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Data Set ID:
NSIDC-0725

MEaSUREs Greenland Annual Ice Sheet Velocity Mosaics from SAR and Landsat, Version 1

This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, contains annual ice velocity mosaics for the Greenland Ice Sheet derived from Synthetic Aperture Radar (SAR) data obtained by TerraSAR-X/TanDEM-X (TSX/TDX) and Copernicus Sentinel-1A and -1B satellites, and from Landsat 8 optical imagery for years 2015 and 2016.

See Greenland Ice Mapping Project (GIMP) for related data.

Note: These data are considered provisional pending a review by the MEaSUREs program. Once the data have been reviewed, this statement will be removed.

Geographic Coverage

Parameter(s):
  • Snow/Ice > Ice Velocity
Spatial Coverage:
  • N: 83, S: 60, E: -14, W: -75

Spatial Resolution:
  • 500 m x 500 m
  • 200 m x 200 m
Temporal Coverage:
  • 1 December 2014 to 30 November 2015
  • 1 December 2015 to 30 November 2016
Temporal Resolution: 1 year
Data Format(s):
  • GeoTIFF
  • ESRI Shapefile
Platform(s) LANDSAT-8, SENTINEL-1A, SENTINEL-1B, TDX, TSX
Sensor(s): C-SAR, OLI, SAR, X-SAR
Version: V1
Data Contributor(s): Ian Joughin
Data Citation

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. 2017. MEaSUREs Greenland Annual Ice Sheet Velocity Mosaics from SAR and Landsat, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/OBXCG75U7540. [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.. 2010. Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping, Journal of Glaciology. 56. 415-430.

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Detailed Data Description

Format

Data are provided at both 200 and 500-m resolutions in GeoTIFF (.tif) format. Six data files are available for each data year and resolution: a velocity magnitude map (vv); separate x- and y-component velocities (vx, vy); separate x- and y-component error estimates (ex, ey), and a browse file (with color log scale velocity saturating at 3000 m/year). In addition, ancillary files are provided for each data year to indicate the source image pairs that were processed to produce the mosaics. These are provided in two separate shapefiles (.shp) for the Landsat 8 (L8) and Synthetic Aperture Radar (SAR) data.

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File and Directory Structure

Data are available via HTTPS: https://daacdata.apps.nsidc.org/pub/DATASETS/nsidc0725_MEASURES_annual_vel_mosaics_v01/.

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File Naming Convention

This section describes the naming convention for this product with an example. Refer to Table 1 for descriptions of the values in the file naming convention.

Naming Convention:

greenland_vel_mosaic[RRR]_[yyyy]_[vv OR vx OR vy]_v1.ext
greenland_vel_mosaic[RRR]_[yyyy]_[ex OR ey]_v1.ext
greenland_vel_mosaic[RRR]_[yyyy]_browse_v1.ext
greenland_vel_mosaic_[yyyy]_[SS]_v1.ext

Example File Names:

greenland_vel_mosaic500_2015_vv_v1.tif
greenland_vel_mosaic500_2015_vx_v1.tif
greenland_vel_mosaic500_2015_vy_v1.tif
greenland_vel_mosaic500_2015_ex_v1.tif
greenland_vel_mosaic500_2015_ey_v1.tif
greenland_vel_mosaic500_2015_browse_v1.tif

greenland_vel_mosaic_2015_L8_v1.shp
greenland_vel_mosaic_2015_L8_v1.dbf
greenland_vel_mosaic_2015_L8_v1.shx
greenland_vel_mosaic_2015_L8_v1.prj

greenland_vel_mosaic_2015_SAR_v1.shp
greenland_vel_mosaic_2015_SAR_v1.dbf
greenland_vel_mosaic_2015_SAR_v1.shx
greenland_vel_mosaic_2015_SAR_v1.prj

Table 1. File Naming Convention
Variable Description
greenland_vel_mosaic Greenland velocity mosaic
RRR Resolution: 500 m or 200 m
yyyy Data year
vv OR vx OR vy Velocity magnitude OR velocity x-direction OR velocity y-direction,
ex OR ey Error x-direction, error y-direction
browse Browse image
SS Source sensor and satellite information:
SAR (TerraSAR-X/TanDEM-X, Sentinel-1A and -1B)
L8 (Landsat-8)
v1 Version 1
.ext GeoTIFF (.tif)
Shapefile (.shp, .dbf, .shx, .prj)

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File Size

Each .tif data file ranges from 6.7 MB to 425 MB. Each shapefile (.shp, .dbf, .shx, .prj) ranges from 255 bytes to 3.8 MB.

