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Fires that rage through forests, consuming vegetation on the ground and spreading into canopies, almost always leave important questions behind when they are done.

Like, what in fact burned? Where did it burn? How intense was the burn? And how far into the canopy did it reach?

Color photo of the Black Hills of South Dakota
The Black Hills of South Dakota. Researchers from the USGS Earth Resources Observation and Science (EROS) Center are conducting research on post-fire recovery using Landsat and Light Detection and Ranging (lidar) data.

Fire scientists at the Earth Resources Observation and Science (EROS) Center say remotely sensed satellite data can answer at least some of those questions by providing what ultimately amounts to a two-dimensional photograph of the fire’s footprint. Depending on the time and resources available, laborious ground surveys can offer very localized insights into the severity as well.

But to take it a step further, EROS staff has begun looking at the potential of three-dimensional (3D) light detection and ranging (lidar) technology to get at what ground surveys and remotely sensed data often cannot—important information on what and where vegetation has been burned above the ground and beneath the top of the canopy.

Using lidar to look at that vegetation structure before and after fires is vital to understanding how future fires in the same area may play out, the fire scientists say. While lidar has certainly been available to assess vegetation characteristics for several decades now, USGS EROS Geographer and Senior Scientist Birgit Peterson says it really hasn’t been integrated into operational burn mapping like it could be.

“There’s a fair amount of experimental work that has gone on, but that bridge to doing something operational really hasn’t occurred,” Peterson said. “I don’t think it’s for lack of trying. I think part of it is just the investment that needs to go into figuring out what works well ... and what could work in different locations relatively consistently. I think that’s where EROS fits in really well.”

Using 3D Technology on the Legion Lake Fire

To explore lidar’s capabilities, Peterson and her colleagues have now made several trips to the Black Hills of western South Dakota to survey landscapes after the December 2017 Legion Lake Fire that affected 54,000 acres in Custer State Park, as well as on private and Federal lands.

As part of its initial work, the EROS team established plots in burned areas and used terrestrial lidar to collect ground measurements. Then, they acquired coincident satellite data from NASA’s ICESat-2 (Ice, Cloud and land Elevation) mission.

That gave them important information at the local footprint level, and at the broad-scale spaceborne level. Now they are working with Unmanned Aerial Systems (UAS) to capture photographs from the sky that will provide 3D Structure-from-Motion point clouds characterizing vegetation or the lack of it that isn’t immediately discernible from terrestrial lidar or satellite images.

“What we hope to tease out is, what can we infer from the interior vegetation structure?” Peterson said. “Where does a shrub layer end and then an actual canopy layer start? That’s the type of thing we want to use this 3D point cloud perspective to kind of pull apart.”

To explore what interior vegetation structure looks like before a fire, as well as the volume of vegetation consumed during the blaze, the EROS team will fly lidar using UAS on prescribed burns planned in the Black Hills to remove potential fire fuel loads.

“These prescribed burns are going to be important to us in the study area,” said EROS software engineer Gail Schmidt, who is also the chief of the Rockerville Volunteer Fire Department in the Black Hills. “Being able to study before and after in those low to moderate severity situations, especially using the 3D technology, is important compared to our traditional satellite imagery that is usually blocked by dense canopy.”

Sending Out Laser Light to Capture Vegetation Structure

Getting at interior vegetation structure with lidar involves sending out laser light that bounces off whatever interferes with its pathway on the way to ground. Thousands of points of light returning to the instrument and taken collectively reveal 3D details of objects with which they interact.

Peterson, Schmidt and their team are getting the chance to use the 3D technology not only on acres burned in Custer State Park, but across private and Federal land management boundaries as well. Historically, getting buy-in from people managing different land interests for fire science work hasn’t always been easy. The fact is, lands abutting one another often have different interests and management objectives. But Schmidt’s connection with different land managers through her firefighting work has enabled important opportunities for the EROS team.

While being able to work across those land management boundaries is important, “the follow-up and ... seeing how the fire area recovers and being able to talk to those local land managers to better understand from their perspective what they’re seeing is valuable, too,” she said.

The most likely beneficiaries of an operational burn mapping system that incorporates 3D imagery of vegetation structure, especially in the lower canopy, are the land managers accessing the EROS products now, Peterson said. Certainly, the USGS’ Landslide Hazards Program may benefit as well from the additional information for the work it does to stem such problems as debris flows caused by rain events on hill slopes laid bare by fire.

In reality, the value of three-dimensional images’ reach will go far beyond that. A land manager with a better understanding of how truly severe a fire was in their location should be able to target what they want to do after the fire more precisely with this 3D assistance, Peterson said.

“From our discussions with some of these local land managers,” Schmidt added, “when they start to see the data that is made available from this project, they will likely have some additional ideas on how to utilize these datasets that maybe we as a team haven’t thought about.”

Potentially Expanding 3D Work Beyond the Black Hills

By demonstrating a prototype in the Black Hills, Peterson hopes their work can be used elsewhere across the country. Obviously, the USGS is not a land management agency, and would have to work in partnership with others to access lands. But if they can develop collaborations with other private and agency land managers, “then we could see, OK, how does this scale? How does this work in different vegetation types, different forest types, different management types?” she said. “Then we could start making assumptions about, ‘OK, can this be something that folds into something that’s operational?’ ”

They aren’t there yet. The team has done a lot of data collections already and has more to do. Peterson said they are in the processing phase now, with analysis to follow.

“Analysis is kind of the fun part, where we can look at what other people have done and learn from that,” she said. “But the main thing is really pulling apart the metrics that we can use to quantify what it is we are looking at. And that’s going to be the challenge for us.”

With publicly available data from ICESat-2—and in time from the GEDI (Global Ecosystem Dynamics Investigation) mission—and with spectral data already being acquired from Landsat and the promise of the 3D lidar technology, Peterson sees EROS playing a key role in taking burn mapping to a new day and a better product at local, regional and national scales.

“What we do is regional to national in scope,” she said. “So yes, I’m excited about what we’re doing. I really think we have a good role to play in bridging that local to national data integration that really hasn’t occurred yet.”

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