by a maximum of approximately 0.5 sq km per grid cell at the corners of the grid. The effect on total extent and total area estimates is, at most, 10,000 sq km (approximately ~0.1%).

Masks and Overlays

The masks and overlays provided here supply the user with many options for concealing unwanted regions or overlaying coastlines and latitude/longitude grids; they are divided into the following categories:

• Land Masks
• Land Overlays
• Missing Data Mask
• Region Masks
• Ocean Masks

Land Masks

The land masks are useful for concealing the land of the Northern and Southern Hemispheres prior to processing of the data. The files are divided into two categories: Northern Hemisphere and Southern Hemisphere.

Note on Land Masks

Several versions of the land masks have been developed, though NSIDC only distrubutes the most current masks.

Beginning with the SMMR era, the first masks NSIDC developed were landmask.ntb and landmask.stb. Other masks were later added for use with SSM/I F8 data. The last set of masks to be added to the series are those beginning with gsfc, which were originally developed for use with SSM/I F11 and F13. However, all of the masks provided here will work across all platforms (SMMR and SSM/I).

The relatively slight differences in numbers of SSM/I-grid pixels masked as land can introduce discrepancies in analyses of time series spanning the SMMR and SSM/I period. One method of addressing this issue is to generate a composite mask in which all pixels mapped as land in any of the masks are coded as land pixels in the composite mask. Use of such a composite mask improves the consistency of the SMMR and SSM/I record at the expense of masking additional ocean areas as land. A composite of the CIA World Data Bank I and II (GSFC-1 and JPL-1) has been produced by J. Maslanik at NSIDC.

Another issue is the effect of contamination of coastal ocean pixels by their proximity to land. Such proximity can modify the brightness temperatures of coastal ocean pixels, producing false ice concentration values along some coasts. These pixel mixing errors are considered in the summer 1996 issue of NSIDC Notes (Issue no. 18). Maslanik et al. (1996) discuss the effects of such land contamination in introducing differences between SMMR and SSM/I time series, and they describe the use of a modified land mask where land areas are extended to mask substantial contamination. The Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I Brightness Temperatures land mask (n3blcmsk.hdf) alleviates this problem whereby ocean pixels adjacent to land or one pixel removed from land are designated as land-contaminated.

The land masks that are availabe are the following:

• amsr_gsfc_6n.hdf and amsr_nic_6s.hdf
• amsr_gsfc_12n.hdf and amsr_nic_12s.hdf
• amsr_gsfc_25n.hdf and amsr_nic_25s.hdf
• gsfc_12n.msk and gsfc_12s.msk
• gsfc_25n.msk and gsfc_25s.msk
• landmask.ntb and landmask.stb
• n3blcmsk.hdf
• pole_n.msk

Land Overlays

The land overlays are useful for overlaying coast line and latitude/longitude grids on top of images and processed data. The files are broken down into two categories: Northern Hemisphere and Southern Hemisphere.

The land overlays that are availabe are the following:

• coast_12n.msk and coast_12s.msk
• coast_25n.msk and coast_25s.msk
• ltln_12n.msk and ltln_12s.msk
• ltln_25n.msk and ltln_25s.msk

Missing Data Mask

The circular section over the Northern Hemisphere pole (known as the pole hole) is never measured due to orbit inclination. The following mask is available for the pole hole region not covered by the SMMR and SSM/I instruments.
 

Region Masks

The region masks are smaller areas than the land masks although they can be used as land masks using the pixel value of 11.
 

Ocean Masks