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Volume

The total volume of the data set is (40 files) is approximately 5.5 GB.

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Spatial Coverage

This data set spans the entire Greenland Ice Sheet.

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

Spatial Resolution

The data are posted at both 500- and 200-m spacing, which should not be confused with the actual resolution. These products are derived as spatially varying averages from source data with resolutions ranging from a few hundred m to 1.5 km, making it difficult to specify the resolution at any point. As some estimates are derived as the average of 30 or more individual measurements, there is some degree of resolution enhancement such that the final resolution is better than that of the individual source products, but not well quantified. 

For work requiring finer resolution, it may be preferable to use the individual TerraSAR-X (TSX)/TanDEM-X (TDX) Landsat data where available (MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR, Version 1; MEaSURES Greenland Ice Velocity: Selected Glacier Site Velocity Maps from Optical Images, Version 1).

Projection and Grid Description

Data are provided in a WGS 84 polar stereographic grid with a standard latitude of 70° N and 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 (EPSG:3413).

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Temporal Coverage

This data set provides annual velocity mosaics for 2015 and 2016. The actual “Year” is broadly defined as 1 December to 30 November, i.e. 1 December 2014 to 30 November 2015 is defined as 2015.

Temporal Resolution

The temporal resolution is one year.

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Parameter or Variable

The parameter for this data set is ice velocity.

Parameter Description

Velocities are reported in meters per year. The vx and vy files contain component velocities 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. Small holes have been filled via interpolation in some areas. Interpolated values are identifiable as locations that have velocity data but no error estimates. Radar-derived velocities are determined using a combination of conventional Interferometric SAR (InSAR) and speckle tracking techniques (Joughin, et. al., 2002).

Sample Data Record

Figure 1. 2016 Greenland Annual Velocity Mosaic Browse Image
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Software and Tools

GeoTIFF files and shapefiles can be viewed with a variety of Geographical Information System (GIS) software packages including QGIS and ArcGIS.

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Data Acquisition and Processing

Derivation Techniques and Algorithms

The data are posted to 200- and 500-m grids, but the true resolution varies between a few hundred m to 1.5 km. Many small glaciers are resolved outside the main ice sheet, but for narrow (<1 km) glaciers, the velocity represents an average of both moving ice and stationary rock. As a result, while the glacier may be visible in the map, the actual speed may be underestimated. For 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 artifacts, but should be used with caution.

Areas with no data correspond either to regions where no data were acquired or where the interferometric or optical correlation was insufficient to produce an estimate, most often in areas with high snow accumulation. The “no data” value for magnitude files is -0.1. The no data value is -2e+9 for the vx, vy, ex, and ey files.

Processing Steps

These mosaics were produced mostly from Copernicus Sentinel-1A and Sentinel-1B data from the European Space Agency and supplemented by TSX/TDX data for coastal outlets. The data were acquired in either 12-day (through Sept 2016) or 6-day repeat cycles (October 2016 forward). In cases of missing acquisitions, the repeat periods may be longer (integer multiples of 6 or 12 days) for some of the image pairs. In addition to the SAR data, during periods when there was sufficient daylight, Landsat 8 velocities were merged with the SAR data.

Although the mosaics represent a full calendar year, they are computed as averages of all available data at each point and weighted by their respective errors (Joughin 2002), so these products do not represent true annual averages. For example, in some places mid-summer may be weighted more heavily than mid-winter due to the seasonal availability of Landsat 8 data. In some regions, clouds or large snow accumulation events may also affect the seasonal distribution of the data. As a result, comparison of adjacent years at any location might produce differences that represent some degree of seasonal variation. Such differences should be small, particularly when examining trends over multiple years.

Unlike earlier SAR acquisitions, Sentinel-1A and -1B provide crossing ascending and descending orbit data over much of the ice sheet. In areas where crossing-orbit data were available, an error-weight range-offset-only solution was included in the velocity product, which eliminated azimuth offsets and reduced the error from ionospheric streaking in the azimuth offsets.

