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 FEATURE ARTICLE
Emerging Technologies For REDD & MRV
Airborne LiDAR For Forestry in Asia
by Cody Benkelman & Scott Stanley
Challenges of REDD/MRV in Remote, Tropical Forests
A general consensus has emerged from the contentious negotiations around climate change that reducing emissions from deforestation and degradation (“REDD”, which generally includes forest conservation, sustainable forest management, and enhancement of forest carbon stocks) is an important mitigation strategy and various countries have committed more than $6 billion for this initiative in key rainforest nations. While use of imagery from existing satellites has been adequate for monitoring large-scale deforestation, finer scale forest clearings and degradation has been challenging to detect and quantify in extensive and remote forests.
The forest management sector has received considerable attention in recent years since substantial emission reductions could be achieved without drastically curtailing timber production. More than 350 million hectares of tropical moist forests are being managed for timber and it is estimated that emissions could be reduced by 160 megatons/year through implementing reduced impact logging techniques and improved management including better road planning and design (Putz et al 2008). However, verifying that these reductions actually took place is problematic. Timber concessionaires selectively log annual compartments that total several thousand hectares in remote areas; in these areas, it would be prohibitively expensive and time intensive to adequately monitor compliance using on-the-ground methods. Many of these forest areas are prone to almost constant cloud cover, which greatly reduces the effectiveness of satellites with optical sensors. As an augmentation to satellite imagery, Credent and its partners, Forest-Carbon (http://forest-carbon.org/), and The Nature Conservancy (http://www.nature.org/) are testing use of airborne LiDAR for inventory and monitoring of remote tropical forests.
Brief Description of Airborne LiDAR Technology
LiDAR is an acronym for “light detection and ranging”, similar to the more familiar term “radar” (for “radio detection and ranging”). Distance or range measurements are made by sending a pulse of laser light out from the LiDAR system, then recording the return pulse(s) and converting round trip time to distance based on the speed of light.
Measurements are made in 3 dimensions (X, Y, and Z), and when operated on an airborne platform (fixed wing airplane or helicopter) with the LiDAR system viewing down, the system can map terrain, land cover, and structures (vegetation, buildings, power lines, etc.) with accuracies measured in centimeters.
The spatial resolution of airborne LiDAR data is determined by the altitude and speed of the aircraft, as well as the number of pulses (samples) collected each second. With modern systems acquiring 100,000 pulses per second or greater, it is not unusual to collect point densities of several samples per square meter. The resulting resolution and accuracy are higher than aerial photography (and much higher resolution than satellite data), resulting in more accurate measurements.
The additional advantage of LiDAR is that all measurements are made in 3 dimensions. And even though the laser pulses cannot penetrate vegetation, each outgoing pulse spreads out as it moves down, and will typically generate more than one return pulse – such as from the top of a tree canopy, branches and understory, as well as the ground.
With accurate placement of each “return” in 3 dimensions, LiDAR can be used to create a highly detailed terrain map (“bald earth” with all vegetation removed), and other valuable information is gained including canopy height, canopy closure or gaps, hydrological flow, and slope maps. This type of detail is typically not resolvable in satellite data, and enables applications that require accurate monitoring of forest constituents.
Forest Profile From LiDAR
One additional consideration for applications requiring forest monitoring is that the high resolution and accuracy of the LiDAR data enable traceable and reproducible measurements, which is invaluable for any applications that require repeat monitoring with regard to change over time, such as MRV (Measuring, Reporting and Verification).
GIS Linkage and Analysis
A variety of software tools are available for LiDAR processing and analysis, and Credent has extensive expertise with this technology. After completing the initial processing on the raw LiDAR data, the information can be easily imported into existing geographic information systems (GIS) enabling additional analysis. In addition, the information can be shared with the public, management, and decision makers at all levels using web services and/or internet tools such as Google Earth.
A Demonstration Collection
Recently, Credent participated in a demonstration LiDAR data collection project in East Kalimantan, Indonesia, over a site being monitored under the REDD forest carbon program. Working in partnership with Forest-Carbon, two flight lines were selected to be flown that would provide a good representation of the type of terrain and land cover found throughout the region.
The data collection was done using an Optech 3100EA LiDAR system flying approximately 300 km/hour from an elevation of 700m above ground level (AGL). The selected site was collected under a single sortie.
To ease the initial processing and future use of the data, the flight lines were broken up into 2km * 2km tiles. From the raw data, the following products were created by Credent:
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Digital Terrain Model (DTM): Utilizing the laser pulses that penetrated the canopy and struck the ground, a “bare earth” terrain model was created.
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Digital Surface Model (DSM): This model represents the first item that the laser pulse reflected off of, which in many cases, is the top of the forest canopy.
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Tree Canopy Closure Map: This is a product of LiDAR data analysis that graphically portrays the size and location of the gaps in the forest canopy (see example below).
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Tree Height Map: This value-added product shows the absolute height of individual or groups of trees.
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Hydrological Flow Direction Map: This map depicts the location and flow direction of the major waterways.
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Slope Map: This map portrays the steepness of slopes by degrees
Preliminary Results and Benefits
Preliminary analysis of the Kalimantan data shows good information was obtained with the LiDAR data set. In a fairly short period of time, Credent was able to produce both the standard products delivered with all LiDAR collections, and also to build the value-added data sets specifically useful to forest managers (see above).
Depending on the exact management objectives and goals for the forest concession, these data sets are extremely useful for both planning and monitoring activities. For example, with detailed terrain information, harvest planning can minimize the amount of skid roads created as well as reduce the impact of stream crossings. The terrain information can easily be viewed as a slope map, allowing managers to easily see steep areas to avoid and allow quick and accurate calculations of the amount of area off limits to logging.
Using the detailed tree height map, foresters can develop their harvest plans by actually looking at the height and diameter of individual trees and determine those which should be cut and those which should remain (see graphic). This enables more efficient and profitable timber management.
In addition, the tree canopy closure map clearly shows and quantifies the existing gaps in the forest canopy, and can be useful when monitoring a concession for changes over time in support of MRV and forest certification activities.
What Next: Additional Research
 LiDAR is still a relatively young technology, and as such, new analytical methods are being developed and proven each year through applied research efforts such as the recent demonstration project. Many new value-added products are under development, and will likely be available in the future – although they are not quite proven today – such as:
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Identification and measurement of tree crowns
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Biomass and carbon content estimates
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Automated change detection
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Species (or genera) identification when LIDAR is paired with multi-band or hyper-band high resolution cameras.
Conclusion
LiDAR is proving to be a valuable tool for the management of tropical forests. By delivering highly accurate 3D data of the forest canopy and the underlying terrain, forest managers can leverage the information derived from just one over flight to develop accurate inventories and detailed management plans, resulting in better communication with all stakeholders and better decisions for long term planning. In addition, as part of a long term monitoring program, multiple LiDAR data sets can provide a precise record of change or stability over time.
For more information on this project or other airborne LiDAR services, please contact Mr. Hon Chuan Lee, Credent Technology: mailto:honchuan@credent-asia.com
Citations
"Improved Tropical Forest Management for Carbon Retention," Francis E. Putz, Pieter A. Zuidema, Michelle A. Pinard, Rene G. A. Boot, Jeffrey A. Sayer, Douglas Sheil, Plinio Sist, Elias, Jerome K. Vanclay, PLoS Biol. 2008 July; 6(7): e166.
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