Department of Geography, University of Southern Mississippi,. Hattiesburg, Mississippi 39406. Dwayne E. Porter. Geographic Information Processing Lab, Belle ...
Temporal Modeling of Bidirectional Reflection Distribution Function (BRDF) in Coastal Vegetation Steven R. Schill The Nature Conservancy, Jacinto Mañon esq. Federico Geraldino, Plaza D’Roca, Suite 401, Ens. Paraiso, Santo Domingo, Dominican Republic
John R. Jensen Department of Geography, University of South Carolina, Columbia, South Carolina 29208
George T. Raber Department of Geography, University of Southern Mississippi, Hattiesburg, Mississippi 39406
Dwayne E. Porter Geographic Information Processing Lab, Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, South Carolina 29208
Abstract: The bidirectional reflection distribution function (BRDF) is a theoretical concept that describes the relationship between a target’s irradiance geometry and the viewing angle of the sensor relative to the target. The BRDF can significantly affect the radiometric quality of remotely sensed data, particularly in off-nadir views. This research used a NASA Sandmeier Field Goniometer (SFG) to collect hourly canopy spectral reflectance at 76 hemispherical angles at two study sites within the North Inlet–Winyah Bay National Estuarine Research Reserve during late winter (March 2000—low live biomass and high dead biomass) and late summer (October 2000—high live biomass and low dead biomass). The objective of this research was to compare and quantify the temporal differences of high spectral and angular resolution BRDF diurnal data for smooth cordgrass (Spartina alterniflora) communities. These data were collected to model and quantify BRDF canopy patterns as they relate to in situ biophysical measurements and phenological change. The hypothesis tested was that temporal changes in LAI, biomass, height, geometry, understory, and tide levels throughout the phenological cycle can be spectrally quantified to provide insight into BRDF research. These data were used to create graphic plots to provide a quantitative assessment of temporal BRDF patterns and biophysical characteristics. This research identified bands that are least impacted by the BRDF, recognized optimal Sun/sensor angles-of-view, and provided insight into radiometrically adjusting remotely sensed data to minimize BRDF effects. Once scientists understand the nature of BRDF in relation to phenological changes within the vegetation canopy, they can begin to apply models to improve the accuracy of information extracted from remotely sensed data.
116 GIScience and Remote Sensing, 2004, 41, No. 2, pp. 116-135. Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved.
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INTRODUCTION With an increasing number of sensors taking advantage of multiangular remote sensing, it is becoming more important to acquire additional ground directional reflectance data for calibration purposes. It has been documented that the intensity of reflected radiance from a target varies considerably depending on the viewing angle of the sensor relative to the target (Deering et al., 1999; Gemmell and McDonald, 2000; Solheim et al., 2000). Directional reflectance is the fundamental and intrinsic physical property governing the reflectance behavior of a scene element and appears to be one of the most diagnostic measurements we can obtain to truly understand a surface’s spectral reflectance characteristics (Nicodemus et al., 1977; Sandmeier et al., 1996). Off-nadir data has been used for validation of forest canopy reflectance models, estimation of biophysical parameters used in forest ecosystem models, derivation of vegetation indices for mapping vegetative vigor, land cover classification, testing geometric registration algorithms, and retrieval of atmospheric properties (Vane and Goetz, 1993). The bidirectional reflectance distribution function (BRDF) is a theoretical concept that describes the relationship between (a) the geometric characteristics of the solar irradiance, and (b) the remote sensing system viewing geometry; hence the bidirectional terminology (Nicodemus et al., 1977; Jensen, 2000). The BRDF, in simple terms, is the relationship between the angle of the observer and the angle of illumination. Objects look differently when viewed from different angles, and when illuminated from different directions. The BRDF is wavelength dependent and is determined by the structural and optical properties of the surface, such as shadowing, scattering, transmission, absorption, orientation distribution, and density (Lucht et al., 2000). The BRDF, fr (sr-1) is formally defined as the ratio of the target radiance dLr (W m-2 sr-1 nm-1) reflected in one direction (Ir,Yr) to the Sun’s incident irradiance dEi (W m-2 nm-1) from direction (Ii, Yi) (Sandmeier, 1999; Sandmeier and Itten, 1999; Jensen and Schill, 2000). dL r ( θ i, ϕ i ; θ r, ϕ r, λ ) f r ( θ i, ϕ i ; θ r, ϕ r, λ ) ≈ --------------------------------------------------dE ( θ , ϕ ; λ ) i
i
i
A hypothetical example of the relationship is seen in Figure 1, where a target is bathed in irradiance (dEi) from a specific Sun zenith and azimuth angle, and the sensor records the radiance (dLr) exiting the target of interest at a specific azimuth and zenith angle (Jensen, 2000). The BRDF can never be measured directly because truly infinitesimal elements of solid angle do not include measurable amounts of radiant flux (Deering, 1999). Unfortunately, it is usually very difficult to acquire BRDF information about a surface because the Sun is continually changing zenith and azimuth angles, and it is difficult to acquire multiple images of the terrain from various angles of view in a short period of time. In order to minimize the Sun’s angle variation and rapidly acquire multiple angular measurements, goniometers have been developed to collect BRDF information about a particular surface (Hosgood et al., 1999). These are specialized instruments that measure spectral reflectance in a specified number of directions distributed throughout the hemisphere above a particular surface in a short
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Fig. 1. Concepts and parameters of the Bidirectional Reflectance Distribution Function (BRDF). A target is bathed in irradiance (dEi) from a specific Sun zenith and azimuth angle, and the sensor records the radiance (dLr) exiting the target of interest at a specific azimuth and zenith angle (Jensen, 2000).
