MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data

Summary

This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, provides annual ice-sheet-wide velocity maps for Greenland, derived using Interferometric Synthetic Aperture Radar (InSAR) data from the RADARSAT-1 satellite. The data set currently contains ice velocity data for the winter of 2000-2001 and 2005-2006, 2006-2007, and 2007-2008 acquired from RADARSAT-1 InSAR data from the Alaska Satellite Facility (ASF), and a 2008-2009 mosaic derived from the Advanced Land Observation Satelitte (ALOS) and TerraSAR-X data. For maps of glacier outlet areas, some of which demonstrated profound velocity changes during the observation period, see the related data set MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR.

Data are available via FTP, in 4-byte big-endian Institute of Electrical and Electronics Engineers (IEEE) floating point and GeoTIFF (.tif) formats.

Note: Although these data sets have been screened for anomalously large errors, at this stage they are Version 1 products and some significant errors may persist (>20 m/yr). See the Error Estimates section of this documentation for details. In addition, these data are considered provisional pending a review by the MEaSUREs program. Once the data have been reviewed, this statement will be removed.

Citing These Data

Literature Citation

We kindly request that you acknowledge the use of this data set by referencing the following peer-reviewed publication.

Joughin, I., B. Smith, I. M. Howat, T. Scambos, and T. Moon. 2010. Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping. Journal of Glaciology, 56 (197), pp. 415-430, doi: 10.3189/002214310792447734.

Data Citation

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Joughin, I., B. Smith, I. Howat, and T. Scambos. 2010. MEaSUREs Greenland Ice Velocity Map from InSAR Data. Boulder, Colorado, USA: NASA DAAC at the National Snow and Ice Data Center. doi:10.5067/MEASURES/CRYOSPHERE/nsidc-0478.001

Overview Table

Category Description
Data formats 4-byte big-endian Institute of Electrical and Electronics Engineers (IEEE) floating point format
GeoTIFF (.tif) format
Spatial coverage and resolution Southernmost Latitude: 60° N
Northernmost Latitude: 83° N
Westernmost Longitude: 75° W
Easternmost Longitude: 14° W

500-meter gridding
Temporal coverage and resolution 2000 to 2001
2005 to 2006
2006 to 2007
2007 to 2008
2008 to 2009
File naming convention greenland_vel_mosaic500_2000_2001.vx
greenland_vel_mosaic500_2000_2001.ey
greenland_vel_mosaic500_2000_2001.tif
greenland_vel_mosaic500_2000_2001.ex.geodat
File size Data files are approximately 64200 KB each.
Parameter(s) Ice sheet surface velocity
Procedures for obtaining data Data are available via FTP.

Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

1. Contacts and Acknowledgments

Investigators

Dr. Ian Joughin
University of Washington
Applied Physics Laboratory
1013 NE 40th Street
Box 355640
Seattle, Washington 98105
USA

Dr. Ben Smith
University of Washington
Polar Science Center Applied Physics Laboratory
University of Washington
Department of Earth and Space Sciences
Seattle, Washington 98195
USA

Dr. Ian Howat
Ohio State University
Byrd Polar Research Center
Scott Hall Room 108
1090 Carmack Road
Columbus, Ohio 43210
USA

Dr. Ted Scambos
University of Colorado
Cooperative Institute for Research in Environmental Science
National Snow and Ice Data Center
CIRES, 449 UCB
Boulder, Colorado 80309
USA

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

Acknowledgements

These data were generated through a grant from the NASA MEaSUREs program.

2. Detailed Data Description

Format

Raw Data Grids

Data files are in 4-byte big-endian IEEE floating point format with the same byte ordering used by non-Intel Macs and most workstations, not the standard PC byteorder. No-data values are represented by -2.0e9. This may require that the user replace these values before applying analysis tools. There are separate files for velocity x (vx) and velocity y (vy) components. There are also accompanying pseudo-error estimates. Refer to the Error Estimates section, below. Accompanying geodat files provide grid descriptions corresponding to each velocity data file.

Grid files are 3010 samples by 5460 lines.

GeoTIFF

The data are also provided in GeoTIFF (.tif) format. For each mosaic, there are three GeoTIFF files, one corresponding to vx data, one for the vy data, and one for the combined velocity data.

