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Data Set ID:
ILVIS2
IceBridge LVIS L2 Geolocated Surface Elevation Product, Version 1
This data set contains surface elevation measurements from NASA's Land, Vegetation, and Ice Sensor (LVIS) over areas including Greenland and Antarctica. The data were collected as part of NASA Operation IceBridge funded campaigns.
Geographic Coverage
| Parameter(s): |
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| Spatial Coverage: |
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| Spatial Resolution: |
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| Temporal Coverage: |
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| Temporal Resolution: | Varies |
| Data Format(s): |
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| Platform(s) | AIRCRAFT, B-200, DC-8, G-V, HU-25C, P-3B, RQ-4 |
| Sensor(s): | ALTIMETERS, LASERS, LVIS |
| Version: | V1 |
| Data Contributor(s): | J. Blair, Michelle Hofton |
| 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.
Blair, J. B. and M. Hofton. 2010, updated 2016. IceBridge LVIS L2 Geolocated Surface Elevation Product, 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/OIKFGJNBM6OO. [Date Accessed].Collapse All / Open All
Detailed Data Description
Format
The LVIS Level-2 Geolocated Surface Elevation Product data files are in ASCII text (.TXT) format. Each data file is paired with an associated XML file. The XML files contain location, platform, and instrument metadata.
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File and Directory Structure
Data files are on the FTP site, https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/ILVIS2.001/ in folders organized by date, for example /2015.10.10/.
2014 ARISE campaign data are included in dates from 02 September 2014 through 02 October 2014, that is, folder names 2014.09.06 through 2014.10.02.
Note: If you downloaded the 2010 Greenland data before 13 April 2012, please download the new Version 01.1 2010 Greenland data files at your convenience. All of the 2010 Greenland data for the IceBridge LVIS Level-2 Geolocated Surface Elevation Product data set have been replaced as of 13 April 2012. Some incorrect values occurred in the previous version of the files, and these are corrected in the Version 01.1 data.
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File Naming Convention
Files are named according to the following convention and as described in Table 1:
File name example:
ILVIS2_GL2013_1114_R1406_043900.TXT
ILVIS2_GL2013_1114_R1406_043900.TXT.xml
ILVIS2_LOYYYY_MMDD_RYYMM_nnnnnn.xxx
Where:
| Variable | Description |
|---|---|
ILVIS2 |
Short name for IceBridge LVIS L2 Geolocated Surface Elevation Product |
LOYYYY |
Campaign identifier. LO = location, where GL = Greenland and AQ = Antarctica. YYYY= four-digit year of campaign |
MMDD |
Two digit month, two-digit day of campaign |
RYYMM |
Date (YY year/ MM month) of the data release |
nnnnnn |
Number of seconds since UTC midnight of the day the data collection started |
.xxx |
Indicates file type: ASCII text (.TXT) or XML (.xml) |
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File Size
Text files range from approximately 351 KB to 89 MB.
XML files range from approximately 10 KB to 159 KB.
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Volume
Data volume for the full data set is approximately 138 GB.
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Spatial Coverage
Spatial coverage for the IceBridge LVIS campaigns includes the Arctic, Greenland, Antarctica, and surrounding ocean areas. In effect, this represents the coverage noted below.
Arctic / Greenland:
Southernmost Latitude: 60° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
Antarctic:
Southernmost Latitude: 90°S
Northernmost Latitude: 53°S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
Spatial Resolution
Spatial resolution is nominally 20 meters, but varies with aircraft altitude. Laser spot size is a function of beam divergence and altitude. Nominal spot spacing is a function of scan rate and pulse repetition rate.
Projection and Grid Description
International Terrestrial Reference Frame (ITRF 2000), WGS-84 Ellipsoid.
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Temporal Coverage
These data were collected as part of Operation IceBridge from 14 April 2009 to the present.
Temporal Resolution
IceBridge campaigns are conducted on an annual repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May, and Antarctic campaigns are conducted during October and November.
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Parameter or Variable
The LVIS Level-2 Geolocated Surface Elevation Products data include mean elevation measurements and other height measurements
Parameter Description
The IceBridge LVIS Level-2 Geolocated Surface Elevation Product ASCII text format data files contain fields as described in Table 2.
| Parameter | Description | Units |
|---|---|---|
| LVIS_LFID | LVIS file identification, including date and time of collection and file number. The second through sixth values in the first field represent the Modified Julian Date of data collection. | n/a |
| SHOTNUMBER | Laser shot assigned during collection | n/a |
| TIME | UTC decimal seconds of the day | Seconds |
| LONGITUDE_CENTROID | Refers to the centroid longitude of the corresponding LVIS Level-1B waveform. | Degrees east |
| LATITUDE_CENTROID | Refers to the centroid latitude of the corresponding LVIS Level-1B waveform. | Degrees north |
| ELEVATION_CENTROID | Refers to the centroid elevation of the corresponding LVIS Level-1B waveform. | Meters |
| LONGITUDE_LOW | Longitude of the lowest detected mode within the waveform | Degrees east |
| LATITUDE_LOW | Latitude of the lowest detected mode within the waveform | Degrees north |
| ELEVATION_LOW | Mean elevation of the lowest detected mode within the waveform | Meters |
| LONGITUDE_HIGH | Longitude of the center of the highest mode in the waveform | Degrees east |
| LATITUDE_HIGH | Latitude of the center of the highest mode in the waveform | Degrees north |
| ELEVATION_HIGH | Elevation of the center of the highest mode in the waveform | Meters |
Sample Data Records
Below is a list of the first five records from ASCII data file ILVIS2_GL2013_1105_R1406_044313.TXT. The twelve columns in each record correspond to the parameters described in Table 2. The column headings and values are wrapped to fit on this page.
