Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors
Summary
Daily ice motion vectors are computed from Advanced Very High Resolution Radiometer (AVHRR), Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and International Arctic Buoy Programme (IABP) buoy data. Daily gridded fields combine data from all sensors, from November 1978 through December 2006. Grids extend from 48.4° N to 90° N, and from 53.2° S to 90° S. Mean gridded fields include yearly, monthly, and weekly means, and a mean for the entire time series. Both daily and mean gridded fields are in 2-byte integer format, reprojected to 25 km Northern and Southern Hemisphere EASE-Grids. Browse images of mean fields are available in Portable Network Graphics (PNG) and Enhanced Postscript (EPS) format. Raw ice motion vectors from each sensor are in tab-delimited ASCII text format.
The complete data set is available via FTP. Registration for data is optional, but encouraged; registered users automatically receive email notification of data updates.
Citing These Data
The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.
The following example shows how to cite the use of this data set in a publication: List the principal investigators, year of data set release, data set title and version number, dates of the data you used (for example, April 1982 - September 1992), publisher: NSIDC, and digital media.
Fowler, C. 2003, updated 2008. Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, [list the dates of the data used]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.
Overview Table
| Category | Description |
|---|---|
| Data format | Daily and mean gridded fields are in 2-byte integer format. Byte order is little-endian for daily and mean gridded fields. Browse images of mean fields are available in Portable Network Graphics (PNG) and Enhanced Postscript (EPS) format. Raw ice motion vectors are in tab-delimited ASCII text format. |
| Spatial coverage and resolution | Spatial coverage extends from 48.4° N to 90° N, and from 53.2° S to 90° S. Actual coverage extends beyond these limits in the grid corners. |
| Temporal coverage and resolution | Temporal coverage varies by sensor. Gridded data are available from November 1978 through December 2006. |
| Tools for accessing data | IDL programs are available via FTP. |
| Grid type and size | Subset of Northern Hemisphere 25 km EASE-Grid: 361 x 361 pixels Subset of Southern Hemisphere 25 km EASE-Grid: 321 x 321 pixels |
| File naming convention |
icemotion.vect.ssmi.yyyyddd.n (and .s) icemotion.vect.avhrr.yyyyddd.n (and .s) icemotion.vect.buoy.yyyyddd.n icemotion.vect.grid.yyyyddd.n (and .s) icemotion.mean.jan-dec.1978-yyyy.n (and .s) icemotion.mean.jan-dec.yyyy.n (and .s) icemotion.mean.<month number>.yyyy.n (and .s) icemotion.mean.week.<week number>.yyyy.n (and .s) |
| File size |
Raw ice motion vectors (uncompressed): 50 KB - 237 KB Daily and mean gridded fields (uncompressed): 781.93 KB north, 618.25 KB south |
| Parameter(s) | Sea ice motion |
| Procedures for obtaining data | Data are available via FTP. |
Table of Contents
- Contacts
- Detailed Data Description
- Data Access and Tools
- Data Acquisition and Processing
- References and Related Publications
- Document Information
1. Contacts
Investigator Name
Charles Fowler
CCAR, 431 UCB
University of Colorado
Boulder, CO USA 80309-0431
Technical Contact
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
2. Detailed Data Description
Format
Raw Ice Motion Vectors
Data are in tab-delimited ASCII text format and each daily file contains a variable number of vectors. At the top of every file is a one-line header containing five numbers as described in Table 1.
| Number | Description |
|---|---|
| First | Specifies the number of vectors (lines) in the file |
| Second | Disregard this number |
| Third | Original grid dimensions (x) |
| Fourth | Original grid dimensions (y) |
| Fifth | Disregard this number |
After the header line, the data is listed in five columns as explained in Table 2.
| Column | Name | Description | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | x | EASE-Grid row number for the start of the vector (vector starts in the center of the grid cell). The upper left corner is represented by x=0. | ||||||||
| 2 | y | EASE-Grid column number for the start of the vector (vector starts in the center of the grid cell). The upper left corner is represented by y=0. | ||||||||
| 3 | u | Vector component in cm/sec. | ||||||||
| 4 | v | Vector component in cm/sec. | ||||||||
| 5 | z | Source of the data (z value varies depending on instrument):
|
See Sample Data Record for an example of a raw vector file derived from SSM/I.
