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Remote Sensing

Lidar monitors environmental changes

Lidar can help map forests, track sea level change, and possibly save lives in a natural disaster.
25 August 2010, SPIE Newsroom. DOI: 10.1117/2.2201008.01

Can a map save the world? Maybe not directly, but the information on a map, and in particular a 3D, highly detailed map, can provide scientists, conservationists, and public servants the information they need to understand an area's biodiversity, geologic history, unexpected terrain differences, and changes over time. Lidar provides that information, and is becoming a stronger and more reliable tool in the quest to save forests, animals, and humans.

LiDAR (Light Detection and Ranging) is an optical remote sensing technology that uses laser pulses to measure properties of scattered light to find the range or other information of a distant target, with high accuracy. Like radar, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. Because lidar uses shorter wavelength laser pulses and is transmitted in a much narrower beam, it can collect much more data in one pass than other longer-wavelength scanners. Data can be collected under a variety of environmental conditions, such as low sun angle, cloudy conditions, and even darkness, resulting in expanded windows for data collection. Data from lidar can also be processed fairly rapidly in comparison to photogrammetry.

Lidar has typically been used for military, police, and security applications. Because of its ability to map and provide data on large areas quickly, lidar has also found many applications in studying forestry, glaciers, coastlines, and cities which all have varying heights and are difficult to navigate by foot and too complicated to measure by satellite.

Canopy heights, biomass measurements, and leaf area can all be studied using airborne lidar systems. The Save the Redwoods League in Northern California is undertaking a project to map the tall redwoods on California's northern coast. Using lidar allows researchers to not only measure the height of previously unmapped trees but to study the factors that may support the biodiversity of the redwood forest.

Figure 1. A drawing of the Coastal Zone Mapping and Imaging Lidar (CZMIL) data acquisition system (DAS), similar to how most lidar data is collected. The CZMIL is designed specifically for ocean and coastal applications. In this scenario, the lidar employs a circular scanner with a fixed incidence angle of 20o. The aircraft is flying parallel to the beach at a height of 400m, with airspeed of 140kts. From SPIE Proc. 76950R.

"The canopy of old-growth redwood forests is highly complex, and supports an amazing biodiversity of species. We want to understand the variability of the structure of the canopy as well as the biodiversity, and whether these two things are related," says Laura Kindsvater, senior conservation planner at the Save the Redwoods League. "Lidar allows us to make management decisions at the landscape level in a more efficient and cost-effective manner. It also gives us a comprehensive picture at the landscape scale. For example, we are using it to prioritize forest restoration for a 25,000-acre young redwood-Douglas-fir forest."

Five government agencies and conservation groups have pitched in to pay the Sanborn Mapping Company and Image Tree Corporation for extremely detailed 3D images of rugged wilderness areas. Using a small, twin-engine plane, the Sanborn flight covered about 100,000 acres, and the Image Tree flight covered about 36,000 acres.

A recent study done in collaboration with the Nature Conservancy, the University of Idaho, and the Pacific Forestry Centre, Canadian Forest Service in Victoria, BC, attempted to characterize forest successional stages across a diverse, mixed-species forest in northern Idaho. Approximately 30,000 hectares on Moscow Mountain was mapped. "The lidar data were fairly low density by today's standards," says lead author Mike Falkowski. "The system was an Optech ATLM30, which operated at 1064 nm and collected up to three returns per pulse. The dataset had a nominal post spacing of 1.95 m (~0.26 pulses per square meter).

"In this particular project we were interested in mapping the successional stage of the forest. The structure of the forest plays a large role in what wildlife species will inhabit the area. In a follow on project we used the successional stage (structure) classifications to map habitat suitability for various cavity nesting birds species. Since theses birds rely on complex food webs, they serve as indicators of biodiversity.

"Lidar data have proven particularly useful for measuring or estimating a suite of forest structural attributes such as canopy height, basal area, and LAI," Falkowski and the other study authors write in the paper. The maps they were able to create using the lidar data, "represents a significant advancement for forest succession modeling and wildlife habitat assessment."

Sassan Saatchi and other researchers at NASA Jet Propulsion Lab have used data collected by the ICESat, Terra and Aqua satellites using Geoscience Laser Altimeter System (GLAS) lidar to create a topographical map that shows the height of forests around the world, from the rain forests of the Amazon to the redwood and sequoia forests of Northern California. Saatchi has already started combining the height data with forest inventories to create biomass maps for tropical forests.

"All we have is a very sparse network of forestry or research plots to measure forest structure and biomass that are different in size, in quality, and in accuracy. GLAS lidar provides millions of shots over the tropics," says Saatchi. The GLAS lidar data is currently being converted to forest height. Complete global inventories of biomass, when they exist, can improve climate models and guide policymakers on how to minimize the human impact on climate with carbon offsets.

GLAS lidar also has its limitations, and there is a need to improve these measurements for a true global carbon mapping and monitoring. "We are currently addressing this need by NASA's new mission DESDynl," says Saatchen. DESDynI is the Deformation, Ecosystem Structure and Dynamics of Ice, which combines InSAR (Interferometric Synthetic Aperture RADAR) and lidar technologies. "DESDynI will have a lidar with much improved spatial resolution and sampling density and a radar polarimetric sensor at (L-band) that can readily take the lidar samples to provide a wall-to-wall map of forest biomass carbon." The information taken from DESDynI will be used to determine the likelihood of earthquakes, volcanic eruptions, and landslides; predict the response of ice sheets to climate change and impact on the sea level; characterize the effects of changing climate and land use on species habitats and carbon budget; and monitor the migration of fluids associated with hydrocarbon production and groundwater resources.

