Data Set ID:
NSIDC-0723

MEaSUREs Greenland Image Mosaics from Sentinel-1A and -1B, Version 2

This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, consists of 6-day and 12-day 50 m resolution image mosaics of the Greenland coastline and ice sheet periphery. The mosaics are derived from C-band Synthetic Aperture Radar (C-SAR) acquired by the Copernicus Sentinel-1A and -1B imaging satellites starting in January 2015.

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

See the Version History section of the documentation for changes made in version 2.1.

Version Summary:

The changes for Version 2 include:

  • Data reprocessed with a small radiometric correction to improve consistency across seam boundaries.
  • Temporal coverage expanded.
  • Dates added to the .gif files.

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Geographic Coverage

Parameter(s):
  • Radar > Radar Backscatter
Spatial Coverage:

  • N: 84, S: 59, E: -15, W: -73
Spatial Resolution:
  • 50 m x 50 m
Temporal Coverage:
  • 1 January 2015
(updated 2018)
Temporal Resolution: 6 days to 12 days
Data Format(s):
  • GeoTIFF
  • JPEG
  • XML
  • ESRI Shapefile
  • GIF
Platform(s) SENTINEL-1A, SENTINEL-1B
Sensor(s): C-SAR
Version: V2
Data Contributor(s): Ian Joughin
Metadata XML: View Metadata Record

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, updated 2018. MEaSUREs Greenland Image Mosaics from Sentinel-1A and -1B, Version 2. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/GPVL3SACPBW4. [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. Moon, and T. Scambos. 2016. A SAR record of early 21st century change in Greenland, Journal of Glaciology. 62. 62-71. https://doi.org/10.1017/jog.2016.10

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Periodic updates to this data set are anticipated as new Sentinel data are collected and processed. We request that users provide feedback to NSIDC (nsidc@nsidc.org) on whether a 25 m product would be desirable.

Detailed Data Description

Format

Each mosaic product spans either a 12-day period (prior to 28 September 2016) or 6-day period (starting on 28 September 2016). Each mosaic is available in Geographic Tagged Image File Format (GeoTIFF) as a set of four tiles geographically divided into North, Northeast, Northwest, and South regions posted at 50 m, which is approximately equivalent to the image resolution after multi-look averaging. In addition, the following ancillary files are available:

  • Shapefiles for each product that provide the dates for the individual images used in each mosaic. In addition to the date, the shapefile specifies the track (relative orbit), orbit (absolute orbit), and particular satellite (S1A/B).
  • Geospatial Data Abstraction Library (GDAL) .vrt files, which allow the 50 m GeoTIFF files to be opened as a single file in geographical information system (GIS) programs
  • OVR files (.vrt.ovr) that contain image pyramids to facilitate rapid viewing of the 50 m mosaics
  • 1-km Quicklook images (.jpg and .jpg.aux.xml) that provide some geolocation information
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File and Directory Structure

Data are available via HTTPS in the following directory:

https://daacdata.apps.nsidc.org/pub/DATASETS/nsidc0723_MEASURES_image_mosaics_v02/

Within this directory there are 130 folders and two video files (.gif). The folders are each named by year-month-day (YYYY-MM-DD), where the date corresponds to the first date of the mosaic epoch. The .gif files are named upernavik_v02.1.gif and greenland_v02.1.gif.

Each folder contains 12 files, where the date corresponds to the first date of the mosaic epoch.

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File Naming Convention

This section explains the file naming conventions with examples of each file type. Tables 1 through 3 describe the variables for each naming convention. Beginning with the November 2017 update, the file names changed to S1_[start_date]_[end_date]_v02.1.[ext] with both S1A and S1B data used in each file. As of February 2018, all file names use this convention.

Shapefiles

Naming Convention: S1_[start_date]_[end_date]_v02.1.[ext]
Example File Names:

  • S1_2015-01-01_2015-01-12_v02.1.dbf
  • S1_2015-01-01_2015-01-12_v02.1.prj
  • S1_2015-01-01_2015-01-12_v02.1.shp
  • S1_2015-01-01_2015-01-12_v02.1.shx

Table 1 describes the variables used in the Shapefile naming convention.

