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Data Set ID:
ABLVIS2

ABoVE LVIS L2 Geolocated Surface Elevation Product, Version 1

This data set contains surface elevation measurements over Alaska and Western Canada taken from the NASA Land, Vegetation, and Ice Sensor (LVIS). The data were collected as part of NASA's Terrestrial Ecology Program campaign, the Arctic-Boreal Vulnerability Experiment (ABoVE).

This is the most recent version of these data.

Version Summary:

New data set.

Parameter(s):
  • TOPOGRAPHY > TERRAIN ELEVATION
Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 72, 
S: 48, 
E: -104, 
W: -158
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:
  • 29 June 2017 to 17 July 2017
Version(s):V1
Temporal ResolutionVariesMetadata XML:View Metadata Record
Data Contributor(s):J. Blair, Michelle Hofton

Geographic Coverage

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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. ABoVE 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/IA5WAX7K3YGY. [Date Accessed].

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Detailed Data Description

The data in this Level-2 product were collected by the NASA Land, Vegetation, and Ice Sensor (LVIS) as part of the Arctic-Boreal Vulnerability Experiment (ABoVE). ABoVE is a NASA Terrestrial Ecology Program field campaign conducted in Alaska and Western Canada. The ABoVE data are used to study environmental change and its implications for social-ecological systems. The data files of this Level-2 product contain canopy top and ground elevations, as well as relative heights derived from the Level-1B data, the ABoVE LVIS L1B Geolocated Return Energy Waveforms. 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.

Format

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

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File Naming Convention

The data files are named according to the following conventions and as described in Table 1.

Example File Names

LVIS2_ABoVE2017_0629_R1803_056233.TXT
LVIS2_ABoVE2017_0629_R1803_056233.TXT.xml

File Naming Convention

LVIS2_ABoVEYYYY_MMDD_RYYMM_nnnnnn.NN

Table 1. File Naming Convention
Variable Description
LVIS2 Short name for LVIS L2 Geolocated Surface Elevation Product
ABoVEYYYY Campaign identifier. ABoVE = acronym for Arctic-Boreal Vulnerability Experiment; 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)
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File Size

The total ASCII text file volume is approximately 104 GB.

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Spatial Coverage

Spatial coverage for the ABoVE LVIS campaigns includes Alaska and Western Canada, as noted by the coverage below.

Alaska / Canada: 
Southernmost Latitude: 48° N
Northernmost Latitude: 72° N
Westernmost Longitude: 158° W
Easternmost Longitude: 104° W

Spatial 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.

Projection and Grid Description

International Terrestrial Reference Frame (ITRF 2008), WGS-84 Ellipsoid.

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Temporal Coverage

29 June 2017 to 17 July 2017

Temporal Resolution

The ABoVE Alaska and Canada campaigns were conducted on 13 days between 29 June and 17 July 2017. Table 2 lists all the flight dates and locations for those days. For more detailed information, visit the NASA ABoVE campaign website.

Table 2. Flight Dates and Locations
Date Location
29 Jun 2017 Saskatoon to Yellowknife
29 Jun 2017 Slave Lake
30 Jun 2017 Yellowknife to Inuvik
30 Jun 2017 Inuvik to Yellowknife
01 Jul 2017 Daring Lake
02 Jul 2017 W and SW Slave Lake
03 Jul 2017 Yellowknife to Whitehorse
03 Jul 2017 Whitehorse to Fairbanks
06 Jul 2017 Kluane
07 Jul 2017 Healy
09 Jul 2017 Fairbanks to Barrow
14 Jul 2017 Fairbanks to Deadhorse via Toolik Lake
14 Jul 2017 Deadhorse to Fairbanks via Fort Yukon
15 Jul 2017 Fort Yukon
16 Jul 2017 Fairbanks to Ketchikan
16 Jul 2017 Ketchikan to Glasgow
17 Jul 2017 Boreal Ecosystem Research and Monitoring Sites (BERMS) Flight
18 Jul 2017 Glasgow to Toledo
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Parameter or Variable

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 3.

Table 3. 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
TLON Longitude of the highest detected signal Degrees East
TLAT Latitude of the highest detected signal Degrees North
TZ 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_ZT Flag indicating LVIS channel waveform contained in the 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

Sample Data Record

Figure 1 shows a sample from the LVIS2_ABoVE2017_0629_R1803_056233.TXT data file.

Figure 1. Sample records from the LVIS2_ABoVE2017_0629_R1803_056233.TXT data file.
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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.

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Data Acquisition and Processing

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, vegetated landcover, ice crevasses, or rocky terrain, produce complex waveforms (right) containing more than one mode. Different modes represent the various surfaces within the footprint, such as the canopy top, the ground, the crevasse bottom, or the top of the broken ice surface, 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.
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, 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. See the NASA LVIS website for more information.

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Derivation Techniques and Algorithms

This data set is derived from the LVIS Level-1B Geolocated Return Laser Waveform product, as described below.

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.
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Sensor or Instrument Description

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.

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References and Related Publications

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).

Document Information

Document Creation Date

May 2018

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