LP DAAC FAQ: GEDI

[Earthdata - wxedward]

Why do GEDI elevation values over my region of interest not match elevation values from another digital elevation model (DEM)?

A known ranging issue affects the absolute elevations of two of the eight beams (Beam0000 and Beam0001). Users are encouraged to read Section 3.7 of the ATBD for GEDI Waveform Geolocation for L1 and L2 products to fully understand the application of geophysical corrections. See Sections 6 and 8 of the Level 1 User Guide for a full description of quality and known issues. Section 6 of the Level 2 User Guide also provides important quality filtering information to ensure the elevation values are not affected by erroneous and/or lower quality returns, and Section 8 provides known issues.

How are GEDI beams split and dithered and what do these two terms mean?

Two of the GEDI lasers are full power, and the third is split into two beams, producing a total of four beams. Beam Dithering Units (BDUs) rapidly change the deflection of the outgoing laser beams. This produces eight ground tracks: four power and four cover tracks. Footprints are separated by 60 m along track and 600 m across track.

How should I quality filter GEDI L1B - L2B data?

Although all L2A and L2B algorithm results are available for every shot with a valid waveform, the science team recommends the following guidelines for selecting “best” data:

GEDI coverage beams (beams 0000, 0001, 0010, and 0011) were designed to penetrate canopies of up to 95% canopy cover under “average” conditions. For this reason, it is recommended to preference the GEDI full power beams in cases of dense vegetation. Second, nighttime GEDI data (solar_elevation < 0) is recommended over daytime acquisitions due to the negative impact of background solar illumination. It is then recommended to use the quality_flag dataset to remove erroneous and/or lower quality returns. For example, a quality_flag value of 1 will indicate that the shot meets criteria based on energy, sensitivity, amplitude, and real-time surface tracking quality.

How do I subset GEDI data spatially and by band/layer?

The LP DAAC has developed a data prep script called the GEDI Spatial and Band/layer Subsetting and Export to GeoJSON (GEDI Subsetter) script. The GEDI Subsetter is a command line executable python script that allows users to spatially subset GEDI files by submitting a GeoJSON or bounding box (Upper Left, Lower Right) region of interest. Users can also perform layer subsetting by defining specific GEDI datasets to be included in their subset output. The script will extract the desired layers, clip to the region of interest, and export as a GeoJSON file of points (shots) that can easily be loaded into GIS and/or Remote Sensing software for further visualization and analysis.

To access the GEDI Subsetter and for additional information on how to use it, visit: https://git.earthdata.nasa.gov/projects/LPDUR/repos/gedi-subsetter/browse.

How do I cite the data?

The following citation information is applicable for the GEDI01_B.002, GEDI02_A.002, and GEDI02_B.002 products:

Dubayah, R., Luthcke, S., J. B. Blair, Hofton, M., Armston, J., Tang, H. (2021). GEDI L1B Geolocated Waveform Data Global Footprint Level V002 [Data set]. NASA EOSDIS Land Processes DAAC. Accessed YYYY-MM-DD from https://doi.org/10.5067/GEDI/GEDI01_B.002.

Dubayah, R., Hofton, M., J. B. Blair, Armston, J., Tang, H., Luthcke, S. (2021). GEDI L2A Elevation and Height Metrics Data Global Footprint Level V002 [Data set]. NASA EOSDIS Land Processes DAAC. Accessed YYYY-MM-DD from https://doi.org/10.5067/GEDI/GEDI02_A.002.

Dubayah, R., Tang, H., Armston, J., Luthcke, S., Hofton, M., J. B. Blair (2021). GEDI L2B Canopy Cover and Vertical Profile Metrics Data Global Footprint Level V002 [Data set]. NASA EOSDIS Land Processes DAAC. Accessed YYYY-MM-DD from https://doi.org/10.5067/GEDI/GEDI02_B.002.

What science datasets does the GEDI Geolocated Waveforms product contain?

The L1B product provides corrected geolocated waveform returns, including transmit and receive housekeeping and relevant instrument parameters, as well as geolocation parameters and geophysical corrections. The detailed product contents are defined in the GEDI L1B Product Data Dictionary.

What science datasets does the GEDI Level 2A product contain?

The L2A product contains information derived from the geolocated GEDI return waveforms, including ground elevation, highest and lowest surface return elevations, energy quantile heights (“relative height” metrics), and other waveform-derived metrics describing the intercepted surface. The detailed product contents are defined in the GEDI L2A Product Data Dictionary.

What science datasets does the Global Ecosystem Dynamics Investigation Level 2B product contain?

