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.

Contacts and Acknowledgments
Detailed Data Description
Data Access and Tools
Data Acquisition and Processing
References and Related Publications
Document Information

Citing These Data

The following example shows how to cite the use of this data set in a publication.

Blair, Bryan, and Michelle Hofton. 2010, updated current year. IceBridge LVIS L2 Geolocated Surface Elevation Product, [list dates of data used]. Boulder, Colorado USA: NASA Distributed Active Archive Center at the National Snow and Ice Data Center. Digital media. 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.

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.

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.

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

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.
A vertical profile of cumulative returned energy is computed. The elevations of the 25, 50, 75 and 100 percent levels are found.
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.
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

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