First Data from NASA’s GEDI Program to 3D Map Forests

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NASA’s new high resolution Light Detection and Ranging (LIDAR) tool is called the Global Ecosystem Dynamics Investigation (GEDI), which was launched in December 2018. It will provide a high, detailed view of forest canopies in 3D by mapping the tops and bottoms of forested regions, providing such imagery for the first time for scientists interested in understanding the relationship of forests with atmospheric CO2.

GEDI: Mapping Forests in 3D

The main idea is for GEDI to map forests in 3D, as well as the overall structure of tree canopies, so that better knowledge of tree density, including branch density, and tree hight can be made. A detailed mapping that provides this data will enable a more accurate estimate of stored carbon held in forested regions. Understanding the structure of forests can also help biologists understand the relationship between forest health and habitat diversity. The instrumentation on GEDI uses 8 lasers that can scan a 4 km wide area, firing photons at 250 times per second to create a detailed profile view of forest canopies. Additionally, the measurements can be used to provide a highly detailed global view of topography, including accurate measurements of dams.[1] All of these measurements will be conducted from the International Space Station, with the project led by University of Maryland scientists and NASA at a total cost of $94 million.[2]

GEDI Produces the First Set of Forest Structure Data

The first set of data that have returned have demonstrated GEDI is able to measure density of forests far better than previous missions, where previous methods were often more estimated or using a combination of instrumentation. Density of branches and leave cover are just some of the outputs revealed by mapping of forests in South Carolina, as detailed in a recent NASA press briefing.[3]Such measures can then be utilised to provide estimates of how much carbon, or CO2, is being contributed to the atmosphere as forests are lost around the world. This will provide greater detail of the carbon cycle, as NASA had previously stated prior to the mission being launched. The mission is expected to last for two years, with GEDI producing over 10 billion images.[4]

GEDI maps out the profile measurements of a South Carolina woodland. Graphic: Joshua Stevens, NASA, 2019.

GEDI maps out the vertical structrure of a South Carolina woodland. Graphic: Joshua Stevens / NASA Earth Observatory, Bryan Blair / NASA Goddard Space Flight Center, Michelle Hofton and Ralph Dubayah / University of Maryland.

One of the main lasers utilised is the High Output Maximum Efficiency Resonator (HOMER), which provides a low particle count laser that is relatively less complex but has great scalability. It is a non-single frequency laser that uses a single oscillator. The lasers have diffractive optical elements that have been designed similar to a previous technology called Lunar Orbital Laser Altimeter (LOLA).[5]


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One proposal had been that GEDI can be fused with TanDEM-X data, which uses interferometric synthetic aperture radar (InSAR) instruments, for measuring ground or surface level structures. This is effectively a type of radar system. Combing datasets between GEDI and TanDEM-X could give scientists an ability to better map ground structure along with forest structure.[6] However, instrumentation in GEDI may not necessitate this as it has proven it is also able to provide a high detailed structural view of areas that are also along open, non-forested surfaces.

The next two years should provide many high resolution, 3D images of forest covers around the world. This will allow scientists to now make better estimates of carbon held in forests as well as how much could be released as forests are destroyed, including the relationships of forests to climate change. Other ecosystem benefits are also possible, with the measurements able to determine the health of forests and better measure elevation across the globe.

References

[1]    For more on GEDI and its capabilities, see:  https://earthobservatory.nasa.gov/images/144818/return-of-the-gedis-first-data.

[2]    For more on the mission and capabilities of GEDI, see: https://gedi.umd.edu/.

[3]    For more on recent mapping of forests in South Carolina, see:  https://www.nasa.gov/feature/goddard/2019/return-of-gedi-s-first-data-shows-forests-topography

[4]    For more on initial statements and mission goals of GEDI, see: https://www.businessinsider.com/nasa-gedi-will-image-the-worlds-forests-in-3d-2015-3?r=UK

[5]    For more on the laser applied in GEDI and its detail, see: Coyle, D. B., Stysley, P. R., Poulios, D., Clarke, G. B., & Kay, R. B. 2015. Laser transmitter development for NASA’s Global Ecosystem Dynamics Investigation (GEDI) lidar. In U. N. Singh (Ed.), : 961208. Presented at the SPIE Optical Engineering + Applications, http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.2191569, April 23, 2019, San Diego, California, United States..

[6]    For more on TanDEM-X and GEDI integration, see:  Qi, W., & Dubayah, R. O. 2016. Combining Tandem-X InSAR and simulated GEDI lidar observations for forest structure mapping. Remote Sensing of Environment, 187: 253–266. https://doi.org/10.1016/j.rse.2016.10.018

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