MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR, Version 3
This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, provides velocity estimates determined from Interferometric Synthetic Aperture Radar (InSAR) data for major glacier outlet areas in Greenland, some of which have shown profound velocity changes over the MEaSUREs observation period. The InSAR Selected Glacier Site Velocity Maps are produced from image pairs measured by the German Aerospace Center's (DLR) twin satellites TerraSAR-X / TanDEM-X (TSX / TDX). The measurements in this data set are provided in addition to the ice sheet-wide data from the related data set, MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data.
Processing steps include a new DEM for images acquired in 2015 and after:
- images created from data acquired on or after January 1, 2015 were processed using the MEaSUREs GIMP DEM V2 (NSIDC-0715)
- for data acquired prior to 2015, images were processed using the MEaSUREs GIMP DEM V1 (NSIDC-0645)
- A field has been added to the .meta file to indicate which DEM was used in the processing.
As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.
Joughin, I., I. Howat, B. Smith, and T. Scambos. 2020. MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR, Version 3. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/YXMJRME5OUNC. [Date Accessed].
Literature Citation
As a condition of using these data, we request that you acknowledge the author(s) of this data set by referencing the following peer-reviewed publication.
Joughin, I., B. Smith, I. Howat, T. Scambos, and T. Moon. 2010. Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping, Journal of Glaciology. 56. 415-430. https://doi.org/10.3189/002214310792447734
Documentation
Created:
16 January 2020
Last modified:
22 September 2020
Data Description
Parameters
Ice velocity is reported in meters per year. The velocity magnitude is reported in the vv files. The vx and vy files contain the velocity components in the x- and y-directions, defined by the polar stereographic grid. These velocities are true values and not subject to the distance distortions present in a polar stereographic grid. In some areas, small gaps have been filled via interpolation. Interpolated values are identifiable as locations where velocity data are present but no error estimates exist. Radar-derived velocities are determined using a combination of conventional InSAR and speckle tracking techniques.
Error estimates are provided for all non-interpolated, radar-derived velocity vectors. These estimates include the statistical uncertainty associated with the phase and speckle tracking error. See Joughin (2002) for more detail on errors and how they are computed.
The missing data value for the velocity magnitude (vv) and error estimate (ex, ey) files is -1 and is set as the attribute in all files. The missing data value for the velocity component (vx, vy) files is -2e+9.
File Information
Format
For each grid and existing time period, the ice velocity magnitude (vv), its components (vx, vy), and the corresponding error estimates (ex, ey) are provided in Geographic Tagged Image File Format (GeoTIFF). A JPEG image of the velocity magnitude is provided for easy visualization. An ASCII formatted metadata file containing source satellite acquistion information is also included.
Naming Convention
Files are named according to the following convention: TSX_[grid]_[startdate]_[enddate]_[hh-mm-ss]_[parameter]_[vXX.X].[ext]
Table 1 describes the existing options in the file naming convention. As an example, below are listed all the files for grid E61.10N for the 19-30 April 2014 period in version 3.0:
whether it is on the East (E), West (W), or South (S) coast
latitude (for E and W) or longitude (for S) in decimal degrees
startdate
Date of first image (DDMMMYY)
enddate
Date of second image (DDMMMYY)
hh-mm-ss
Nominal time for pair
parameter
Velocity magnitude, velocity component, or error estimate
vv: velocity magnitude
vx: x component of velocity
vy: y component of velocity
ex: error of x component
ey: error of y component
vXX.X
Version of the data set
.ext
File extensions:
.tif = GeoTIFF formatted file
.jpg = JPEG file; visualization of the velocity magnitude
.meta = ASCII text file; contains the Central Julian date and nominal time (HH:MM:SS) for the pair, the date for each image, production date, sensor combinations, and geographical information
Spatial Information
Coverage
This data set contains velocity data for most of the outlet glaciers for the Greenland Ice Sheet. It is presented by study sites, with a total of 55 grids. The spatial coverage map in Figure 1 shows the locations of all grids on a map of Greenland.
Figure 1. This map shows the locations of all the grids around Greenland (click image for high-resolution version).
Resolution
100 meters
Projection
Data are provided in subregions of a polar stereographic grid with a standard latitude of 70° N and a rotation angle of -45° (sometimes specified as a longitude of 45° W). With this convention, the y-axis extends south from the North Pole along the 45° W meridian.
The origin specifies the polar stereographic coordinates for the center of the lower left pixel, i.e., the first sample in the file. This specification, using the middle of the reference pixel, differs from that used in some GeoTIFF and other formats where the reference coordinates are specified for the outer corner of the reference pixel.
Temporal Information
Coverage
12 June 2008 to present.
Data for 2008 were only obtained for three grids on a trial basis. Most grids have data starting in 2009.
This data set undergoes periodic updates as new data are collected and processed. Please check theTemporal Coverage table (available as an Excel spreadsheet under the Technical References tab) for a complete list of available dates by grid and by year.
Resolution
The temporal resolution varies between 11, 22, and 33 days on an 11-day repeat cycle.