Daily and Mean Gridded Fields
Daily and mean gridded fields are projected to Northern and Southern Hemisphere EASE-Grids. Data are in 2-byte integer binary format (little-endian). A pixel value of 0 in the third variable indicates no vectors at that location. Each vector represents three variables:
- u component (cm/sec, scaled by 10)
- v component (cm/sec, scaled by 10)
- third variable (explained below)
For the daily gridded data, the third variable contains the square root of the estimated error variance, scaled by 10, at a given location. The error variance is the estimated error of that vector obtained from the optimal interpolation process. It is dependent upon the distance from correlation functions of the input vectors and the variance of the input vectors.
If the closest input vector was greater than 1250 km, then a value of 1000 is added to this variable. Because interpolation was applied to a surface map from passive-microwave data, coastlines may contain false ice. In this case, the third variable was assigned a negative value to allow users to remove these vectors near coastlines (within 25 km). As an example, a value of -1035 indicates all of the following conditions:
- The vector was near a coastline
- The nearest sampled vector was further than 1250 km
- The vector had a σ value of 3.5 or the estimation error variance (σ2) is 12.25
For the mean grids, the third variable is the number of vectors that contributed to the mean value. The information contained in this third variable provides a means of characterizing data quality, in addition to the "near coastline" check described above. For example, a user might choose to filter out vectors with error variances above a certain level, or values for which the nearest observed vector was beyond a particular distance.
Row, Column to Latitude, Longitude
The files north_x_y_lat_lon and south_x_y_lat_lon are in tab-delimited ASCII text format with four columns as described in Table 3.
| Column | Name | Description |
|---|---|---|
| 1 | x | Grid row number |
| 2 | y | Grid column number |
| 3 | lat | Corresponding latitude |
| 4 | lon | Corresponding longitude |
File and Directory Structure
The following diagram refers to the FTP site where data are available.
|
Directory Descriptions
|
Files are compressed and packaged in Unix tar files to facilitate FTP transfer. To uncompress and untar files, use the commands in the following example.
% uncompress 1978_weekly_means_s.tar.Z
% tar xvf 1978_weekly_means_s.tar
x icemotion.mean.week45.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week46.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week47.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week48.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week49.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week50.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week51.1978.s, 618246 bytes, 1208 blocks
x icemotion.mean.week52.1978.s, 618246 bytes, 1208 blocks
%
File Naming Convention
ASCII Ice Motion Vectors
icemotion.vect.ssmi.yyyyddd.n (and .s)
icemotion.vect.avhrr.yyyyddd.n (and .s)
icemotion.vect.buoy.yyyyddd.n
Daily Gridded Fields
icemotion.vect.grid.yyyyddd.n (and .s)
Example: icemotion.vect.grid.1978132.n
Mean Grid for Entire Time Series
icemotion.mean.jan-dec.1978-yyyy.n (and .s), where yyyy is the 4-digit year for the last year of data available.
Example:icemotion.mean.jan-dec.1978-2006.n
Yearly Mean Grids
icemotion.mean.jan-dec.yyyy.n (and .s)
Example: icemotion.mean.jan-dec.1978.n
Monthly Mean Grids
icemotion.mean.<month number>.yyyy.n (and .s)
Example: icemotion.mean.11.1978.n
Weekly Mean Grids
icemotion.mean.week.<week number>.yyyy.n (and .s)
Example: icemotion.mean.week.13.1978.n
Browse images of mean grids have .png and .eps file extensions.
