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MEaSURES Greenland Ice Velocity: Selected Glacier Site Velocity Maps from Optical Images, Version 1
This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, consists of mean monthly velocity maps for selected glacier outlet areas. The maps are generated by tracking visible features between optical image pairs acquired by the Landsat 7 Enhanced Thematic Mapper Plus (ETM+), the Landsat 8 Operational Land Imager (OLI), and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER).
See Greenland Ice Sheet Mapping Project (GIMP) for related data.
Geographic Coverage
Spatial Coverage: |
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Spatial Resolution: |
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Temporal Coverage: |
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Temporal Resolution: | 1 month |
Parameter(s): |
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Platform(s) | LANDSAT-7, Landsat 8, TERRA |
Sensor(s): | ASTER, ETM+, OLI |
Data Format(s): |
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Version: | V1 |
Data Contributor(s): | Ian Howat |
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.
Howat, I. 2016. MEaSURES Greenland Ice Velocity: Selected Glacier Site Velocity Maps from Optical Images, 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/EYV1IP7MUNSV. [Date Accessed].Detailed Data Description
This data set is organized into 49 study sites. The following files are provided for each site:
- velocity browse image (PNG)
- component velocity in the x-direction (GeoTIFF)
- component velocity in the y-direction (GeoTIFF)
- error estimates in the x-direction (GeoTIFF)
- error estimates in the y-direction (GeoTIFF)
- metadata file (ASCII text)
Study sites in this data set correspond to geographical sub-regions of Greenland. The name of each sub-region reflects its center latitude. Table 2 under the Spatial Coverage section of this document lists the name (center latitude) of each sub-region, the latitude and longitude of its lower left corner, and the geographical features it contains. Data are available via HTTPS in the following directories:
These directories are divided into folders for each study site. Data files are stored in subfolders labeled by date of acquisition.
This section explains the file naming convention used for this data.
Example File Names:
OPT_E61.10N_1999-09.png
OPT_E61.10N_1999-09.vx.tif
OPT_E61.10N_1999-09.vy.tif
OPT_E61.10N_1999-09.ex.tif
OPT_E61.10N_1999-09.ex.tif
OPT_E61.10N_1999-09.meta
Naming Convention:
- OPT_[sub-region]_[date].[datum].[ext]
The following table describes the variables used in this data set's file naming convention:
Variable |
Description |
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OPT |
Velocities derived from optical image pairs acquired by Landsat 8 OLI, Landsat 7 ETM+, ASTER, or a combination. |
sub-region |
Sub-region names are defined as follows:
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datum |
Component velocity or component error estimate:
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ext |
File extension:
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GeoTIFF files are approximately 1.5 MB each. Browse images and metadata files are less than 50 KB each.
The total data volume is approximately 21 GB.
This data set contains velocity maps for most of the outlet glaciers on the Greenland Ice Sheet. 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
100 m
Projection and Grid Description
Data are organized into 49 sub-regions of a 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. Table 2 lists the name (center latitude) of each sub-region, the latitude and longitude of its lower left corner, and the glaciers it contains.
Data span July 1999 through September 2015.
Temporal Resolution
Monthly
Parameter Description
This data set reports the following parameters:
- Ice velocities (x- and y-components)
- Error estimates (x- and y-components)
Corresponding ASCII text metadata files (.meta) are also provided that contain geographical information plus dates and codes that reveal the sensor combinations of the images used to create the monthly mean. The sensor combination key is provided in Table 4:
Sensor | Code |
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Landsat 8 OLI | LC8 |
Landsat 7 ETM+ | LE7 |
ASTER | AST |
The following sensor combinations are possible: LE7/LE7, LE7/LC8, AST/AST, LC8/LC8.
Software and Tools
GeoTIFF files can be viewed with a variety of Geographical Information System (GIS) software packages including:
Data Acquisition and Processing
Level 1 imagery for the Landsat 7 ETM+ and Landsat 8 OLI was obtained from the U. S. Geological Survey (USGS | Landsat Level 1 Standard Data Products). ASTER (AST14DMO) imagery was obtained from the NASA Land Processes Distributed Active Archive Center (LPDAAC).
