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IceBridge LVIS L2 Geolocated Surface Elevation Product

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.

Operation IceBridge products may include test flight data that are not useful for research and scientific analysis. Test flights usually occur at the beginning of campaigns. Users should read flight reports for the flights that collected any of the data they intend to use. Check IceBridge campaign Flight Reports for dates and information about test flights.

Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Blair, Bryan, and Michelle Hofton. 2010, updated 2012. IceBridge LVIS L2 Geolocated Surface Elevation Product, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/ilvis2.html.

Overview

Platform

NASA DC-8
NASA P-3B
NASA HU-25C Falcon Jet
NSF/NCAR G-V
NASA LARC King Air B-200

Sensor

Land, Vegetation, and Ice Sensor (LVIS)

Spatial Coverage

Greenland, Antarctica

Spatial Resolution

Nominally 20 meters

Temporal Coverage

Periodic, ongoing, from 14 April 2009 to the present.

Temporal Resolution

Seasonal

Parameters

Mean surface elevation

Data Format

ASCII text files
Big-endian binary files

Metadata Access

View Metadata Record

Data Access

FTP

1. Contacts and Acknowledgments

Investigator(s) Name and Title

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

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

This work was supported by NASA Grant Number NNX11AH69G, LVIS: A Topographic Mapping Capability for IceBridge.

2. Detailed Data Description

Format

The most recent LVIS Level-2 Geolocated Surface Elevation Product data files are in ASCII text (.TXT) format. Some older data files are in big-endian binary (.lge) format. Over time the older binary format files are being converted to the text format.

File and Directory Structure

Data files are organized on the FTP site, ftp://n4ftl01u.ecs.nasa.gov/SAN2/ICEBRIDGE_FTP/, as described in Figure 1.

directory structure

Figure 1. Directory Structure

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.

File Naming Convention

Binary files are named according to the following convention and as described in Table 2:

File name example:

LVIS_GL_20090416_VECT_20100120.lge

LVIS_LOC_YYYYMMDD_VECT_yyyymmdd.lge

Where:

Table 2. Binary File Naming Convention
Variable Description
LVIS Instrument: Land, Vegetation, and Ice Sensor
LOC Mission location. ANT = Antarctica. GL = Greenland
YYYY four-digit year of data collection
MM two-digit month of data collection
DD two-digit day of data collection
VECT vector x, y, z values
yyyy four-digit data release year
mm two-digit data release month
dd two-digit data release day
.lge indicates LVIS ground elevation file

ASCII text files are named according to the following convention and as described in Table 3:

Example File Name:

LVIS_GL2011_MJD55666_LEVEL2_20110915_C.TXT

LVIS_LOYYYY_MJDnnnnn_LEVEL2_YYYYMMDD_A.TXT

Where:

Table 3. ASCII File Naming Convention
Variable Description
LVIS Instrument: Land, Vegetation, and Ice Sensor
LOYYYY Campaign identifier. For LO, GL = Greenland and ANT = Antarctica. YYYY = year of campaign.
MJDnnnnn Modified Julian Date of data collection = nnnnn
LEVEL2 Level 2 data processing
YYYY four-digit data release year
MM two-digit data release month
DD two-digit data release day
A Sub-file designation. Alphabetical ordering of split file from original data file. See Note below.
.TXT indicates ASCII text file

NOTE on Sub-file designation:
The original 2010 and 2011 Greenland ASCII data files exceeded the NSIDC FTP server size limit. These data files have been split into smaller ASCII sub-files. For example, the three files:
LVIS_GL2011_MJD55667_LEVEL2_20110915_A.TXT
LVIS_GL2011_MJD55667_LEVEL2_20110915_B.TXT
LVIS_GL2011_MJD55667_LEVEL2_20110915_C.TXT
are parts 'A', 'B', and 'C' of the original LVIS_GL2011_MJD55667_LEVEL2_20110915 data file.

File Size

Binary files range from approximately 189 MB to 888 MB.

Text files range from approximately 14 MB to 888 MB.

Volume

Data volume for the full data set is approximately 75 GB.

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.

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.

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 4.

Table 4. ASCII Text File Parameter Description
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

The IceBridge LVIS Level-2 Geolocated Surface Elevation Product big-endian binary data files contain fields as described in Table 5, and illustrated in Figure 1.