File Size
Raw ice motion vectors (uncompressed): 50 KB - 237 KB
Daily and mean gridded fields (uncompressed): 781.93 KB north, 618.25 KB south
Spatial Coverage
Spatial coverage of gridded data extends from 48.4* degrees to 90 degrees north latitude, and from 53.2* degrees to 90 degrees south latitude. The actual coverage extends beyond these limits in the grid corners. Table 4 and 5 summarize the values of corner pixels for the Northern and Southern Hemispheres, respectively.
| Corner | Center of corner pixel |
Outer edge of corner pixel |
|---|---|---|
| Upper left | 29.89694° N, 135.00000° W | 29.71270° N, 135.00000° W |
| Upper right | 29.89694° N, 135.00000° E | 29.71270° N, 135.00000° E |
| Lower left | 29.89694° N, 45.00000° W | 29.71270° N, 45.00000° W |
| Lower right | 29.89694° N, 45.00000° E | 29.71270° N, 45.00000° E |
| Corner | Center of corner pixel |
Outer edge of corner pixel |
|---|---|---|
| Upper left | 37.13584° S, 135.00000° W | 36.95776° S, 135.00000° W |
| Upper right | 37.13584° S, 135.00000° E | 36.95776° S, 135.00000° E |
| Lower left | 37.13584° S, 45.00000° W | 36.95776° S, 45.00000° W |
| Lower right | 37.13584° S, 45.00000° E | 36.95776° S, 45.00000° E |
*For the Northern Hemisphere, the center of the tangent pixels is 48.42649° N and the outer edge is 48.40237° N. For the Southern Hemisphere, the center of the tangent pixels is 53.21244° S and the outer edge is 53.18868° S.
Spatial Coverage Map
Click on the thumbnails below to see detailed coverage maps.
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Spatial Resolution
Source data are regridded to Northern and Southern Hemisphere EASE-Grids with 25 km pixel spacing.
Projection
Data are georeferenced to the EASE-Grid projection, an azimuthal equal area projection. Please review the Summary of EASE-Grid Map Projection Parameters for details on the EASE-Grid projection.
Grid Description
The northern grid is 361 x 361, centered on the geographic North Pole. The southern grid is 321 x 321, centered on the geographic South Pole. Pixel size is 25 km. Grid coordinates begin in the upper left corner of the grid. These grids are subsets of the Northern and Southern EASE-Grids. Please see Summary of EASE Grid Map Projection Parameters for more information, or All About EASE-Grid for more information on related products and tools.
Temporal Coverage
The temporal coverage varies by type of data and/or by sensor, as shown in Table 6.
| Type/Sensor | Start Date | End Date |
|---|---|---|
| Daily Gridded Fields | 01 November 1978 | 31 December 2006 |
| Daily ASCII Ice Motion Vectors | ||
| AVHRR | 24 July 1981 | 30 December 2000 |
| Buoy | 18 January 1979 | 30 December 2005 (North only) |
| SSM/I | 25 October 1978 | 31 December 2006 |
| Mean Gridded Fields | ||
| All | 1978 | 2006 |
| Climatological monthly means | November 1978 | December 2006 |
| Weekly means | Week 45 in 1978 | Week 52 in 2006 |
| Yearly means | 1979 | 2006 |
Temporal Resolution
The temporal resolution varies by sensor, as shown in Table 7.
| Sensor | Resolution |
|---|---|
| AVHRR | Four satellite passes are used each day when available. |
| Buoys | The 12:00 Greenwich Mean Time (GMT) buoy positions were used to compute 24-hour mean velocities. |
| SMMR | Data are available every two days for any given pixel. |
| SSM/I | Data are available every day for any given pixel. |
Sample Data Record
Following is a sample of raw vectors derived from SSM/I data. The first seven lines of icemotion.vect.ssmi.2003078.n are shown. The first line is the header and indicates that this file contains 1679 vectors and that the original grid was 1805 x 1805 pixels. For a description of the data columns see the Raw Ice Motion Vectors Format section of this document.
1679 1 1805 1805 0.0691199973
747.50 267.50 0.00 0.00 3.00
897.50 267.50 0.00 0.00 3.00
912.50 267.50 0.00 0.00 3.00
882.50 282.50 9.05 7.24 3.00
897.50 282.50 0.00 3.62 3.00
912.50 282.50 0.00 0.00 3.00
Quality Assessment
See the following for accuracy estimates of ice motion from each sensor.
Ice motion from SMMR and SSM/I
3. Data Access and Tools
Data Access
Data are available via FTP.
