Data Description

This Level-2 data set contains measurements taken by NASA's Land, Vegetation, and Ice Sensor (LVIS) in support of the NASA Arctic-Boreal Vulnerability Experiment (ABoVE) and the NASA Global Ecosystem Dynamics Investigation (GEDI). ABoVE is a NASA Terrestrial Ecology Program conducted in Alaska and Western Canada. The ABoVE data are used to study environmental change and its implications for social-ecological systems. GEDI was launched to the International Space Station (ISS) in December 2018 to measure forest canopy height, structure, and surface elevation to improve characterization of important carbon and water cycle processes, biodiversity, and habitats. These flights provide data for ABoVE science investigations, as well as calibration and validation of GEDI mission in the United States, Canada, and Central America.

Two LVIS instruments were co-mounted and operated during flights, with data products referred to as LVISC (from the LVIS-Classic instrument) and LVISF (from the LVIS-Facility instrument). This data set contains measurements taken by the LVIS-Classic instrument, whereas the corresponding Level-2 LVISF data set, LVIS Facility L2 Geolocated Surface Elevation and Canopy Height Product, contains data from the co-mounted LVIS-Facility instrument. These two LVIS instruments differ in the laser footprint size and spacing on the ground. The Level-1B versions of these data sets, LVIS Classic L1B Geolocated Return Energy Waveforms and LVIS Facility L1B Geolocated Return Energy Waveforms, contain geolocated return energy waveforms used to create the Level-2 data sets.

This data set is also closely related to the ABoVE LVIS L2 Geolocated Surface Elevation Product data set, which provides ABoVE data, and the AfriSAR LVIS L2 Geolocated Surface Elevation Product data set, which provides GEDI calibration and validation data.

Parameters

All of the parameters contained in the data files are described in Table 1.

Note:
^(*) This parameter was newly added. The rest of the parameters are the same as in the AfriSAR LVIS L2 Geolocated Surface Elevation Product data set.

File Information

Format

The data files are in ASCII text format (.TXT). Each data file is paired with an associated XML file (.xml), which contains additional metadata.

Naming Convention

Example file names:

LVISC2_ABoVE2019_0715_R2002_086485.TXT
LVISC2_ABoVE2019_0715_R2002_086485.TXT.xml

LVISC2_GEDI2019_0521_R2002_073401.TXT
LVISC2_GEDI2019_0521_R2002_073401.TXT.xml

Files are named according to the following convention, which is described in Table 2:

LVISC2_CAMPYYYY_MMDD_RYYMM_nnnnnn.ext

Spatial Information

Coverage

Spatial coverage for this data set currently includes parts of Alaska, Western Canada, the continental United States, and central America, as noted by the spatial extents below:

Southernmost latitude: 9° N
Northernmost latitude: 72° N
Westernmost longitude: 168° W
Easternmost longitude: 81° W

Resolution

The nominal spatial resolution of the LVISC data sets is 20 m, but varies slightly with aircraft altitude and speed. Laser spot size is a function of beam divergence and altitude. Nominal spot spacing is a function of scan rate, pulse repetition rate, and airplane ground speed. The instrument resolution and footprint size are comparable to those in the AfriSAR LVIS L2 Geolocated Surface Elevation Product data set, but are lower than in the ABoVE LVIS L2 Geolocated Surface Elevation Product and the LVIS Facility L2 Geolocated Surface Elevation and Canopy Height Product data sets.

Geolocation

International Terrestrial Reference Frame 2008 (ITRF08), WGS-84 ellipsoid

Temporal Information

Coverage

21 May 2019 to 08 August 2019

Resolution

Varies

Instrumentation

NASA's LVIS is an imaging lidar sensor suite for precise and accurate large-area surface mapping and characterization. LVIS uses airborne lidar scanning laser altimeters to collect topography and vegetation coverage data over land, ocean, and ice surfaces, along with downward-looking, high-resolution camera imagery. The LVIS instruments differ in laser footprint size and spacing on the ground but generate near-identical data products.

Laser altimeters send a laser beam toward a target object and measure the time it takes for the signal to reflect back from the surface. Knowing the precise round-trip time for the reflection to return allows the distance, or range, to the target to be calculated. Range is combined with the pointing and positioning of the laser at the time of each laser shot to determine the location of each laser footprint on the ground relative to a reference ellipsoid (e.g.: Hofton et al., 2000). LVIS employs a signal digitizer with a very precise oscillator to measure both the transmitted and reflected laser pulse energies versus time. These digitized and captured histories are known as waveforms (i.e., the transmitted and return waveforms). The outgoing signal represents the profile of the individual laser pulse versus time; the return pulse comprises the interaction of that transmitted pulse with the target surface versus time.