Variable Description
Table 1. Naming Convention for Shapefiles
S1 Sentinel-1 satellites A and B
start_date date of first image used in the mosaic period (YYYY-MM-DD)
end_date date of last image used in the mosaic period (YYYY-MM-DD)
v02.1 version number
.ext File extension. The shapefile format consists of four files:
.dbf (database file)
.prj (projection information)
.shp (shapes)
.shx (shape indices)

Browse JPEGs and Metadata files, GDAL Virtual Files, and GDAL Image Pyramid Files

Naming Convention: S1_[start_date]_[end_date]_mosaic_v02.1.[ext]
Example File Name:

  • S1_2015-01-01_2015-01-12_mosaic_v02.1.jpg
  • S1_2015-01-01_2015-01-12_mosaic_v02.1.vrt
  • S1_2015-01-01_2015-01-12_mosaic_v02.1.vrt.ovr
  • S1_2015-01-01_2015-01-12_mosaic_v02.1.jpg.aux.xml

Table 2 describes the variables used in this data set's Browse file naming convention.

Variable Description
Table 2. Naming Convention for Browse Files
S1 Sentinel-1 satellites A and B
start_date date of first image used in the mosaic period (YYYY-MM-DD)
end_date date of last image used in the mosaic period (YYYY-MM-DD)
mosaic denotes that the file is a multiyear mosaic file
v02.1 version number
.ext File extensions:
.jpg (JPEG)
.vrt (GDAL virtual format)
.vrt.ovr (GDAL OVR pyramid format)
.jpg.aux.xml (Extensible Markup Language)

GeoTIFFs

Naming Convention: S1_[start_date]_[end_date]_[region]_v02.1.tif
Example File Names:

  • S1_2015-01-01_2015-01-12_south_v02.1.tif
  • S1_2015-01-01_2015-01-12_northwest_v02.1.tif
  • S1_2015-01-01_2015-01-12_northeast_v02.1.tif
  • S1_2015-01-01_2015-01-12_north_v02.1.tif

Table 3 describes the variables used in this data set's GeoTIFF file naming convention.

Variable Description
Table 3. Naming Convention for GeoTIFF Files
S1 Sentinel-1 satellites A and B
start_date date of first image used in the mosaic period (YYYY-MM-DD)
end_date date of last image used in the mosaic period (YYYY-MM-DD)
region regions of Greenland: north, northeast, northwest, or south
v02.1 version number
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File Size

File sizes vary depending upon file types:

  • GeoTIFF (.tif) files range from 16 MB to 195.4 MB
  • Browse images (.jpg) range from 83 KB to 365 KB
  • Geolocation (jpg.aux.xml) files range from 1 KB to 2 KB each
  • Shapefiles range from 1 KB to 3 KB
  • GDAL virtual (.vrt) files are 3 KB each
  • GDAL OVR (.vrt.ovr) files are approximately 503 MB

The total data volume of this data set is approximately 106 GB.

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Spatial Coverage

The study area lies within the following bounding box:

  •  Southernmost Latitude: 60° N
  •  Northernmost Latitude: 82° N
  •  Easternmost Longitude: 20° W
  •  Westernmost Longitude: 70° W

Spatial Resolution

The spatial resolution is 50 m x 50 m.

Projection and Grid Description

Projection

GeoTIFFs 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).

Grid

The 50 m mosaics are provided as a set of 4 tiles, divided and named by geographic region: North, Northeast, Northwest, South.

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Temporal Coverage

Start date: 01 January 2015 (ongoing)

This data set undergoes periodic updates as new Sentinel data are collected and processed.

Temporal Resolution

The temporal resolution for data acquired prior to 28 September 2016 is 12 days. The temporal resolution for data acquired after 28 September 2016 is 6 days.

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Parameter or Variable

The mosaics provided in this data set consist of uncalibrated C-SAR radar backscatter.

Parameter Description

In SAR applications, backscatter is the ratio between the power of a radar pulse transmitted to the target and the power scattered back towards the antenna.

Sample Data Record

Figure 1. Sample mosaic of data collected from 25 January 2015 to 02 February 2015 (file name: S1A_2015-01-25_2015-02-05_mosaic_v01.jpg). Contains modified Copernicus Sentinel data (2015-2017), acquired by the European Space Agency, distributed through the Alaska Satellite Facility, and processed by Joughin, I.
Figure 2. Individual frame from 28 October 2016 from the sample animation upernavik_v02.1.gif, focusing on the glacier termini near Upernavik, Greenland. Contains modified Copernicus Sentinel data (2015-2017), acquired by the European Space Agency, distributed through the Alaska Satellite Facility, and processed by Joughin, I.

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Software and Tools

GeoTIFF files can be viewed with a variety of Geographic Information System (GIS) software packages including:

For details about the shapefile format, see the ESRI Shapefile Technical Description white paper (PDF, 124 KB). The VRT format is discussed in detail on the GDAL Virtual Format Tutorial Web page. The ArcGIS Resource Center's Raster Pyramids and OVR Pyramid Files help pages provide more information about raster data set downsampling.