Table 5. Binary File Parameter Description
Parameter Bytes Type Description Units
LVIS_LFID 4 Unsigned long integer LVIS file identification, including date and time of collection and file number. The third through seventh values in the first field represent the Modified Julian Date of data collection. n/a
SHOTNUMBER 4 Unsigned long integer Laser shot assigned during collection n/a
TIME 8 Double UTC decimal seconds of the day Seconds
GLON 8 Double Refers to the longitude of the lowest detected mode within the corresponding LVIS Level-1B waveform. Degrees East
GLAT 8 Double Refers to the latitude of the lowest detected mode within the corresponding LVIS Level-1B waveform. Degrees north
Zg 4 Float Refers to the elevation of the lowest detected mode within the corresponding LVIS Level-1B waveform. Meters
rh25 4 Float Height (relative to Zg) at which 25% of the waveform energy occurs Meters
rh50 4 Float Height (relative to Zg) at which 50% of the waveform energy occurs Meters
rh75 4 Float Height (relative to Zg) at which 75% of the waveform energy occurs Meters
rh100 4 Float Height (relative to Zg) at which 100% of the waveform energy occurs Meters

Figure 1 illustrates the elevation parameters in the Level-2 binary data files. Zg is the elevation of the lowest detected return pulse in the Level-1B waveform. Data parameters rh25, rh50, rh75, and rh100 are measured relative to Zg. In this illustration, the rh25 energy height is slightly negative, for example below the Zg elevation, while the rh50, rh75, and rh100 are positive.

Figure 1. Illustration Describing the Elevation Parameters in the Level-2 Binary Data Files

Sample Data Records

Below is a list of the first five records from ASCII data file LVIS_GL2010_MJD55300_LEVEL2_20110922_C.TXT. The twelve columns in each record correspond to the parameters described in Table 3. The column headings and values are wrapped to fit on this page.

Below is a list of the first five records from binary data file LVIS_GL_20090416_VECT_20100120.lge after it was processed using the LVIS C reader software. The ten columns correspond to the parameters described in Table 4. Note that for Operation Ice Bridge released data, most or all of the rh values are intentionally assigned to zero because the relevance of these products to ice sheet studies is not yet established.

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

NSIDC provides an LVIS C reader that reads a binary data file from the Operation IceBridge LVIS instrument and prints the records to standard output.

4. 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.

Figure 2. Sample Level-1B Product Waveforms, from which the Level-2 Products are Derived

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.

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.

  1. 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.
  2. 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.
  3. 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.

The following processing steps are performed by the data provider to produce the binary format Level-2 data.

  1. Proceeding from the Level-1B waveform, a background or threshold return energy level is first determined. This forms the datum to which the next calculation is referenced.
  2. A vertical profile of cumulative returned energy is computed. The elevations of the 25, 50, 75 and 100 percent levels are found.
  3. Next the location and elevation of the lowest mode in the Level-1B waveform is determined. The elevation is shown as Zg in Figure 1. The lowest mode represents the lowest reflecting surface within the laser footprint.
  4. The difference between Zg and the 25, 50, 75 and 100 percent elevations are reported as rh25, rh50, rh75, and rh100, respectively.

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.

5. References and Related Publications

Blair, J. B., Rabine, D. L., and Hofton, M. A. 1999. The Laser Vegetation Imaging Sensor: a medium-altitude, digitisation-only, airborne laser altimeter for mapping vegetation and topography, ISPRS Journal of Photogrammetry and Remote Sensing, 54, 115-122.

Hofton, M. A., Blair, J. B., Luthcke, S. B., and Rabine, D. L. 2008. Assessing the performance of 20-25 m footprint waveform lidar data collected in ICESat data corridors in Greenland, Geophysical Research Letters, 35: L24501, doi:10.1029/2008GL035774.

Related Data Collections

Related Web Sites

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 6.

Table 6. Acronyms and Abbreviations
Acronym Description
ASCII American Standard Code for Information Interchange
CIRES Cooperative Institute for Research in Environmental Science
ECS NASA Earth Observing System Data and Information System (EOSDIS) Core System
FTP File Transfer Protocol
GPS Global Positioning System
IMU Inertial Measurement Unit
ITRF International Terrestrial Reference Frame
Level-1B Level 1B processing
Level-2 Level 2 processing
LFID LVIS File ID
Lidar LIght Detection And Ranging
LVIS Land, Vegetation, and Ice Sensor
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
URL Uniform Resource Locator
WGS 84 World Geodetic System 1984

Document Creation Date

17 July 2012

Document Revision Date

Document URL

http://nsidc.org/data/docs/daac/icebridge/ilvis2/index.html