A PORTABLE LIDAR SYSTEM FOR RAPID DETERMINATION OF FOREST CANOPY STRUCTURE 

Geoffrey G. Parkera, David J. Hardingb, and Michelle L. Bergera

aSmithsonian Environmental Research Center, Edgewater, MD, USA; bNASA Goddard Space Flight Center, Greenbelt, MD, USAContact email: parkerg@si.edu

BiasesThe Riegl LD90-3100 has two characteristics the can affect structural measurements. It always averages at least 5 measurements together for every output resulting in potentially fictitious targets, and its beam is not perfectly thin, as is assumed by the Macarthur-Horn (1969) transformation. We did a number of tests to characterize these biases.Beam Size: The emergent beam is 12.4 cm2 and has an angular divergence of 2 mrad. Our tests found that the laser can detect targets of many sizes and orientations and can pass through most holes found in the forest canopy.Averaging: We manually averaged a string of raw data in progressively larger groups and compared the resulting distribution to that of the raw data. Averaging had little effect on the distribution of the data when only 5 measurements were averaged and when the laser was moved at a walking pace.

Limitations• Awkward in forests with dense understory or many fallen logs.• Computer and laser can be sensitive to high moisture.• Absolute positioning of each point in space is not precise.

Uses• Rapidly assess structure at a range

of spatial scales.• Quantify structural changes.• Characterize forest complexity or

biotic habitats.• Rapidly survey damage in remote

locations.

AbstractSignificant functional characteristics of forests are related to the organization of

their canopies. However, understanding of the relation between canopy structure and function has been limited by a lack of methods to determine structure at scales consistent with the footprints of function measurements. We describe a portable system, assembled from commercially-available components, for acquiring measurements of distances to overhead plant surfaces that can be aggregated to assess canopy structure at scales of ecological interest. Deployed by a person from the forest floor, the system includes a narrow-beam, rapidly-pulsed, first-return laser rangefinder coupled with a data recording system.

From tests in an age-sequence of broad-leaved, closed-canopy forests we found the system provides results significantly more rapidly than previous methods, at spatial scales as small as 1 m in all dimensions. The estimated mean vertical canopy structure is consistent with that found from more laborious, manual approaches, such as the “Foliage Height Profile” method. The system has some biases due to beam width and range averaging but from a variety of tests we found these have relatively little effect on the structure estimates. Various field sampling schemes and methods of aggregating the measurements yield a variety of representations of structure, including mean profiles, tomographic sections, three-dimensional distributions of canopy surface density, and maximum height surfaces. Derivable summary measures include canopy cover and area index, porosity, the size distribution of overhead openings, and indices of structural complexity. Moreover, the approach can provide estimates of spatial variability and covariance not previously obtainable. Portable LIDAR systems such as the one we describe provide a new tool for measurement of small-scale forest structure useful in various canopy research and forest management applications.

AcknowledgementsThis project was supported with a grant from the National Science Foundation panel on Biological Infrastructure (DBI-0096656), by the NASA Terrestrial Ecology Program, the Global Canopy Programme, and by the Smithsonian Environmental Research Center. Advice from Bryan Blair, Barry Coyle, James Getter, Christof Hug, Bob Knox, Ross Nelson, David Rabine, Jim Ritchie, and especially Michael Lefsky contributed to the development of this system. Saharah Moon Chapotin, Alix Hui, Beth Reichart, Brigid Franey, and Rehanna Chaudhri tested early versions of the system. Edward Dick constructed the platform and Jerry Pritchard built testing equipment. George Rasberry, Ed Schmitt, Bob Peel, and Joyce Schick sampled the field test plots. Kevin Bach, Bill Brinley, Art Ellison, Russ Goodrich, Roger Hurst, John Niemel, Reggie Reid, David Thouin, and Michael Winings assisted in developing this system.

Future Work• Test an unaveraged small beam size rangefinder.• Develop software for structural metrics and visualization.• Relate ground-based metrics to airborne LIDAR metrics, stand biomass and production, and habitat structure and complexity.

The System

• Riegl LD90-3100HS 1 Khz first-return laser.

• Small laptop computer for data acquisition.

• Custom frame and straps for carrying laser.

The Approach

(A) As the system is walked along a transect, the laser continuously measures the distance to surfaces overhead. Laser pulses that pass through the canopy and do not return (“sky hits”) are indicated by an error code in the data stream.

(B) The raw data are grouped into vertical and horizontal bins.

(C) The MacArthur-Horn transformation (MacArthur and Horn 1969) yields the relative surface area index, the proportion of surface area in each horizontal bin at each height.

(D) The relationship between cover, which is easily calculated from the laser data, and Leaf Area Index (LAI) is used to estimate the actual amount of surface area in each vertical bin (Canopy Area Index – CAI).

B Binned Data

C Relative Surface Area Index D Estimated Surface

Area Density

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ReferencesParker, G.G., Harding, D.J., and Berger, M.L. 2004. A portable LIDAR system for rapid determination of forest canopy structure.

Journ. Appl. Ecol., 41, 755-767.MacArthur, R.H. and Horn, H.S. 1969. Foliage profile by vertical measurements. Ecology, 50, 802-804.

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Visualizations

(A) The resulting estimates of surface area density in each voxel can be plotted as a contour graph.

(B) The data from an entire transect can be summed into a Canopy Height Profile of mean and standard error of surface area density.

(C) If numerous parallel transects are walked, the local maximum heights can be compiled into a 3-dimensional map of the outer canopy surface.

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