As the aircraft travels over a target area, the laser beam and the telescope field-of-view scan a pattern along the surface perpendicular to the aircraft heading. LVIS instruments have a scan angle of approximately 12° (+-6° around nadir), allowing them to cover 2 km swaths from an altitude of 10 km. The typical diameter of the laser footprint on the ground is 10 m to 25 m, depending on the aircraft altitude, as well as laser repetition rate and divergence. Laser positioning at the time of each laser shot is provided by GPS satellite data. Laser pointing information is provided by an Inertial Measurement Unit (IMU) attached directly to the LVIS instrument.

Data Acquisition and Processing

Background

Figure 1 shows two examples of return waveforms: a simple waveform (left) and a complex waveform (right). The simple waveform occurs when the surface is relatively smooth within the laser footprint, thus generating a laser return waveform that consists of a single mode. The detection threshold is computed relative to the mean noise level and is used to detect the return signals that are geolocated for Level-2 data products. Complex waveforms containing more than one mode are produced when the laser beam hits multilayered surfaces, such as forests, vegetated land cover, ice crevasses, or rocky terrain. Different modes represent the various surfaces within the footprint, such as the canopy top, the ground, the crevasse bottom, or the top of broken ice surface, and are distributed according to their relative elevations within the footprint.

Acquisition

The primary Level-1B data product is the geolocated laser return waveforms, representing the vertical distribution of reflecting surfaces within the area of the laser footprint over the sampled terrain. For vegetated terrain these surfaces include tree canopies, branches, other forms of vegetation, and open ground. For cryospheric areas these surfaces comprise 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 many different products to be extracted during data post-processing, such as the data presented in the Level-2 data 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 additional information. See the LVIS website at NASA Goddard Space Flight Center for more information.

Processing Steps

This data set is derived from the Level-1B data set, LVIS Classic L1B Geolocated Return Energy Waveforms. The following processing steps are performed by the data provider to produce the Level-2 data:

1. A background, or threshold, return energy level is determined from the Level-1B waveform data. This threshold forms the datum to which the subsequent measurements are referenced.
2. The centroid of the waveform above the threshold is computed. The centroid represents the mean location and elevation of all reflecting surfaces within the laser footprint.
3. All modes in the waveform are identified, followed by a selection of the highest and lowest modes for output. These modes correspond to the mean elevation of the highest and lowest reflecting surfaces, respectively, within the laser footprint.

For more details see Hofton et al. (2000).

Quality, Errors, and Limitations

Obvious lower quality data, such as data collected in areas with clouds and cloud-obscured returns, were removed; however, spurious returns may still be present. Data collected in aircraft turns have been removed from this data set. It is recommended that users review the waveforms for their specific areas of study to verify ground return and canopy top identification. It is possible that some anomalies are still present in the data.

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 supported by the LVIS team that reads the LVIS Level-2 data into an IDL structure.

Related Data Sets

LVIS Classic L1B Geolocated Return Energy Waveforms
LVIS Facility L1B Geolocated Return Energy Waveforms
LVIS Facility L2 Geolocated Surface Elevation and Canopy Height Product
ABoVE LVIS L1B Geolocated Return Energy Waveforms
ABoVE LVIS L2 Geolocated Surface Elevation Product
AfriSAR LVIS L1B Geolocated Return Energy Waveforms
AfriSAR LVIS L2 Geolocated Surface Elevation Product

Related Websites

LVIS website at NSIDC
LVIS website at NASA Goddard Space Flight Center
ABoVE website at NASA
GEDI website

Contacts

Bryan Blair
Laser Remote Sensing Laboratory, Code 61A
NASA Goddard Space Flight Center
Greenbelt, MD 20771

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

Acknowledgments

This work was supported through funding from NASA. GEDI flights were supported by the GEDI mission. ABoVE flights were supported by Hank Margolis (NASA - SMD - ESD Terrestrial Ecology).

References

Hofton, M. A., Blair, J. B., Minster, J.-B., Ridgway, J. R., Williams, N. P., Bufton, J. L., & Rabine, D. L. (2000). An airborne scanning laser altimetry survey of Long Valley, California. International Journal of Remote Sensing, 21(12), 2413–2437. https://doi.org/10.1080/01431160050030547