Processing Steps
These data were created using orthorectified Landsat Level L1T or L1G and ASTER (AST14DMO) imagery. Orthorectified images were received in UTM projection and converted to Polar Stereographic using Geographic Data Abstraction Library (GDAL) software. ASTER visible bands 1-3 were reduced to a single grayscale principle component image. The panchromatic band was used for Landsat. Velocity fields were constructed using images from the same sensor or combinations of Landsat 7, Landsat 8, and ASTER images. In the case of Landsat pairs, only images from the same path/row were used to reduce the impact of terrain-dependent errors.
Velocity fields were produced by an automated cross-correlation of sequential images using the Multi-Image Multi-Chip (MIMC) algorithm described in Ahn and Howat (2011) and updated in Jeong et al. (in revision). The MIMC utilizes a range of image filters and search window sizes as well as both backward and forward matching to generate 64 matches per sample. Neighborhood statistics and an a priori velocity field, consisting of radar-derived velocities closest in time to the image dates from the MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data data set, were used to select the highest confidence solution and its uncertainty.
This velocity field was then corrected for image re-registration errors by subtracting the average displacement over bedrock or very slow moving ice (< 10 m/yr), which is located using the a priori velocity field. The residual deviation of velocities over bedrock then provides the registration error (see Error Sources). Individual velocity image pairs within each region were sampled to the same grid and stacked into monthly medians at each grid point, providing a monthly sampling. The median error was also obtained.
Note: Monthly means are calculated from images which may have acquisition dates from the preceding or succeeding month. For the naming convention, the month is determined from where the midpoint Julian dates fall. For example, September monthly means may have been generated from images that were acquired in August or in October but the midpoint Julian date between the images falls within September. The exact dates used are included in the meta file.
Error Sources
Uncertainty in the velocity solution results from uncertainty in the match solution and uncertainty in image co-registration. Match solution uncertainty is estimated at each grid point from the sample of individual velocity solutions that results from the MIMC procedure. This error is typically on the order of one-third of a pixel. Co-registration error, the dominant source of uncertainty, is estimated from the residual velocities obtained over bedrock and very slow ice after the mean is removed. These errors vary considerably, but are typically on the order of 100 m/year.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) obtains high-resolution (15 to 90 square meters per pixel) images of the Earth in 14 different wavelengths of the electromagnetic spectrum, ranging from visible to thermal infrared light. ASTER was launched in December 1999 onboard Terra, the flagship satellite of NASA's Earth Observing System (EOS). For more information, see Terra | ASTER.
The Enhanced Thematic Mapper Plus (ETM+) instrument on board Landsat 7 is a fixed “whisk-broom,” eight-band, multispectral scanning radiometer capable of providing high-resolution imaging information of the Earth’s surface. Orbiting at an altitude of 705 km, the instrument detects spectrally-filtered radiation in visible near-infrared, short-wave near-infrared, long-wave near-infrared, and panchromatic bands from the sun-lit Earth in a 183 km wide swath. Visit NASA Landsat Science | The Enhanced Thematic Mapper Plus for more information.
The Operational Land Imager (OLI) on Landsat 8 is an enhanced version of Landsat 7’s ETM+ that adds two new spectral bands: a deep blue visible channel (band 1) specifically designed for water resources and coastal zone investigation; and a new infrared channel (band 9) to detect of cirrus clouds. For more information, see USGS | Landsat 8.
References and Related Publications
Contacts and Acknowledgments
Dr. Ian Howat
Ohio State University
Byrd Polar Research Center
Scott Hall Room 108
1090 Carmack Road
Columbus, Ohio 43210
USA
These data were generated through a grant from the NASA MEaSUREs program.
Document Information
DOCUMENT CREATION DATE
March, 2016
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