The L2B product contains biophysical information derived from the geolocated GEDI return waveforms, including total and vertical profiles of canopy cover and Plant Area Index (PAI), the vertical Plant Area Volume Density (PAVD) profile, and Foliage Height Diversity (FHD). The detailed product contents are defined in the GEDI L2B Product Data Dictionary.

Why is the uneven spacing between beams across track?

While the design of the instrument and its operational implementation are to provide even cross-track spacing of the beams, several factors cause the beams to have small differences in the cross-track spacing. These factors include slight variations in beam alignment in the instrument frame, changes in altitude of the ISS, and changes in ISS attitude orientation, specifically yaw.

How do I extract elevation and height metrics for alternative algorithm setting groups in L2A?

Only the suggested result for each laser footprint is stored in the root group of the L2A product for each beam. This is currently set to the output of algorithm setting group 1 (see Table 5 of the Level 2 User Guide) and will be updated as post-launch cal/val progresses. Elevation and height metrics outputs for all algorithm setting groups can be found in the geolocation subgroup of the L2A data product. For example, elev_lowestreturn_a<n> is the elevation of lowest return detected using algorithm setting group <n>, relative to reference ellipsoid; and rh_a<n> are the relative height metrics at 1% intervals using algorithm <n> (in cm). See Section 5 of the ATBD for GEDI Waveform Geolocation for L1 and L2 Products for additional details.

How do I extract cover and vertical profile metrics for alternative algorithm setting groups in L2B?

Only the suggested result for each laser footprint is stored in the root group of the L2B product for each beam. The suggested result corresponds to the L2A algorithm setting group set in /BEAMXXX/selected_l2a_algorithm and will be updated as post-launch cal/val progresses. In contrast to the L2A data, only a select set of L2B algorithm outputs is stored for each L2A algorithm setting group. These outputs can be found in the /BEAMXXXX/rx_processing subgroup and include the directional gap probability (pgap_theta_a<n>), canopy (rv_a<n>) and ground (rg_a<n>) waveform integrals, and the results of the extended Gaussian fit, fit to the ground waveform (rg_eg_*_a<n>), where <n> is the algorithm setting group <n> (see Table 5 of the Level 2 User Guide). These outputs enable rapid recalculation of L2B vertical profiles for different L2A algorithm setting groups. Examples of this recalculation are being prepared by the GEDI Science Team as Python Jupyter Notebooks.

In L2A, why are Relative Height (RH) values sometimes negative?

Relative Height is calculated by the following equation: elev_highestreturn - elev_lowestmode. The lower RH metrics (e.g., RH10) will often have negative values, particularly in low canopy cover conditions. This is because a relatively high fraction of the waveform energy is from the ground and below elev_lowestmode. For example, if the ground return contains 30% of the energy, then RH1 through 15 are likely to be below 0 since half of the ground energy from the ground return is below the center of the ground return, which is used to determine the mean ground elevation in the footprint (elev_lowestmode). The RH metrics are intended for vegetated surfaces. Results over bare/water surfaces are still valid but may present some confusing results. See Section 6 Level 2 User Guide for more detailed information.

Is GEDI still an active instrument?

As of March 2023, the GEDI instrument has paused acquisition of science data for a period of 13 to 18 months in anticipation of its move to an alternate ___location on the International Space Station (ISS).

The ISS offers a unique hosting opportunity for Earth science payloads where NASA can test new instrument approaches that can make major contributions to understanding the changing planet.

NASA’s GEDI instrument aboard the space station is one of multiple instruments from the agency and others providing information about the Earth system and effects of climate change.

Demand is high for external operations on station, and GEDI is scheduled to be replaced by a Department of Defense (DOD) payload after more than four years of operations. However, the agency is exploring an option to keep the instrument in space through the life of the space station.

The proposed solution calls for temporarily moving GEDI to an alternate ___location, where it will remain offline while a DOD’s technology payload completes its mission. In 2024, GEDI will return to its original ___location, and resume operations on station.

What format does GEDI data come in?

The Level 1 and Level 2 GEDI datasets that are distributed by the LP DAAC are available as HDF5 (.h5). ORNL DAAC also distributes Level 3 and Level 4 GEDI datasets. The Level 3 and Level 4B datasets are gridded Tiff (.tif) and the L4A are HDF5 (.h5). Third party tools with HDF5 support include IDL and Matlab.

The LP DAAC also provides several Data Prep Scripts and Tutorials for GEDI data that you might be interested in. Please visit https://github.com/nasa/GEDI-Data-Resources to learn more.

Asking additional GEDI questions?
Have other GEDI questions? GEDI scientists can respond directly to your questions through the NASA Earthdata forum.