Data Description

The data in this Level-2 product were collected as part of Operation IceBridge campaigns by the NASA Land, Vegetation, and Ice Sensor (LVIS). The corresponding Level-1B data set, IceBridge LVIS L1B Geolocated Return Energy Waveforms, contains the spatially geolocated laser return waveforms. The LVIS Level-2 data products are derived from these waveforms and provide measurements for a variety of scientific applications. See the Related Data Collections section for links to other LVIS data sets.

Parameters

This data set includes mean elevation and other height measurements. All the parameters contained in the data files are described in Table 1.

Note: The LVIS facility is working toward establishing a standard set of data parameters that are applicable to a range of fields. Thus, not all parameters contained in the Level-2 files are currently relevant to every field of research. Additionally, some parameter names were changed; see the Version History section for more information.

File Information

Format

The data files are in ASCII text format. Each data file is paired with an associated XML file, which contain additional metadata.

File Contents

Figure 1 shows the first five records from the ASCII data file ILVIS2_GL2017_0825_R1805_052154.TXT.

Naming Convention

Example file names:

ILVIS2_GL2017_0825_R1805_052154.TXT
ILVIS2_GL2017_0825_R1805_052154.TXT.xml

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

ILVIS2_LOYYYY_MMDD_RYYMM_nnnnnn.xxx

File Size

The total data file volume is approximately 241 GB.

Spatial Information

Coverage

Spatial coverage for this data set includes areas of the Arctic and Greenland, as noted below:

Arctic / Greenland:
Southernmost Latitude: 60° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Resolution

Spatial resolution for the LVIS-Facility instrument is nominally 8 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.

Geolocation

The following table provides the geolocation details for this data set.

Temporal Information

Coverage

25 August 2017 to 20 September 2017

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 typically conducted in October and November.

Data Acquisition and Processing

Background

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. The mean noise level provides the threshold relative to which all signal processing is referenced. 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 is also typically returned from multilevel vegetation land cover such as a forest.

Acquisition

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 elevations of 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.

Processing

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.

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. 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.
3. Several other elevation and relative height products are derived and output, including the elevation of the highest detected surface and the heights of energy levels above the lowest mode. These provide additional information on the 3D surface structure within the laser footprint.

Quality, Errors, and Limitations

The data have received limited quality assurance checking. Obvious low-quality data were removed, such as clouds and cloud-obscured returns. Currently, there are no known errors or limitations in this data set.

Instrumentation

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.

Software 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.

A tutorial on accessing and understanding LVIS data can be found at: https://lvis.gsfc.nasa.gov/Home/GabonWorkshop.html

Version History

The data for 2009 through 2015 are stored in Version 1 of this data set.

Version 2 of this data set contains more parameters than Version 1. In addition, some parameters were renamed or removed. Essentially, Version 2 comprises the following changes:

• LVIS_LFID is now called LFID.
• LONGITUDE_LOW, LATITUDE_LOW, and ELEVATION_LOW were renamed to GLON, GLAT, and ZG.
• LONGITUDE_HIGH, LATITUDE_HIGH, and ELEVATION_HIGH were renamed to HLON, HLAT, and ZH.
• LONGITUDE_CENTROID, LATITUDE_CENTROID, and ELEVATION_CENTROID were removed.
• TLON, TLAT, and ZT were added.
• RH10 through RH100 were added.
• AZIMUTH, INCIDENTANGLE, RANGE, and COMPLEXITY were added.
• CHANNEL_ZT, CHANNEL_ZG, and CHANNEL_RH were added.

Related Data Sets

• Antarctic 5-km Digital Elevation Model from ERS-1 Altimetry
• GLAS/ICESat 500 m Laser Altimetry Digital Elevation Model of Antarctica
• GLAS/ICESat L1B Global Elevation Data
• IceBridge ATM L1B Qfit Elevation and Return Strength
• IceBridge ATM L2 Icessn Elevation, Slope, and Roughness
• IceBridge LVIS L1B Geolocated Return Energy Waveforms
• Pre-IceBridge ATM L2 Icessn Elevation, Slope, and Roughness

Related Websites

• LVIS website at NASA Goddard Space Flight Center
• IceBridge data website at NSIDC
• IceBridge website at NASA
• ICESat/GLAS website at NASA Wallops Flight Facility
• ICESat/GLAS website at NSIDC
• ICESat2 website at NSIDC
• ICESat2 website at NASA

Contacts and Acknowledgments

Bryan Blair
Geodesy and Geophysics Laboratory, Code 61A
NASA Goddard Space Flight Center
Greenbelt, MD 20771

Michelle Hofton
Department of Geographical Sciences
2181 LeFrak Hall
University of Maryland
College Park, MD 20742

Acknowledgments

The NASA LVIS Facility is funded by NASA Headquarters.

References

Blair, J. B., Rabine, D. L., & 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(2–3), 115–122. https://doi.org/10.1016/s0924-2716(99)00002-7

Hofton, M. A., Blair, J. B., Luthcke, S. B., & 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(24). https://doi.org/10.1029/2008gl035774