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This data set contains surface elevation measurements from NASA's Land, Vegetation, and Ice Sensor (LVIS) over areas including Greenland, Alaska, and Antarctica. The data were collected as part of NASA Operation IceBridge funded campaigns.
Data Set ID:
ILVIS2
IceBridge LVIS L2 Geolocated Surface Elevation Product, Version 1
There is a more recent version of these data.
Version Summary:
Initial release
Overview
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Citing These Data
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 2018. 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: https://doi.org/10.5067/OIKFGJNBM6OO. [Date Accessed].Documentation
Detailed Data Description
Note: If you downloaded the 2010 Greenland data before 13 April 2012, please download the new Version 1.1 2010 Greenland data files at your convenience. All of the 2010 Greenland data for this data set were replaced on 13 April 2012. Some incorrect values occurred in the previous version of these files, and were corrected for the Version 1.1 data.
In 2014, only the ARISE Alaska campaign was flown (no Greenland or Antarctica campaigns). These data from 02 September 2014 to 02 October 2014 are stored in the folders /2014.09.02/ through /2014.10.02/.
Format
The data files are in ASCII text format. Each data file is paired with an associated XML file, which contain additional metadata.
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File Naming Convention
Example file names:
ILVIS2_GL2013_1114_R1406_043900.TXTILVIS2_GL2013_1114_R1406_043900.TXT.xml
The data files are named according to the following convention and as described in Table 1:
ILVIS2_LOYYYY_MMDD_RYYMM_nnnnnn.xxx
| 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 data release |
nnnnnn |
Number of seconds since UTC midnight of the day on which the data collection started |
.xxx |
Indicates file type:
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(*)NOTE: The 2014 ARISE Alaska campaign file names contain the same campaign identifier as the Greenland file names (GL).
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File Size
The total data file volume is approximately 130 GB.
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Spatial Coverage
Spatial coverage for this data set includes the Arctic, Greenland, Alaska, 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
Alaska:
Southernmost Latitude: 72° N
Northernmost Latitude: 75° N
Westernmost Longitude: 160° W
Easternmost Longitude: 140° W
Spatial Resolution
Spatial resolution is nominally 20 m, 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
14 April 2009 to 31 October 2015
Temporal Resolution
IceBridge campaigns are conducted on an annually repeating basis. Arctic, Greenland, and Alaska campaigns are typically conducted in March, April, and May; Antarctic campaigns are typicallly conducted in October and November.
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Parameter or Variable
This data set includes mean elevation and other height measurements.
Parameter Description
The ASCII text files contain the fields described in Table 2.
| Parameter | Description | Units |
|---|---|---|
| LVIS_LFID | LVIS file identification, including date and time of collection and file number. Values three through seven in the first field represent the Modified Julian Date of the data collection. | n/a |
| SHOTNUMBER | Laser shot assigned during collection | n/a |
| TIME | UTC decimal seconds of the day | Seconds |
| LONGITUDE_CENTROID | Centroid longitude of the corresponding LVIS Level-1B waveform | Degrees east |
| LATITUDE_CENTROID | Centroid latitude of the corresponding LVIS Level-1B waveform | Degrees north |
| ELEVATION_CENTROID | Centroid elevation of the corresponding LVIS Level-1B waveform | Meters |
| LONGITUDE_LOW | Longitude of the center of the lowest mode in the waveform | Degrees east |
| LATITUDE_LOW | Latitude of the center of the lowest mode in the waveform | Degrees north |
| ELEVATION_LOW | Elevation of the center of the lowest mode in 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
Figure 1 shows the first five records from the ASCII data file ILVIS2_GL2013_1105_R1406_044313.TXT. The twelve columns in each record correspond to the parameters described in Table 2.
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Data Access and Tools
The data files can be opened by any software that reads ASCII text files.
Also available: read_ilvis2.pro, an IDL program that reads the LVIS Level-2 data into an IDL structure.
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Data Acquisition and Processing
A laser altimeter is an instrument that measures the range from the instrument to a target object or surface. The device sends a laser beam toward the target, and measures the time it takes for the signal to reflect back from the surface. Knowing the precise round-trip time for the signal 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 m in diameter). The 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; however, the primary products are the range from the instrument to the Earth's surface and the distribution of reflecting surfaces within the laser footprint area. For vegetated terrain, these surfaces include tree canopies, branches, other forms of vegetation, and open ground. For cryospheric data, these surfaces are snow, ice, crevasses, snowdrifts, and 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. For more information, see the NASA LVIS website.
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Derivation Techniques and Algorithms
Processing Steps
This data set is derived from the LVIS Level-1B Geolocated Return Laser Waveform product. The following processing steps are performed by the data provider to produce the Level-2 data in ASCII text format.
- 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.
- 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.
- 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
LVIS is an airborne lidar scanning laser altimeter used by NASA to collect surface topography and vegetation coverage data. LVIS uses a signal digitizer with oscillator to measure transmitted and reflected laser pulse energies versus time and capture photon histories as waveforms. The laser beam and telescope field of view scan a raster pattern along the surface perpendicular to the aircraft heading as the aircraft travels over a target area. LVIS has a scan angle of approximately 12°, and can cover 2 km swaths from an altitude of 10 km. A typical collection size is 10 m to 25 m spots. In addition to waveform data, GPS satellite data are recorded at ground tie locations and on the airborne platform to precisely reference the aircraft position. An 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
Contributor(s) / Investigator(s):
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
July 2012
Document Revision Date
March 2019