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Data Acquisition and Processing

Theory of Measurements

The interactions between radar signals and the ground depend upon many factors, such as the density and dielectric properties of surface materials, vegetation cover, surface roughness at the scale of the signal's wavelength, topographic variations, and the instrument's look angle and signal polarization. The image resolution is particularly affected by signal strength, chirp pulse length and bandwidth, return signal integration time, and the time between pulse transmissions.

For a detailed discussion of SAR theory, see SAR Theory/Interpreting Images (PDF, 82.4 KB). For general information about the mathematical derivations and theories behind SAR processing algorithms, see Scientific SAR User's Guide (PDF, 296 KB).

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Data Acquisition Methods

Copernicus Sentinel-1A and -1B satellite imagery were acquired by the European Space Agency (ESA) and archived and distributed through the Alaska Satellite Facility.

From 01 January 2015 to 15 September 2016, data was acquired from Sentinel-1A only. Beginning 28 September 2016, data was acquired from both platforms, Sentinel-1A/-1B.

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Derivation Techniques and Algorithms

Processing Steps

The Alaska Satellite Facility provided all source data as Level-1 burst Single-Look Complex (SLC) data, including digitized voltage values, instrument calibration constants, satellite timing, attitude, and position information. The SLC burst data were processed to full multi-burst SLC scenes using the GAMMA Modular SAR Processor (MSP) package.

The data have been mapped to grey scale values in the range of 0 - 255 (bytes) using a nonlinear stretch to allow visual discrimination of topographical features within the images. As a result, the pixel values for each year are not directly comparable; however, if no change has occurred, they should be relatively consistent from scene to scene.

The mosaics are produced from four descending and two ascending tracks. In some cases, the image tracks overlapped, but at any given pixel, only one image was used. If the overlapping images were ascending (Tracks 74 and 90) and descending (Tracks 26, 112, 141, 170), the descending image was used (see the shapefile attribute field to determine track direction). If two parallel images overlapped, the top polygon in the shapefile should correspond to the image that was included in the mosaic. In some locations, it is possible that one mosaic will have ascending geometry and another descending geometry because of missed acquisitions (i.e., a consistent geometry is used whenever sufficient data are acquired). In such areas, mountains and other topography features will appear to be illuminated from opposite sides. If the images are flickered between, the different viewing geometries can give the impression of large shifts, even though the data are generally well registered. For regions with like-viewing geometry, the co-registration is far better than the 50-m posting, except in cases where the topography is extreme.

In instances where no viable data were available, a blank tile (GeoTIFF (.tif)) was still generated. The JPEG file (.jpg) that accompanies each four-tile mosaic provides a quicklook of expected data output when all four tiles are combined.

Error Sources

Except in regions of extreme terrain, geometric accuracy is better than a single 50 m pixel (Joughin et al, 2016).

Terrain Correction

The data were terrain corrected with MEaSUREs Greenland Ice Mapping Project (GIMP) Digital Elevation Model, Version 1, (Joughin et al, 2016). Due to this correction, some slight differences in registration relative to MEaSUREs Greenland Ice Sheet Mosaics from SAR Data, Version 1 mosaics exist.

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Sensor or Instrument Description

For more information on the SAR satellites, Sentinel-1A and -1B, please see the European Space Agency's Copernicus Sentinel-1 site.

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Version History

Table 4. Version History
Version Description Effective Date
V2.1 An additional track (Sentinel 1, track 83) is included to improve coverage in the southern part of the ice sheet. This will change the geometry for a small region in the southwest from ascending to predominantly descending from May 2017 forward when track 83 is available.  May 2017 v2.0 and June 2017 v2.0 files were replaced. July 2017 data were added. September 2017
V2
  • Data were reprocessed with a small radiometric correction to improve consistency across seam boundaries
  • Temporal coverage was expanded through 31 May 2017
  • Dates added to the .gif files
 July 2017
V1 Initial Release June 2017
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References and Related Publications

Joughin, I., B. E. Smith, I. M. Howat, T. Moon, and T. A. Scambos. 2016. A SAR record of early 21st century change in Greenland. Journal of Glaciology 62(231): 62–71. doi: http://dx.doi.org/10.1017/jog.2016.10.

Related Data Collections

Related Websites

Contacts and Acknowledgments

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

Acknowledgments: 

These mosaics were generated through a grant from the NASA MEaSUREs program. Contains modified Copernicus Sentinel data (2015-2018), acquired by the European Space Agency, distributed through the Alaska Satellite Facility, and processed by Joughin, I.

The WMO Polar Space Task Group is acknowledged for its role in coordinating the routine Copernicus Sentinel-1 Greenland acquisition plans.

Document Information

DOCUMENT CREATION DATE

June 2017

DOCUMENT REVISION DATE

August 2018

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