Software and Tools
Several IDL programs are available via FTP to read ice motion data, and create Postscript plots or display data to a screen. The IDL prcedures and descriptions are listed in Table 8.
| IDL Procedure | Description |
|---|---|
| display_nogrid_north.pro | SSM/I and AVHRR vectors, North |
| display_nogrid_south.pro | SSM/I and AVHRR vectors, South |
| disp_ice_motion.pro | IDL procedure for creating animations of gridded and non-gridded sea ice motion vectors. |
The programs display_nogrid_north.pro and display_nogrid_south.pro do not work with buoy-derived ASCII vectors.
The map files nsidc_north_map and nsidc_south_map are required for the IDL programs listed in Table 8 to run. These map files are provided in the tools directory: Following is an example of running an IDL program:
IDL> display_nogrid_north, 'icemotion.nogrid.1978132.n'
The files north_x_y_lat_lon and south_x_y_lat_lon are latitude and longitude grids with 25 km pixel spacing, for overlay with daily and mean gridded vector fields.
Animation
The file disp_ice_motion.pro animates daily and mean gridded data by day, week, month, or year. The map files nsidc_north_map and nsidc_south_map must be in the same directory as the IDL program. Following is an example of how to animate northern daily ice motion grids from 01 November 1978 through 01 December 1978:
IDL> disp_ice_motion
% Compiled module: DISP_ICE_MOTION.
Enter time category (1 = Daily Raw Sensor Data or Grids)
(2 = Weekly Mean Grids)
(3 = Monthly Mean Grids)
(4 = Yearly Mean Grids)
: 1
Enter start and end dates for animation (yyyymmdd, e.g., 19950610).
Start Date: 19781101
% Compiled module: JULDAY.
End Date: 19781201
Enter the data type (1 = AVHRR)
(2 = Buoy)
(3 = SMMR)
(4 = SSM/I)
(5 = grid)
: 5
Enter the hemisphere (1 = northern)
(2 = southern)
: 1
Enter the full name of the directory that the ice motion files are in.
(Note: must correctly use upper and lower case letters.)
: <enter relative or full directory path here>
Related Data Collections
- AVHRR Polar Pathfinder Twice-Daily 5 km EASE-Grid Composites
- AVHRR Polar Pathfinder Twice-Daily 25 km EASE-Grid Composites
4. Data Acquisition and Processing
Theory of Measurements
Measurement of sea ice movement is accomplished with frequent repeat coverage of remotely-sensed imagery. Ice motion computed from satellite imagery represents the displacement between the acquisition times of two images with the same spatial coverage. Researchers identify a feature (such as an ice floe) on two registered images and measure its pixel displacement. Ice velocity vectors are computed based on the pixel resolution and time span between images.
A more automated method is to measure the correlation of groups of pixels between image pairs. A small target area in one image is correlated with several areas of the same size in a search region of the second image. The displacement of the ice is then defined by the location in the second image where the correlation coefficient is the highest. This spatial correlation method is used to produce ice motion vectors for this data set. This approach is generally valid over short distances away from the ice edge in areas where ice conditions are relatively stable from day to day. Spatial correlation methods cannot, however, find matches between images where a complete knowledge of ice dynamics is needed; for example, in areas where ice is deforming or in the ice margins near the open ocean where ice can deform and rotate -- for example, areas where the spatial or spectral characteristics of the ice within a pixel are changing rapidly (Emery, Fowler, and Maslanik 1995).
Sensor or Instrument Description
Please refer to the following documents for details of each sensor:
- SMMR Instrument Description
- SSM/I Instrument Description
- AVHRR Polar Pathfinder Twice-Daily 5 km EASE-Grid Composites
- International Arctic Buoy Program Air Droppable RAMS (ADRAMS) Buoy
Data Source
AVHRR Data
AVHRR Global Area Coverage (GAC) images at 5 km gridded resolution were used to estimate ice motion over the Arctic and Antarctic, because they were available for nearly the entire time series, they provide an intermediate spatial resolution between passive microwave and buoys, they provide finer time sampling than microwave data, and they are not subject to the same error sources as the other data sets.
