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Data Set ID: 
AFLVIS2

AfriSAR LVIS L2 Geolocated Surface Elevation Product, Version 1

This data set contains surface elevation data over Gabon, Africa. The measurements were taken by the NASA Land, Vegetation, and Ice Sensor (LVIS), an airborne lidar scanning laser altimeter. The data were collected as part of a NASA campaign, in collaboration with the European Space Agency (ESA) mission AfriSAR.

This is the most recent version of these data.

Version Summary: 

Initial release

STANDARD Level of Service

Data: Data integrity and usability verified

Documentation: Key metadata and user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools

See All Level of Service Details

Parameter(s):
  • TOPOGRAPHY > TERRAIN ELEVATION
Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 1, 
S: -2, 
E: 12, 
W: 8
Platform(s):AIRCRAFT, B-200, C-130, DC-8, G-V, HU-25C, P-3B, RQ-4
Spatial Resolution:
  • Varies x Varies
Sensor(s):ALTIMETERS, LASERS, LVIS
Temporal Coverage:
  • 20 February 2016 to 8 March 2016
Version(s):V1
Temporal ResolutionVariesMetadata XML:View Metadata Record
Data Contributor(s):J. Blair, Michelle Hofton

Geographic Coverage

Other Access Options

Other Access Options

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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. 2018. AfriSAR 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/A0PMUXXVUYNH. [Date Accessed].
Created: 
15 October 2018
Last modified: 
7 March 2019

Data Description

The data in this Level-2 product were collected as part of the AfriSAR mission by the NASA Land, Vegetation, and Ice Sensor (LVIS) in collaboration with the European Space Agency (ESA) and the Gabonese Space Agency. The AfriSAR mission was an airborne campaign that collected radar and field measurements of tropical forests in Gabon, West Africa. The AfriSAR data is a precursor to upcoming spaceborne missions that examine the role of forests in Earth's carbon cycle. The data are also distributed in the Level-1B format, through the AfriSAR LVIS L1B Geolocated Return Energy Waveforms data set. The Level-1B data files contain geolocated laser waveform data for each laser footprint. Other related LVIS data sets include Level-0, Level-1B, and Level-2 products collected as part of the Operation IceBridge campaigns. See the Related Data Collections section for links to these data sets.

Parameters

The data files contain canopy top and ground elevations and relative heights derived from the Level-1B data.

Parameter Description

Parameters contained in the ASCII text files are described in Table 1.

Table 1. ASCII Text File Parameters

Parameter

Description

Units

LFID LVIS file identification. The format is XXYYYYYZZZ, where XX identifies instrument version, YYYYY is the Modified Julian Date of the flight departure day, and ZZZ represents the file number. N/A
SHOTNUMBER LVIS shot number assigned during collection. Together with LFID, it provides a unique identifier to every LVIS laser shot. N/A
TIME UTC decimal seconds of the day Seconds
GLON Longitude of the lowest detected mode within the waveform Degrees East
GLAT Latitude of the lowest detected mode within the waveform Degrees North
ZG Mean elevation of the lowest detected mode within the waveform Meters
HLON Longitude of the center of the highest detected mode within the waveform Degrees East
HLAT Latitude of the center of the highest detected mode within the waveform Degrees North
ZH Mean elevation of the highest detected mode within the waveform Meters
TLON Longitude of the highest detected signal Degrees East
TLAT Latitude of the highest detected signal Degrees North
ZT Elevation of the highest detected signal Meters
RH10 Height (relative to ZG) at which 10% of the waveform energy occurs Meters
RH15 Height (relative to ZG) at which 15% of the waveform energy occurs Meters
RH20 Height (relative to ZG) at which 20% of the waveform energy occurs Meters
RH25 Height (relative to ZG) at which 25% of the waveform energy occurs Meters
RH30 Height (relative to ZG) at which 30% of the waveform energy occurs Meters
RH35 Height (relative to ZG) at which 35% of the waveform energy occurs Meters
RH40 Height (relative to ZG) at which 40% of the waveform energy occurs Meters
RH45 Height (relative to ZG) at which 45% of the waveform energy occurs Meters
RH50 Height (relative to ZG) at which 50% of the waveform energy occurs Meters
RH55 Height (relative to ZG) at which 55% of the waveform energy occurs Meters
RH60 Height (relative to ZG) at which 60% of the waveform energy occurs Meters
RH65 Height (relative to ZG) at which 65% of the waveform energy occurs Meters
RH70 Height (relative to ZG) at which 70% of the waveform energy occurs Meters
RH75 Height (relative to ZG) at which 75% of the waveform energy occurs Meters
RH80 Height (relative to ZG) at which 80% of the waveform energy occurs Meters
RH85 Height (relative to ZG) at which 85% of the waveform energy occurs Meters
RH90 Height (relative to ZG) at which 90% of the waveform energy occurs Meters
RH95 Height (relative to ZG) at which 95% of the waveform energy occurs Meters
RH96 Height (relative to ZG) at which 96% of the waveform energy occurs Meters
RH97 Height (relative to ZG) at which 97% of the waveform energy occurs Meters
RH98 Height (relative to ZG) at which 98% of the waveform energy occurs Meters
RH99 Height (relative to ZG) at which 99% of the waveform energy occurs Meters
RH100 Height (relative to ZG) at which 100% of the waveform energy occurs Meters
AZIMUTH Azimuth angle of laser beam Degrees
INCIDENTANGLE Off-nadir incident angle of laser beam Degrees
RANGE Distance along laser path from the instrument to the ground Meters
COMPLEXITY Complexity metric for the return waveform N/A
CHANNEL_L1B Flag indicating LVIS channel waveform contained in the matching Level-1B file N/A
CHANNEL_ZG Flag indicating LVIS channel used to locate ZG N/A
CHANNEL_RH Flag indicating LVIS channel used to calculate RH metrics N/A