period of time. Field goniometers have been used in recent years to assess the BRDF of natural and manmade surfaces (Walter-Shea et al., 1993; Sandmeier and Itten, 1999; Jensen and Schill, 2000). The Swiss Field Goniometer System (FIGOS) (Sandmeier and Itten, 1999) was a benchmark instrument for BRDF acquisition that spurred the development of the second-generation Sandmeier Field Goniometer (SFG) (Sandmeier, 2000). The Systems Engineering Division at NASA Ames Research Center, Moffett Field, CA, constructed the SFG under the commission of the Commercial Remote Sensing Office at John C. Stennis Space Center (Turner, 1998). The portable SFG is shown in Figure 2 and consists of three parts: (a) the zenith arc (2-meter radius); (b) the azimuth ring (2-meter radius); and (c) the spectroradiometer sled. A GER 3700 (GER, 1997) spectroradiometer is attached to a moving sled mounted on the zenith arc and maintains a constant 2-meter hemispherical pointing distance between the sensor and the target. The spectroradiometer records the amount of radiance leaving the target in 704 bands at any position between 0° and 75° view zenith angle and 0° and 360° view azimuth angle. BRDF estimations for a particular surface are most useful when Sun angle variation is minimized; therefore the SFG was designed to acquire 76 hemispherical measurements within a short time period (8–10 minutes) (Turner, 1999). RESEARCH OBJECTIVE NASA has recently identified the collection of multiangular BRDF spectral measurements as a key remote sensing research issue (Demircan et al., 2000). In addition, there is a need within the BRDF research community for increased in situ validation
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Fig. 2. The Sandmeier Field Goniometer (SFG) collecting smooth cordgrass (Spartina alterniflora) BRDF measurements at North Inlet, SC. Spectral measurements are made at Sun zenith angle of Ii and Sun azimuth angle of Yi and a sensor zenith angle of view of Ir and sensor azimuth angle of Yr. A GER 3700 spectroradiometer is attached to the moving sled mounted on the zenith arc, which records the amount of radiance leaving the target in 704 bands at 76 angles (adapted from Sandmeier, 1999; Jensen and Schill, 2000).
studies (Kimes et al., 1994; Lucht, 1998). The objective of this research was to collect hyperspectral (400–2500 nm sampled approximately every 3 nm) and multiangular (every 15° along the zenith arc and every 30° along the azimuth base) resolution BRDF data at one-hour intervals for Smooth Cordgrass (Spartina alterniflora). This research was the first field campaign of the SFG and the first attempt to document the BRDF in coastal vegetation. Spartina is an ecologically important intertidal species found along the eastern seaboard of North America from Newfoundland to northern Florida, and in the Gulf of Mexico from Florida to southern Texas (Mendelssohn and Morris, 2001). BRDF data were acquired for two biophysically different canopies in a South Carolina coastal marsh during two different growing seasons (March and October 2000). BRDF spectral reflectance measurements were collected under cloudless sky conditions at one-hour intervals throughout daylight hours using the SFG. The collection of hyperspectral and multiangular BRDF diurnal data for Spartina communities provides insight into how the BRDF behaves in different canopy structures and growing seasons. Hyperspectral BRDF data of Spartina in North Inlet, South Carolina was collected for the purpose of: (a) creating a database of BRDF measurements acquired at multiple Sun zenith and azimuth angles for Spartina alterniflora; (b) comparing the BRDF measurements acquired in two contrasting
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Fig. 3. A. BRDF measurements were collected at two locations in wetlands near the Belle Baruch Marine Field Laboratory, part of the North Inlet–Winyah Bay National Estuarine Research Reserve, located near Georgetown, SC (1994 USGS DOQQ 1 × 1 m. B. Boardwalk site. C. Clambank site (1999 ADAR 5500 0.7 × 0.7 m). The reserve includes 2,630 ha of Spartina alterniflora (Loisel.) marsh and 463 ha of tidal creeks separated from the Atlantic Ocean by barrier islands.