Sea Change

DESDynI is just one example of lidar being used to measure coastlines and river basins, seacliff volume and beach sediment change. One of the organizations working with Save the Redwoods League is using the lidar data collected "to generate a high resolution digital elevation model (DEM), and from the DEM are modeling stream flows and potential for erosion so that they can guide their watershed restoration work," says Kindsvater.

Figure 2. A sample of a floodplain review in Pierce County, WA. The LiDAR TIN information has a 40% Transparency so the property owner can visualize their building locations in relation to where floodwater could back up behind the highway. Credit Dennis Dixon, Pierce County Public Works.

Roger Fuller of the Nature Conservancy of Skagit County, WA, has been using lidar mapping to measure the tidal level changes seen in local estuaries. Fuller says the lidar measurement will help the Nature Conservancy better understand the impacts of climate change on tidal level changes. While it is a slow process and still needs to be compared to ground measurements taken by hand, Fuller is optimistic that enough data points will be collected over the next few seasons to show any changes in water elevation change in the estuaries of Skagit and Island Counties in northwest Washington.

The Pierce County, WA, Public Works and Utilities department uses lidar to aid in flood-plain management along the major rivers that flow through heavily populated areas and farmland. "We look for channel migration patterns, and whether there are historic geomporphology studies, such as where old trees were," says Dennis Dixon, SWM Engineer, Public Works and Utilities. "It's been a huge help trying to figure out where ravines are. We'll often start with Department of Natural Resources maps, trying to find ravines, but the maps are plus or minus 500 feet from where streams might actually be. The lidar gives us most times a really good idea where the flood channel is."

Lidar is good for measuring the rivers' edges, but does have some limitations. "The big problem with lidar is it doesn't behave well in the water," says Dixon. "Plus, it doesn't work well in really thick vegetation. So the two places that are the most critical for me are where lidar is the weakest. But it's a whole lot better than anything else we have."

Lidar has even been used in Spain and South Africa to survey and track air pollution. Read the SPIE Newsroom article "Lidar probes air pollution," by Jose Moreno et al., and "Lidar Research in South Africa" by Sivakumar Venkataraman, to learn more.

Disaster Preparedness

Figure 3. Perkins Lane in Seattle, WA, has been called "the ground zero of Seattle landslides." Lidar image of the area (upper) taken by the Puget Sound Lidar Consortium. Non-lidar image (lower) from Google maps.

Lidar can also be used to predict how natural disasters will affect cities and residential areas. Carol Dunn, Disaster Risk Communicator for the Emergency Preparedness Division of Bellevue, WA, has used lidar to do several safety assessment on residential areas all over the Pacific Northwest.

In one study, Dunn compared lidar maps to historical maps of damage to old structures caused by earthquakes that have hit the Seattle region over the past 100 years, and determined that structures over 100 years old were most at risk to fire or structural damage due to their "balloon frame" construction. "Factors that can influence how much damage is caused by an earthquake include a building's construction style and materials, ground type, and the intensity of the shaking," says Dunn.

"Lidar is great because it reveals information that we miss when we are just looking around" at surface features and using traditional surveying methods, says Dunn. "Over the coming months we are hoping to use the lidar to identify old landslides that were missed by previous geological assessments."

Lidar is also helping to keep residents safe in hurricane-prone parts of the United States. Lidar mapping has been used in several areas to determine if structures such as residential homes have enough elevation to withstand an approaching storm. Residents of Santa Rosa County, FL, for example, can access an interactive website, located at http://maps.roktech.net/santarosa/evacuation/, to learn which category of storm will trigger an evacuation recommendation for their property.

Lidar's potential

Lidar data requires a lot of computer power to process, and a skilled technician to collect it. In the last computer processing power has increased dramatically, and more vendors who offer lidar collection services to smaller organizations. Lidar itself has also become much more detailed than even a few years ago, making it suitable for more and more complex terrain.

"At this point, I know that I am not using the remote sensing information to its full potential," says Dunn. But cost, access, and knowledge of how to interpret the data are all still major concerns for many people interested in using Lidar data. The main issues holding Dunn back are specific training and a suitable software program that is easy to use and interpret the data with.

Dixon finds the interpretation of the data less of a barrier, and instead is more interested in seeing lidar expand its repertoire of readable terrain. "The holy grail would being able to penetrate into water," he says.

For researchers who are interested in using lidar, Falkwoski recommends, "Do your research to determine exactly what to ask for from lidar vendors. This includes everything from data specs to accuracy requirements to specific deliverables."

Figure 1 complete photo credit: From Grady Tuell, Kenneth Barbor, and Jennifer Wozencraft, "Overview of the coastal zone mapping and imaging lidar (CZMIL): a new multisensor airborne mapping system for the U.S. Army Corps of Engineers," Proc. SPIE 7695, 76950R (2010). doi:10.1117/12.851905

Lidar for Archaeology and Preservation

Lidar is also used by archaeologists to survey hard-to-get-to land or mapping features beneath forest canopy, identify old roads and compounds that aren't identifiable by ground surveying, and to see layers of cultural debris buried just below the surface.

University of Central Florida researchers Arlen and Diane Chase led a NASA-funded research project in April 2009 over the jungles of Belize, and detected thousands of new Mayan structures, 11 new causeways, tens of thousands of agricultural terraces and many hidden caves - results beyond anyone's imagination. The data also confirm the size of a city spread over 177 square kilometers or 68 square miles, and corroborate the Chases' previous estimates for the size of the population, at least 115,000 people in A.D. 650.

Using lidar not only saves time and money, but also protects forests and helps governments and universities determine what areas need to be protected and preserved.

Beth Kelley is a science and technology writer living in Seattle.