Passive Microwave Data
NSIDC provided NIMBUS-7 SMMR Pathfinder Brightness Temperatures at 37 GHz (25 km gridded resolution). Because of satellite limitations, full Arctic coverage was only available every two days with SMMR. NSIDC also provided DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures at 37 GHz (25 km resolution) and 85 GHz (12.5 km resolution) vertical and horizontal polarizations. These data essentially provide all-sky coverage, whereas AVHRR data are limited by cloud cover.
Buoy Data
International Arctic Buoy Program (IABP) "C" data were used to calculate ice motion vectors from buoys. IABP provides buoy location information through satellite tracking of buoys placed on sea ice. Several buoy locations are determined each day, and corresponding ice motions are calculated. Ice motion from buoys is very accurate, but it is limited since the numbers and locations of buoys are driven by cost and logistics. Also, buoys have not been placed on ice in the Eastern Arctic.
Processing Steps
See the following for detailed information about the methods Fowler used to compute ice motion fields:
Ice motion from SMMR and SSM/I
Mean Fields
Fowler computed mean ice motion from the daily gridded ice motion data. The northern and southern polar regions have several mean fields: weekly, monthly, annual, and mean for the entire time series (November 1978 to March 2003). For the northern region, Fowler calculated a mean for October through June -- from freezing to melting seasons. In some cases, this may be of more use than the annual mean in the Arctic.
For the weekly means, at least five out of seven days were needed to compute each vector mean. For the monthly means, at least 20 days were needed. For any mean greater than one month, at least 40 days were needed.
Weekly means for each year start at 01 January for consistency. The last day of each year, or two days if in a leap year, were not used.
5. References and Related Publications
Cracknell, A. 1997. The Advanced Very High Resolution Radiometer. London: Taylor and Francis.
Emery, W., C. Fowler, and J. Maslanik. 1995. Satellite Remote Sensing of Ice Motion, in Oceanographic Applications of Remote Sensing, ed. Motoyoshi Ikeda and Frederic W. Dobson. CRC Press, Boca Raton.
Isaaks, E., and R. M. Srivastava. 1989. An Introduction to Applied Geostatistics. New York: Oxford University Press.
Kidwell, K. 1995. NOAA Polar Orbiter Data User's Guide. U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, NESDIS.
Maslanik, J., C. Fowler, J. Key, T. Scambos, T. Hutchinson, and W. Emery. 1998. AVHRR-based Polar Pathfinder Products for Modeling Applications. Annals of Glaciology 25:388-392
Rosborough, G., D. Baldwin, and W. Emery. 1994. Precise AVHRR Image Navigation. IEEE Transactions in Geosciences and Remote Sensing 32(3):644-657.
Schweiger, A., C. Fowler, J. Key, J. Maslanik, J. Francis, R. Armstrong, M. J. Brodzik, T. Scambos, T. Haran, M. Ortmeyer, S. Khalsa, D. Rothrock, and R. Weaver. 1999. P-Cube: A Multisensor Data Set for Polar Climate Research. Proceedings on the 5th Conference on Polar Meteorology and Oceanography, American Meteorological Society, Dallas, TX, 15-20 Jan., 136-141.
6. Document Information
Acronyms and Abbreviations
The acronyms and abbreviations used in this document are listed in Table 9.
| Acronym | Description |
|---|---|
| ASCII | American Standard Code for Information Interchange |
| AVHRR | Advanced Very High Resolution Radiometer |
| CCAR | Colorado Center for Astrodynamics Research |
| EASE-Grid | Equal Area Scalable Earth-Grid |
| FTP | File Transfer Protocol |
| GAC | Global Area Coverage |
| GISMO | Graphical Interface for Subsetting, Mapping, and Ordering |
| GMT | Greenwich Mean Time |
| IABP | International Arctic Buoy Programme |
| MCC | Maximum Cross Correlation |
| NSIDC | National Snow and Ice Data Center |
| NOAA | National Oceanic and Atmospheric Administration |
| RMS | Root mean square |
| SMMR | Scanning Multichannel Microwave Radiometer |
| SSM/I | Special Sensor Microwave/Imager |
| URL | Uniform Resource Locator |
Document Creation Date
May 2003
Document Revision Date
February 2008
September 2007
Document URL
http://nsidc.org/data/docs/daac/nsidc0116_icemotion.gd.html
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