Spartina phenological cycles (March and October); (c) documenting BRDF differences between two biophysically different Spartina canopies; and (d) investigating the physical mechanisms that appear to drive the observed BRDF measurements based on the observed Spartina canopy biophysical characteristics. Once scientists understand the nature of BRDF in coastal environments, they can apply BRDF measurements to imagery for maximum feature extraction. STUDY AREA This research was conducted in the North Inlet–Winyah Bay National Estuarine Research Reserve located on Hobcaw Barony, near Georgetown, South Carolina, 90 km northeast of Charleston, SC (Fig. 3). This area is representative of an undeveloped high-salinity non-riverine estuary that covers about 80 km2 and consists of barrier islands, intertidal salt marsh, and low-lying coastal forest. The reserve includes 2,630 ha of Spartina alterniflora (Loisel.) marsh and 463 ha of tidal creeks separated from the Atlantic Ocean by barrier islands. Other marsh plants within the community
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Fig. 4. Terrestrial views of the study sites representing two biophysically different Spartina canopies acquired in October 1999. A. The high marsh Boardwalk site is characterized as a short, dense canopy (35 cm) with slow tides. B. The low marsh Clam Bank site is characterized as a tall, leafy canopy (100 cm) with rapid tides.
include Spartina patens (Muhl.), Juncus roemerianus (Scheele), Salicornia virginica (L.), and Iva frutescens (L). At mean tide, Spartina alterniflora (Loisel.) marsh comprises 73.0%, tidal creeks 20.6%, oyster reefs 1.0%, and exposed mud flats 5.4% of the low marsh–estuarine zone (Dame and Kenny, 1986). Hydrographic characteristics of the North Inlet estuary include an annual seasonal salinity range of 30–34 ppt, an average channel depth of 3 m, and a seasonal water temperature range of 30°33°C (U.S.E.S, 1997). The BRDF sampling took place at two locations within the North Inlet–Winyah Bay Reserve. These sites were selected based on canopy structure and accessibility for SFG assembly and placement. Terrestrial views of the study sites representing two biophysically different Spartina canopies are shown in Figure 4. The Boardwalk site is characterized as a short canopy (35 cm) located in the high marsh where slow and stagnant tides provide minimal nutrient input. In contrast, the Clam Bank site represents a tall, leafy canopy (100 cm) located in the low marsh where tidal waters rapidly flow nutrients in and out of the stand at regular tidal cycles. METHODOLOGY In Situ Data Collection This research is the first to investigate temporal BRDF patterns as they relate to biophysical canopy changes in intertidal coastal vegetation. It is also the first to document seasonal differences in the BRDF as they relate to changes in the biophysical properties of coastal plant canopies. Data were strategically collected in March and October according to the phenological cycle of Spartina in South Carolina, where peak senescence occurs in March and biomass reaches full growth in October (Cranford et al., 1989; Dame and Kenny, 1989; Jensen et al., 1998). In order to achieve the full benefit of field BRDF data, it was essential to carefully plan and collect detailed in situ biophysical canopy information in order to validate and verify all BRDF measurements. In addition, environmental and atmospheric conditions can alter the structure and orientation of tall vegetation surfaces and thus impact the
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Table 1. Reported in situ Biophysical Measurements for March and October Data Collections from both the Boardwalk and Clam Bank Research Sites Date and location
LAI
March, Boardwalk October, Boardwalk March, Clambank October, Clambank
2.07 2.27 2.02 2.25
Height, Dry weight, Dry weight, Pct. live/ Chlorophyll cm m2 (g/m2) live (g/m2) dead (g/m2) 35 35 90 100
37.72 47.46 223.28 155.61
364.5 68.57 397.03 130.32
9.5 41 36 54
0.95 1.29 1.062 2.22