Annual mosaic for 2015 (Dec 1, 2014 - Nov 30, 2015)

Sentinel-1A data acquisitions began in 2015, but the acquisition rates were not as regular as later years. As a result, these data tend to be somewhat noisier than the 2016 data, particularly in the middle of the ice sheet. In addition, the sampling of coastal regions is more irregular (there are gaps in the temporal coverage where TSX/TDX data was not acquired by the satellite for a month or more), which reduces the averaging of seasonal variation.

Annual mosaic for 2016 (Dec 1, 2015 - Nov 30, 2016)

For this year, the six Sentinel-1A tracks that image the majority of the Greenland coast were collected for almost every 12-day satellite repeat cycle. Beginning in October 2016, Sentinel-1B started acquiring data over Greenland in an orbit that lags Sentinel-1A by six days, providing better coverage and thus more correlations in the data. As a result, the accuracy for this mosaic is considerably better than the mosaic for 2015 for most regions.

Baseline Fits

Each image pair used in the mosaic requires a 4- to 6-parameter fit for the baseline parameters. The data are fit to a common set of ground control points as described by Joughin et al., 2010.  For years where data is not well controlled (sparse ground control points), control points from other years with adequate controls are used. This greatly improves consistency of the data from year to year. While this could mask some true change, the errors without this procedure are far larger than any change likely to occur.

These data should not be used to determine inter-annual change for interior regions of the ice sheet (roughly defined as areas above 2,000 m). In outlet glaciers close to the coast where the baselines are well constrained by bedrock, the velocity mosaics are well suited to this task. However, care should be exercised in interpreting any change observed in intermediate regions (roughly 1,000 m to 2,000 m), i.e. avoid areas where the observed changes seem to follow a satellite swath boundary. Refer to Figure 5 in Phillips et al., 2013 for an example. Note that baseline errors are not included in the formal error estimates, and actual errors can be substantially larger than stated. In particular, where estimates are derived from an average of 20 or more estimates, the reported errors are extremely low (<0.5 m/year). With baseline errors included, the actual error ranges from 1 to 3 m/year in most cases.

Interpolated Points

Small holes in the final maps have been filled via interpolation. These points can be identified as those that have valid velocity data but no corresponding error estimate.

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

Error Sources and Limitations

Due to the large volume of averaged source data, the overall quality of the data set is quite good. While the spatial coverage is generally improved in the southeast relative to earlier Greenland Ice Mapping Project (GIMP) MEaSUREs products, the results are considerably noisy relative to other regions of the ice sheet. High snow accumulation in the southeast greatly reduces image-to-image correlation, resulting in higher noise. Additionally, in these regions there may be coherent displacement signals (e.g., vertical displacement associated with compacting snow) that are not associated with horizontal ice motion. If such displacement occurs with characteristics other than that assumed in the solution (e.g., predominantly vertical instead of horizontal displacement), then the results will be incorrectly mapped to horizontal motion, thereby contributing to the overall level of noise.

Error Estimates

Error estimates are provided for all non-interpolated, radar-derived velocity vectors in separate GeoTIFF files appended with _ex.tif and _ey.tif. Formal errors agree reasonably well with errors determined by comparison with GPS data (Joughin, et al, 2002). The values, however, underestimate true uncertainty in several ways, and as such should be used more as an indication of relative quality rather than absolute error.

In general, the error estimates represent the average behavior of the data. This means that errors may be much lower than reported in some areas and much greater in others; care should be taken if assigning statistical significance based on the errors, especially given that the errors can be been correlated over large areas. For example, even if the errors are correct in a global sense, one might compare two mosaics and find a large difference over 5% of the ice sheet. However, because errors can be spatially correlated over broad areas, one should not assume significance at the 95% confidence level; this might be precisely the 5% that statistically should exceed the errors because the errors are not uniformly distributed. By contrast, if the errors were completely uncorrelated, one could average over neighborhoods to reduce the error.

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Sensor or Instrument Description

Descriptions of  the instruments used to construct the mosaics from which this data set is derived are at the mission sites:

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References and Related Publications

Contacts and Acknowledgments

Investigator(s) Name and Title

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

Acknowledgments: 

This project was supported by a grant from the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) Program.

Contains modified Copernicus Sentinel data (2014-2016), acquired by the European Space Agency, distributed through the Alaska Satellite Facility and processed by Joughin, I.

Document Information

DOCUMENT CREATION DATE

September 2017

DOCUMENT REVISION DATE

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