File and Directory Structure

Data are available on the FTP site in the ftp://sidads.colorado.edu/pub/DATASETS/nsidc0478_MEASURES_greenland_V01/ directory. Within this directory, there are folders for each season of data, for example, the folder 2000 contains data for 2000-2001. Each folder contains one Readme file and eight data files for each season of data as described in Table 1.

Table 1. Example File List for 2000-2001 Data
Filename Description
greenland_vel_mosaic500_2000_2001.vx
greenland_vel_mosaic500_2000_2001.vy
Velocity products in 4-byte IEEE floating point format
greenland_vel_mosaic500_2000_2001.vx.tif
greenland_vel_mosaic500_2000_2001.vy.tif
greenland_vel_mosaic500_2000_2001.tif
Velocity products in GeoTIFF (.tif) format
greenland_vel_mosaic500_2000_2001.vx.geodat
greenland_vel_mosaic500_2000_2001.vy.geodat
Grid description files in .geodat format
greenland_vel_mosaic500_2000_2001.ex
greenland_vel_mosaic500_2000_2001.ey
Error values (1 sigma standard deviation)

File Naming Convention

Files are named according to the following convention and as described in Table 2:

Example: greenland_vel_mosaic500_2000_2001.vx and greenland_vel_mosaic500_2000_2001.vy provide velocity data for the entire continent of Greenland, at a 500 meter resolution for the years 2000 to 2001. Additional error and location information relating to these files are available in the corresponding files with the same prefix.

greenland_vel_mosaic500_yyyy_yyyy.xx

Where:

Table 2. File Naming Convention
Variable Description
greenland Location
vel Type of measurement (velocity)
mosaic500 Type and resolution of map
yyyy_yyyy Winter season of data
xx File extension
  • vx: velocity x values
  • vy: velocity y values
  • ex: error, x values
  • ey: error, y values
  • geodat: file extension for location data files
  • tif: file extension for GeoTIFF files

File Size

Data files are approximately 64,200 KB each.

Spatial Coverage

This data set covers the entire expanse of Greenland.

Southernmost Latitude: 60° N
Northernmost Latitude: 83° N
Westernmost Longitude: 75° W
Easternmost Longitude: 14° W

Spatial Resolution

500 meter

Projection

Data are provided in polar stereographic coordinates with a standard latitude of 70° N and rotation angle of -45° (sometimes specified as a longitude of 45° E). With this convention, the x-axis extends south from the North Pole along the 45° E meridian.

Location information is provided by the geodat files, in the following format:

Note. The origin specifies the polar stereographic coordinates for the center of the lower left pixel, the first sample in the file. This specification using the middle of the reference pixel is different than some GeoTIFF and other formats where the reference coordinates are specified for outer corner of the reference pixel.

Sample Geodat file:

# 2
3010.0000 5460.0000
500.00000 500.00000
-645.00000 -3370.0000
&

Temporal Coverage

The data set currently provides one velocity mosaic for the winters of 2000 to 2001, 2005 to 2006, 2006 to 2007, 2007 to 2008, and 2008 to 2009.

Parameter or Variable

The vx and vy files yield values of velocity in meters per year, in the x and y directions defined by the polar stereographic grid. The velocities are true values and not subject to the distance distortions present in a polar stereographic grid. In some areas small holes have been filled via interpolation. Interpolated values are identifiable as the locations where there is velocity data but no error estimate. Radar-derived velocities are determined using a combination of conventional InSAR and speckle tracking techniques (Joughin, et. al., 2002).

Error Estimates

These data sets have been screened for anomalously large errors, but at this stage they are version 1 products and some significant errors may persist (>20 m/yr). In particular, a bad InSAR baseline fit may bias the ice velocities over wide regions (for example across single or multiple adjacent swaths) by tens of meters per year. Once the first round of processing is complete, the investigators will perform an inter-annual comparison to help screen out or correct problem (for example outlier) tracks. Users who find such anomalies should report them to NSIDC and the PI at ian@apl.washington.edu, to facillitate their removal in the next round of processing.

In addition to errors, please note that velocities in the interior (roughly >100 km from the coast) are determined using baseline fits to a common set of ground control points (balance and GPS velocities at elevations >2000 m), under the assumption that slow-moving ice far from the coast is changing only slowly (Joughin et al, 2010). While less than ideal, this assumption produces far smaller errors than using no control. Caution should be taken when interpreting any apparent changes in ice flow speed in areas >100 km from the coast. In coastal regions, the velocities are referenced to rock and more accurately represent ice velocity and its temporal evolution. Users should be aware the potential for anomalously large errors exists. If in doubt, please contact NSIDC and the PI at ian@apl.washington.edu for verification.