BRDF patterns. For this reason, air temperature (F°), humidity (%), tide stage (m), and wind where measured at hourly intervals. The Spartina biophysical canopy parameters that were measured in this study included total above-ground biomass (g/m2), canopy height (cm), leaf area index (L), and chlorophyll concentration (mg/g). Total above-ground biomass was harvested within a 0.25 m2 quadrat area based on techniques described in Gross et al. (1987). Canopy height measurements were based on the average height measured from the tidal flat and measured approximately 35 cm at the Boardwalk site while the Clam Bank site measured between 90 and 100 cm. Leaf-area-index (LAI) and chlorophyll concentration are two important biophysical canopy parameters that indicate the structure and health of a plant. LAI measurements for this study were collected using a Decagon AccuPAR™ Ceptometer, which measures photosynthetically active radiation (PAR) (400–700 nm) above and below the canopy (Decagon, 1994). Chlorophyll content was measured from finely ground leaf samples in which the pigment was extracted in 100 ml of 85% acetone (DMSO). Reported biophysical measurements for March and October data collections from both locations are summarized in Table 1. Figure 5 shows an example of the hourly environmental conditions including air temperature, humidity, thermal temperature at the target, Sun zenith angle, and tide level that were measured for the March 22, 2000 collection at the Boardwalk location. A Minolta/Land Cyclops™ 330 thermal gun was used to measure infrared radiation (8– 13 µm) from the target canopy. Goniometer Field Deployment The collection of BRDF data in a marsh environment is logistically challenging; consequently a stable platform had to be constructed to raise the SFG to the level of the Spartina canopy. At each site, six-inch diameter PVC pipes were used to raise the SFG to the height of the canopy. The platform consisted of eight PVC pipes that were placed approximately 1.5 m into the mud and leveled. The SFG has several advantages over existing field goniometers (Turner, 1998). The SFG is a fully automated instrument designed to measure BRDF in 15° ncrements across a half sphere. This is accomplished by using a laptop-controlled GER 3700 spectroradiometer that travels on a belt-driven instrument sled that rides on a half circle truss called the Zenith Arc. The spectroradiometer covers the spectrum between 300 and 2500 nm in 704 bands with a resolution of 1.5 nm (300–1050 nm), 6.2 nm (1050–1840 nm), and 8.6 nm
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Fig. 5. Hourly environmental conditions including air temperature, humidity, thermal temperature at the target, Sun zenith angle, and tide level that were measured for the March 22,2000 collection at the Boardwalk location.
(1950–500 nm), respectively. The Zenith Arc rotates in 15° ncrements on a base ring assembly called the Azimuth Ring. Rotation is accomplished using a motorized drive sled that is driven across a belt on the base ring. A custom foreoptic right-angle telescope lens is connected to the spectroradiometer to decrease shadowing effects that would occur if the instrument body were placed between the Sun and the target (Fig. 6). The instrument sled, which supports the GER 3700, incorporates an alignment plate to allow for fine-tuning of the instrument’s optical path relative to the target. A 0.4-hp brushless DC motor powers the instrument sled that supports the GER 3700. An identical motor that is located on the Base Ring Drive Sled drives the zenith arc. Laboratory tests show the pointing accuracy between –60° and +60° on the Zenith Arc to be ±1 cm (Sandmeier and Itten, 1999). To periodically adjust for any changing atmospheric conditions, a Spectralon® reflectance reference panel measurement is acquired at nadir (0°) during each of the six zenith collection series. Additional technical descriptions of the GER 3700 spectroradiometer calibration procedures and results are outlined in Schaepman (1998). Additional explanations of the mechanical and positioning accuracy and limitations of the SFG can be found in Sandmeier (2000). Target sampling was conducted at regular one-hour intervals throughout the day. Changes in the Sun’s zenith position during the March field campaign ranged from ~12° shift in the early (8:00–9:00 a.m.) and late (4:00–5:00 p.m.) hours, and ~3° shift during the mid-day (12:00–1:00 p.m.) hours. Azimuthal shifts in the Sun’s position were inversely related to zenith changes. They ranged from a ~10° shift in the early (8:00–9:00 a.m.) and late (4:00–5:00 p.m.) hours, and ~40° shift during the mid-day (12:00–1:00 p.m.) hours. In October, the zenith shift was slightly less during the mid-day hours (~2°), but changes in the azimuth direction were much less (~22°). The
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Fig. 6. A custom fore optic right-angle telescope lens is connected to the spectroradiometer to decrease shadowing effects that would occur if the instrument body were placed between the Sun and the target. The instrument sled, which supports the GER 3700, incorporates an alignment plate to allow for fine-tuning of the instrument’s optical path relative to the target (Turner, 1998).