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.

File Contents

Figure 1 shows a sample from the LVIS2_Gabon2016_0220_R1808_038024.TXT data file.

Figure 1. A sample of records from the LVIS2_Gabon2016_0220_R1808_038024.TXT data file. The column headings and values are wrapped to fit into the figure.

Naming Convention

Example file names:

LVIS2_Gabon2016_0220_R1808_038024.TXT
LVIS2_Gabon2016_0220_R1808_038024.TXT.xml

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

LVIS2_GabonYYYY_MMDD_RYYMM_nnnnnn.NN

Table 2. File Naming Convention
Variable Description
LVIS2 Short name for LVIS L2 Geolocated Surface Elevation Product
GabonYYYY Campaign identifier. Gabon = location of AfriSAR mission; YYYY= four-digit year of campaign
MMDD Two digit month, two-digit day of campaign
RYYMM Date (YY year / MM month) of the data release
nnnnnn Number of seconds since UTC midnight of the day the data collection started
NN Indicates file type: .TXT (ASCII text file) or .TXT.xml (XML metadata file)

File Size

The total ASCII text file volume is approximately 15 GB.

Spatial Information

Coverage

The data set covers rainforests in Gabon, Africa, as noted by the coverage below.

Northernmost Latitude: 1° N
Southernmost Latitude: 2° S
Westernmost Longitude: 8° E
Easternmost Longitude: 12° E

Resolution

The spatial resolution is on average 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.

Geolocation

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

Temporal Information

Coverage

20 February 2016 to 08 March 2016

Resolution

The AfriSAR campaign was conducted on nine days between 20 February and 08 March 2016. Table 3 lists all the flight dates and locations for those days. For more detailed information, visit NASA's AfriSAR ORNL DAAC web page.

Table 3. Flight Dates and Locations
Date Location
20 Feb 2016 Mabounie site
22 Feb 2016 TanDEM-X and GEDI lines
23 Feb 2016 Biomass transect 1
25 Feb 2016 Mondah site
02 Mar 2016 Lope site
03 Mar 2016 Mondah site -2
04 Mar 2016 Pongara site
07 Mar 2016 RABI site
08 Mar 2016 Fill in: Biomass, Mondah, Pongara sites

Data Acquisition and Processing

Background

As described on the NASA LVIS website, 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 reflection to return yields the range to the target.

Figure 2 shows two example return waveforms. A simple waveform (left) occurs when the surface is relatively smooth within the laser footprint, which generates 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. Multilayered surfaces, such as forests or vegetated landcover, produce complex waveforms (right) containing more than one mode. Different modes represent the various surfaces within the footprint, such as the canopy top or the ground, and are distributed according to their relative elevations within the footprint.

Figure 2. Sample Level-1B product waveforms illustrating some possible distributions of reflected light.

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, 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.

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 website for more information.

Processing

This data set is derived from the AfriSAR LVIS L1B Geolocated Return Laser Waveforms product. 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.

Quality, Errors, and Limitations

Currently, there are no known errors or limitations in this data set.

Instrumentation

As described on the NASA LVIS website, 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 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 m to 25 m 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 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 supported by the LVIS team that reads the LVIS Level-2 data into an IDL structure.

Related Data Sets

Related Websites

Contacts and Acknowledgments

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 through funding from Hank Margolis (NASA - SMD - ESD Terrestrial Ecology).

References

Blair, J. B., D. L. Rabine., and M. A. Hofton. 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., J. B. Blair, J. B. Minster., J. R. Ridgway, N. P. Williams, J. L Bufton, and D. L. Rabine. 2000. An Airborne Scanning Laser Altimetry Survey of Long Valley, California, International Journal of Remote Sensing, 21(12): 2413-2437.

Hofton, M. A., J. B. Blair, S. B. Luthcke, and D. L. Rabine. 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.

No technical references available for this data set.

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