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Software and Tools
The data files can be opened by any software that reads ASCII text files.
Also available: an IDL program that reads the LVIS Level-2 data into an IDL structure: read_ilvis2.pro.
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Data Acquisition and Processing
As described on the NASA LVIS Web site, a laser altimeter is an instrument that measures range from the instrument to a target object or surface. The device sends a laser beam towards the target, and measures the time it takes for the signal to reflect back from the surface. Knowing the precise round-trip time it takes for the reflection to return yields the range to the target.
Figure 2 shows two examples of return energy waveforms. A simple waveform occurs where the ice surface is relatively smooth within the footprint of the laser pulse (approximately 20 meters in diameter). "Mean noise level" provides the threshold relative to which the centroid and all modes are computed. A complex waveform might be returned from a rougher ice surface and could contain more than one mode, originating from different reflecting surfaces within the laser footprint such as crevasse sides and bottom, open water, large snowdrifts, and other steep or multiple slopes. A complex waveform would be more typically returned from multilevel vegetation land cover such as a forest.
Data Acquisition Methods
LVIS employs a signal digitizer, disciplined with a very precise oscillator, to measure both the transmitted and reflected laser pulse energies versus time. These digitized and captured photon histories are known as waveforms. For the outgoing pulse, it represents the profile of the individual laser shot, and for the return pulse it records the interaction of that transmitted pulse with the target surface.
Processing of these waveforms yields many products, but the primary product is range from the instrument to the Earth's surface and the distribution of reflecting surfaces within the area of the laser footprint. For vegetated terrain these surfaces are tree canopies, branches, other forms of vegetation, and open ground. For cryospheric data these surfaces are snow, ice, crevasses, snowdrifts, sea ice possibly interspersed with open ocean, exposed rock, and water.
LVIS uses a waveform-based measurement technique to collect data instead of just timing detected returns of the laser pulse. The return signal is sampled rapidly, and stored completely for each laser shot. Retaining all waveform information allows post processing of the data to extract many different products. With the entire vertical extent of surface features recorded, metrics can be extracted about the sampled area. An advantage of saving all of the waveform data is that new techniques can be applied to these data long after collection to extract even more information. See the NASA LVIS Web site.
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Derivation Techniques and Algorithms
The LVIS Level-2 Geolocated Surface Elevation Product is derived from the LVIS Level-1B Geolocated Return Laser Waveform product, described in the Processing Steps section.
Processing Steps
The following processing steps are performed by the data provider to produce the ASCII text format Level-2 data.
- Proceeding from the Level-1B waveform, a background or threshold return energy level is first determined. This threshold forms the datum to which the subsequent measurements are referenced.
- Next the centroid of the waveform above the threshold is computed. The centroid represents the mean location and mean elevation of all reflecting surfaces within the laser footprint.
- Finally, all modes in the waveform are identified, followed by selection of the highest and lowest modes for output. These correspond to the mean elevation of the highest and lowest reflecting surfaces, respectively, within the laser footprint.
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Sensor or Instrument Description
As described on the NASA LVIS Web site, the Land, Vegetation, and Ice Sensor is an airborne Lidar scanning laser altimeter used by NASA for collecting surface topography and vegetation coverage data. LVIS uses a signal digitizer with oscillator to measure transmitted and reflected laser pulse energies versus time capturing photon histories as waveforms. The laser beam and telescope field of view scan a raster pattern along the surface perpendicular to aircraft heading as the aircraft travels over a target area. LVIS has a scan angle of approximately 12 degrees, and can cover 2 km swaths from an altitude of 10 km. Typical collection size is 10 to 25 meter spots. In addition to waveform data, GPS satellite data is recorded at ground tie locations and on the airborne platform to precisely reference aircraft position. An Inertial Measurement Unit (IMU) is attached directly to the LVIS instrument and provides information required for coordinate determination.
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References and Related Publications
Contacts and Acknowledgments
Bryan Blair
Laser Remote Sensing Laboratory, Code 694
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Michelle Hofton
Department of Geography
2181 LeFrak Hall
University of Maryland
College Park, MD 20742
Acknowledgments:
This work was supported by NASA Grant Number NNX11AH69G, LVIS: A Topographic Mapping Capability for IceBridge.
Document Information
Document Creation Date
17 July 2012
Document Revision Date
18 August 2014
14 March 2016
09 June 2016
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