logic of the SFG sampling polar coordinate system is found in Figure 7. The SFG begins data collection in the Solar Principle Plane (SPP), where the Sun and the sensor are aligned in one plane. A series of hotspot readings are first acquired at an angular resolution of 2° and in an angular range of ±10° centered on the hotspot. The collection of BRDF data for this study was acquired at a constant 2-meter distance with a 2° field-of-view of the spectroradiometer. The target area was approximately 10.5 cm at nadir (0°). Field measurements were taken under clear and stable atmospheric conditions. In order to reduce the effect of changing illumination and other environmental conditions that may affect the field BRDF data, sampling occurred while observing the following precautions (Sandmeier, 1999): (1) visible clouds were minimal; (2) no measurements were acquired under hazy conditions or in the presence of cirrus clouds; (3) reflectance-reference measurements were acquired frequently in the course of surface reflectance measurements; (4) measurements were acquired during calm wind days to minimize the effects of canopy orientation displacement. The Anisotropy Factor BRDF data are influenced by the spectral reflectance variability of a target. In order to concentrate on BRDF-related effects, bidirectional reflectance is often normalized to a standard reflectance signature of the respective targets (Jackson et al., 1990; Sandmeier et al., 1998b). The bidirectional reflectance data obtained in this research was processed to a bidirectional reflectance factor (R) and then to an anisotrophy factor (ANIF). R is computed using the following equation:
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Fig. 7. The spectral sampling logic of the SFG polar coordinate system. The SFG begins data collection in the Solar Principle Plane (SPP), where the sun and the sensor are aligned in one plane. A series of hotspot readings are first acquired at an angular resolution of 2° and in an angular range of ±10° centered on the hotspot. The SPP moves throughout the day corresponding to the shift in the Sun’s azimuth angle. After the hotspot has been sampled, 66 hemispherical angles are acquired with resolutions of 15° and 30° in zenith and azimuth directions, respectively (adapted from Sandmeier, 1999, 2000; Jensen and Schill, 2000).
dL r ( θ i, ϕ i ; θ r, ϕ r, λ ) - × R ref ( θ i, ϕ i ; θ r, ϕ r, λ ) R ( θ i, ϕ i ; θ r, ϕ r, λ ) ≈ ---------------------------------------------------------dL ( θ , ϕ ; θ , ϕ , λ ) ref
i
i
r
r
where dLr is the energy reflected from a surface in a specific direction divided by the radiance dLref, reflected from a loss-less Lambertian reference panel measured under identical illumination geometry. The Rref is a calibration coefficient determined for the spectral reflectance panel used. The bidirectional reflectance factor (R) is then normalized to ANIF to analyze the spectral variability in BRDF data. The ANIF allows separation of spectral BRDF effects from the spectral signature of a target. The ANIF is calculated by normalizing bidirectional reflectance data R to nadir reflectance, Ro using the equation (Sandmeier et al., 1998a; Sandmeier and Itten, 1999): R ( θ i, ϕ i ; θ r, ϕ r, λ ) ANIF ( θ i, ϕ i ; θ r, ϕ r, λ ) ≈ ---------------------------------------------R o ( θ i, ϕ i ; λ )
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BRDF Data Visualization The BRDF data collected were plotted in three-dimensional diagrams based on the polar coordinate system described in Figure 7. All three-dimensional graphs were created with Golden Software, Inc. SURFER™ 7.02 statistical software (Golden Software, 2000) and represent the nadir-normalized anisotropy factor values. Linear kriging was used to interpolate between BRDF sample points. When computing the interpolation weights, the algorithm considered the inherent length scale of the data as well as the inter-data spacings. Depending on the range of anisotropy factors, the z-axis had to be adjusted in scale to accommodate the data spread. In order to process the tremendous volume of BRDF data that was generated (e.g., 535,040 bands for a 10-hour collection day), an automated graphing application was developed using Golden Software, Inc. SCRIPTER™ software. The program was designed to read in the positions of the previously computed anisotropy factors and create single-bit (black and white) image files for each of the 704 bands. RESULTS The BRDF data presented in this research is intended to improve our understanding of hyperspectral bidirectional reflectance patterns acquired from two biophysically different salt marsh canopies in two different seasons under a variety of sensor angles and illumination conditions. The four data collection periods include: (1) March 21–22, 2000, Boardwalk Site, 8:00 a.m.