Error estimates are included for all non-interpolated radar-derived velocity vectors, in the files greenland_vel_mosaic500_YYYY_YYYY.ex and greenland_vel_mosaic500_YYYY_YYYY.ey. These estimates include the statistical uncertainty associated with the phase and speckle tracking error. Formal errors agree reasonably well with errors determined by comparison with GPS data (Joughin, et al, 2002). These estimates, however, underestimate true uncertainty in several ways and as such should be used more as an indication of relative quality rather than absolute error.

Swath data are broken into overlapping sections during processing and estimates may be double averaged in regions of overlap, causing errors to be underestimated by 1/sqrt(2) in these regions. Errors are derived from local statistics, so errors may be overstated near shear margins and other areas with strong velocity gradients. Errors associated with the baseline solutions are not included in the error estimates. These are generally small (<5 m/yr) but can be larger in interior areas away control points derived from rock on the coast or balance velocities near the divides. The error estimates also do not account for ionospheric streak errors, which tend to be the worst in northwest Greenland near the magnetic north pole. These errors range from severe (>10 m/yr) in the 2000-2001mosaic (near the solar maximum) to almost negligible in the years after 2005. Absolute errors in the DEM used for processing introduce only small errors, but slope errors in the DEM can produce errors of up to about three percent of the observed speed. The formal errors plus three percent are a good rule of thumb to use except in cases where streaks suggest substantially larger errors.

See Joughin, et al, 2002 for more detail on errors and how they are computed.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

Total volume of the data set is approximately 500 MB.

Software and Tools

Reading data with Matlab

The following routines will read the data in Matlab.

% This function reads a standard mosaiced velocity data
% set and returns arrays with the data and the geodat info.
%
function [vx,vy,xd]= readvelocity(rootname)
%
% Get geodat info
%
xd=readgeodat(strcat(rootname,'.vx.geodat'));
%
% Vx component
%
fid = fopen(strcat(rootname,'.vx'),'r','ieee-be');
[vx,count]=fread(fid,[xd(1,1) xd(1,2)],'float32');
fclose(fid);
%
% Vy component
%
fid = fopen(strcat(rootname,'.vy'),'r','ieee-be');
[vy,count]=fread(fid,[xd(1,1) xd(1,2)],'float32');
fclose(fid);

%
% Read a geodat file
%
function xgeo=readgeodat(filein)
fid = fopen(filein,'r');
xgeo=zeros(3,2);
i=1;
while ~feof(fid),
  line=fgets(fid);
  [A,count]=sscanf(line,'%f %f',[1 2]);
  if(count == 2)
     xgeo(i,:)=A;
     i=i+1;
  end
end
fclose(fid);=

Reading data with IDL

The following code fragment can be used to write a complete program to read the data with IDL.

;
; Similar to matlab program
;
get_lun,lun
openr,lun,filename
buffer=fltarr(xs)
image=fltarr(xs,ys)
for i=0,ys-1 do begin
   readu,lun,buffer
   buffer=swap_endian(buffer,/swap_if_little_endian)  ; This is needed on a PC
   image(*,i) = buffer
endfor

 

Related Data Collections

4. Data Acquisition and Processing

Theory of Measurements

The velocity maps in this data set were created using SAR data from the RADARSAT satellite. The methods included a combination of speckle tracking and conventional interferometry. Annual velocity maps were produced by mosaicking multiple strips of InSAR-derived data. The individual mosaic images were selected to satisfy two criteria: first, the images came from the same time of year, and second, the images for each individual year were collected in as short a period of time as possible. For more detail, please see Joughin, et. al,. 2002.

Sensor or Instrument Description

2000 to 2001 Mosaic

In late 2000 and early 2001, during the RADARSAT-1 Modified Antarctic Mapping Mission, the Canadian Space Agency (CSA) acquired nearly complete coverage of Greenland with multiple passes suitable for InSAR (September 3 to January 24). All of the available data for Greenland were used to produce the 2000 to 2001 mosaic. In cases where the data quality was too poor, some products were discarded. All source data were obtained from the Alaska Satellite Facility (ASF).