Data Acquisition and Processing
Theory of Measurements
The ice velocity maps in this data set were created using Synthetic Aperture Radar (SAR) data from the German Aerospace Center's (DLR) twin satellites TerraSAR-X / TanDEM-X (TSX/TDX). The methods include a combination of speckle tracking and conventional interferometry. See Joughin (2002) for more detail.
Sensor / Instrument Description
The twin satellites TerraSAR-X / TanDEM-X fly in close formation only a few hundred meters apart. For each time period in this data set, velocities were estimated from a pair of images. For any given pair, the images are obtained from either satellite and could have any of the possible sources: TSX/TSX,TSX/TDX, or TDX/TDX.
Error Sources
Error estimates are provided for all non-interpolated, radar-derived velocity components (vx, vy). They include the statistical uncertainty associated with the phase and speckle tracking error inherent in the SAR data. Formal errors agree reasonably well compared with errors determined from GPS data (Joughin, 2002). However, the true uncertainty is likely larger and these estimates should be used as an indication of relative quality rather than as absolute error.
Software and Tools
GeoTIFF files can be viewed with a variety of Geographical Information System (GIS) software packages, including QGIS and ArcGIS.
Version History
Version 3 was released in August 2020. Refer to Table 2 for the data set version history:
Table 2. Version History
Version
Release Date
Description of Changes
V1
May 2011
Initial release
V1.1
February 2016
GeoTIFF file format added; binary format discontinued; contains improved temporal sampling for the Jakobshavn Isbrae, Helheim, and Kangerdlugssuaq glaciers. The improved sampling addresses previous artifacts related to slope discontinuities at these glaciers' termini for the years 2009 – 2016
V1.2
May 2017
Renamed files to include the nominal time for pair; added 3 TSX subdirectories missing from their respective region directories; removed extraneous files from several TSX subdirectories; included .meta files for metadata
V2
February 2020
1. Full reprocessing with accumulated minor updates. Output should be generally consistent with previous versions.
2. The tiffs are now cloud optimized and include scale-down by 2 and 4 pyramids.
3. Velocity magnitude is now included as a separate tiff to be consistent with other velocity products (so now there are vx, vy, and vv tiffs).
4. Correction of browse images (distortions, color bar placement, watermark, and color-scale consistency.)
5. Addition of consistent NoData values
6. Temporal coverage was extended.
V3
August 2020
Processing steps include a new DEM for images acquired in 2015 and after. For consistency with the MEAsUREs Greenland Ice sheet Mapping Project (GIMP) Sentinel-1 product (NSIDC-0723), and to account for the evolving ice sheet geometry, images created from data acquired on or after January 1, 2015 were processed using the MEaSUREs GIMP DEM V2 (NSIDC-0715). For images acquired prior to 2015, the processing steps included the MEaSUREs GIMP DEM V1 (NSIDC-0645). A field has been added to the .meta file to indicate which DEM was used in the processing. As a result of this change, there could be geolocation and other systematic differences when comparing images prior to 2015 with images from 2015 and after. Such artifacts are most likely to be found in regions with strong elevation changes or where there are large changes in terminus position. Since all of this product’s V2 data were produced with the MEaSUREs GIMP V1 DEM (NSIDC-0645), such artifacts should also be present when comparing V2 and V3 images for the same date if in 2015 and later. Other input data, algorithms, processing steps, and uncertainty estimates remain the same as in V2.
Ian Joughin
Applied Physics Laboratory
University of Washington
Ian Howat
Byrd Polar Research Center
Ohio State University
Ben Smith
Polar Science Center Applied Physics Laboratory
University of Washington
Ted Scambos
National Snow and Ice Data Center
Cooperative Institute for Research in Environmental Science
University of Colorado
References
Yushin Ahn, & Howat, I. M. (2011). Efficient Automated Glacier Surface Velocity Measurement From Repeat Images Using Multi-Image/Multichip and Null Exclusion Feature Tracking. IEEE Transactions on Geoscience and Remote Sensing, 49(8), 2838–2846. https://doi.org/10.1109/tgrs.2011.2114891
Howat, I. M., Box, J. E., Ahn, Y., Herrington, A., & McFadden, E. M. (2010). Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland. Journal of Glaciology, 56(198), 601–613. https://doi.org/10.3189/002214310793146232
Joughin, I. (2002). Ice-sheet velocity mapping: a combined interferometric and speckle-tracking approach. Annals of Glaciology, 34, 195–201. https://doi.org/10.3189/172756402781817978
Joughin, I., Abdalati, W., & Fahnestock, M. (2004). Large fluctuations in speed on Greenland’s Jakobshavn Isbræ glacier. Nature, 432(7017), 608–610. https://doi.org/10.1038/nature03130
Joughin, I., Smith, B. E., Howat, I. M., Scambos, T., & Moon, T. (2010). Greenland flow variability from ice-sheet-wide velocity mapping. Journal of Glaciology, 56(197), 415–430. https://doi.org/10.3189/002214310792447734
Joughin, I., Kwok, R., & Fahnestock, M. (1996). Estimation of ice-sheet motion using satellite radar interferometry: method and error analysis with application to Humboldt Glacier, Greenland. Journal of Glaciology, 42(142), 564–575. https://doi.org/10.3189/s0022143000003543
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