NASA data lets scientists map forests (and the trees)

LIDAR and MODIS data can measure carbon cycles, predict forest fire patterns

Scientists have developed the world’s first global forest height data map using satellite data from NASA.

Michael Lefsky of Colorado State University created the map using data collected by NASA's ICESat, Terra, and Aqua satellites. The map will be used to determine how much carbon the world’s forests store and how fast that carbon cycles through ecosystems and back into the atmosphere. It also will help scientists predict the spread and behavior of fires and understand the suitability of species to specific forests.

To create the map, Lefsky combined data collected from Light Detection And Ranging (LIDAR) laser technology and the Moderate Resolution Imaging Spectroradiometer (MODIS), a satellite instrument aboard both the Terra and Aqua satellites.

LIDAR uses laser pulses to determine distance -- in this case, the distance between the ground and the top of the trees.

“LIDAR is unparalleled for this type of measurement,” Lefsky said. LIDAR can capture in seconds an amount of data that would take weeks or months to collect in the field by counting and measuring tree trunks.

Lefsky collected seven years’ worth of LIDAR data through more than 250 million laser pulses. LIDAR directly measured 2.4 percent of the Earth’s forested surfaces.

While LIDAR provided height, MODIS provided width. The MODIS satellite technology views the entire Earth’s surface every one to two days, acquiring data in 36 spectral bands. 

Lefsky spent years working on techniques to fuse the data. One reason for the difficulty was that the LIDAR data came from a satellite designed for the study of the topography of ice sheets, not vegetation.

Lefsky described the map as a first draft. A proposed NASA satellite called DESDynI (Deformation, Ecosystem Structure and Dynamics of Ice), would significantly improve map resolution.

“We’ve never been able to look at a map and say, 'here’s how tall the canopy is' before,” said Ralph Dubayah, one of the DESDynI project scientists. “This map is a big step forward, and it really helps set the stage for DESDynI and shows what’s possible.”

The map depicts average height over 5 square kilometers (1.9 square miles), not the maximum heights any one tree or small patch of trees might attain. For any patch of forest, the height shown means that 90 percent or more of the trees in the patch are that tall or taller.

Information gleaned from the map can help scientists determine whether the planet can continue to soak up carbon emissions. A colleague of Lefsky, Sassan Saatchi of NASA’s Jet Propulsion Laboratory, is combining the height data with forest inventories to create biomass maps of tropical forests. Biomass maps can be used to improve climate models and develop measures to minimize human climate impact.


About the Author

Kathleen Hickey is a freelance writer for GCN.

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