2005 to 2006 Mosaic

In 2005 and 2006, RADARSAT-1 imaged most of Greenland four consecutive times, producing three InSAR pairs. All of the data were processed. Poor coherence passes were screened out, and the remaining data were used to assemble the mosaic.

2006 to 2007 Mosaic

The 2006/2007 mosaic was produced with RADARSAT-1 fine beam data collected from 18 December 2006 and 15 April 2007. Fine beam data are intended for applications which require the best spatial resolution available from RADARSAT. The azimuth resolution is 8.4m, with range resolution 9.1m to 7.8m; the incidence angle range is 37 to 47 degrees.

2007 to 2008 Mosaic

The 2007/2008 mosaic was produced with RADARSAT-1 fine beam data, in the same manner as the 2006/2007 mosaic.

2008 to 2009 Mosaic

The 2008/2009 mosaic was produced with ALOS and TerraSAR-X data.

Derivation Techniques and Algorithms

Annual mosaics were created using data collected over an approximately 96-day period during the winter. Areas with no data correspond to either regions where no data were acquired or where the interferometric correlation was insufficient to produce an estimate, most often in areas with high snow accumulation. Regions with data represent the average of between one and three estimates (larger numbers may occur in regions of swath overlap, especially at higher latitudes).

The data are posted on a 0.5-kilometer grid, but the true resolution varies between 0.5 and 1 km. Many small glaciers are resolved outside the main ice sheet, but it is important to remember that for narrow (<1km) glaciers, the velocity represents an average of both moving ice and stationary rock, so while the glacier may be visible in the map, its speed is likely underestimated. Also on some of the smaller glaciers, interpolation produces artifacts where the interpolated value is derived from nearby rock, causing apparent stationary regions in the middle of otherwise active flow. The data have been screened to remove most of these, but please proceed with caution.

5. References and Related Publications

Joughin, I. 1995. Estimation of Ice-Sheet Topography and Motion Using Interferometric Synthetic Aperture Radar. (PhD Dissertation, University of Washington).

Joughin, I. 2002. Ice-Sheet Velocity Mapping: A Combined Interferometric and Speckle-Tracking Approach. Annals of Glaciology 34: 195-201. doi:10.3189/172756402781817978.

Joughin, I., S. Tulaczyk, R. Bindschadler, and S. F. Price. 2002. Changes in West Antarctic Ice Stream Velocities: Observation and Analysis. Journal of Geophyscial Research-Solid Earth, 107(B11), 2289. doi:10.1029/2001JB001029.

Joughin, I., W. Abdalati and M. Fahnestock. 2004. Large Fluctuations in Speed on Greenland's Jakobshavn Isbrae Glacier. Nature, 432(7017): 608-610. doi:10.1038/nature03130.

Moon, T. and I. Joughin 2008. Retreat and Advance of Greenland Tidewater Glaciers from 1992 to 2007. Journal of Geophysical Research Earth Surface, 113: F02022, doi:10.1029/2007JF000927.

Rignot, E. and P. Kanagaratnam. 2006. Changes in the Velocity Structure of the Greenland Ice Sheet. Science, 311(5763): 986-990. doi:10.1126/science.1121381.

Joughin, I., B. Smith, I. M. Howat, T. Scambos, and T. Moon. 2010. Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping. Journal of Glaciology, 56 (197), pp. 415-430, doi:10.3189/002214310792447734.

The following related documents are available

Table 3. Related Documents
Document Description URL
Alaska Satellite Facility ASF Web site http://www.asf.alaska.edu/
MEaSUREs Data Web site NSIDC data Web site http://nsidc.org/data/measures/
NASA MEaSUREs Web site NASA Measures Projects Web site http://earthdata.nasa.gov/our-community/community-data-system-programs/measures-projects

6. Document Information

Acronyms

The following acronyms are used in this document.

Table 4. Acronyms and Abbreviations
Acronym Description
FTP File Transfer Protocol
InSAR Interferometric Synthetic Aperture Radar
MEaSUREs Making Earth System Data Records for Use in Research Environments
NSIDC National Snow and Ice Data Center
URL Uniform Resource Locator

Document Creation Date

23 September, 2010

Document Revision Date

24 May, 2012

Document URL

http://nsidc.org/data/docs/measures/nsidc0478_joughin/index.html