–1:00 p.m. and 2:00 p.m.–5:00 p.m.; (2) March 24, 2000, Clam Bank Site, 8:00 a.m.–5:00 p.m.; (3) October 12, 2000, Boardwalk Site, 9:30 a.m.–5:00 p.m.; (4) October 11, 2000, Clam Bank Site, 2:00 p.m.–5:00 p.m. All spectroradiometer data were originally recorded in digital numbers (DN) and subsequently converted to radiance values using band specific calibration techniques found in Sandmeier (1999). These radiance values were then converted to nadir-normalized anisotropy factors that were used to create three-dimensional plots based on the SFG polar coordinates. While three-dimensional plots are extremely useful for visualizing the general BRDF characteristics of a surface, two-dimensional graphs provide a more quantitative analysis since they facilitate reading exact bidirectional factor values (Sandmeier, 2000). Due to the effects of energy absorption by various gases in the atmosphere, the following bands were removed from the analysis: 300– 400 nm, 1750–2080 nm, and 2430–2500 nm. For simplicity, and for the enormous volume of bands to present, several of the two-dimensional graphs were constructed using an aggregated eight-class band classification system to represent the following spectrum ranges: 400–499 nm (blue), 500–599 nm (green), 600–699 nm (red), 700– 759 (NIR), 760–899 nm (NIR), 900–1549 nm (NIR), 1550–1749 nm (SWIR), and 2080–2430 nm (SWIR). A complete listing and discussion of all summarized BRDF univariate statistics can be found in Schill (2001). BRDF Variation from Changing Sensor and Solar Angles Several general patterns were observed throughout all acquired BRDF datasets. A major trend observed at all Sun angles and spectral bands was lower reflectance
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readings in near-nadir view angles and increasing reflectance with increasing offnadir view angles for all azimuth directions. Lower reflectance values at near-nadir angles are the result of viewing lower shaded canopy layers and the viewing of different proportions of layer components at varying sensor view angles. The general pattern observed is an increase of reflectance with lower sensor zenith angles (off-nadir views) and lower Sun zenith angles (Kimes, 1983). This concept is demonstrated in Jensen and Schill (2000), where the most dramatic differences in reflectance occurred in the region from 800 to 1400 nm . Kimes (1983) suggested that reflectance distributions in most canopies tend to be azimuthally symmetric because the leaf transmittance is nearly equal to the leaf reflectance for most wavelengths. This varies depending on canopy structure, optical properties, and leaf orientation characteristics. BRDF Differences Attributed to Phenological Change Several diurnal and seasonal Spartina canopy BRDF patterns are shown in Figure 8. In March, when a majority of the canopy has senesced, the greatest anisotropy factor ranges occur in green and red energy (600–800 nm) (Fig. 8A). During March, the highest level of variance in the visible range occurred at 5:00 p.m. in the red (600– 700 nm) region of the spectrum. However, the greener October canopy exhibited the greater increase in BRDF effects in the early morning hours (Fig. 8B). In addition, there is a tremendous difference in the range of hourly anisotropy factors observed between March and October. In March, the highest anisotropy factor in the visible range was 6.58 measured at approximately 620 nm (red), while the highest October factor was 77.61 at 403 nm (blue). This means that in visible light, senesced Spartina canopy exhibits more BRDF variance in red light, while healthy green canopies have greater differences in blue light. Senesced canopy appears to have minimal BRDF fluxuations in near-infrared energy. The lowest anisotropy factor ranges occurred from 1100–1300 nm. The greatest increase in BRDF response occurred in the late hours of the day as the Sun gradually moved to lower zenith angles. BRDF Variation Attributed to Canopy Differences The dense, more complete canopy at the Boardwalk site resulted in BRDF patterns that were less sporadic and more predictable. In contrast, the Clam Bank site had much less interpretable patterns. The reason for this may be the leafy, sparse canopy which results in more BRDF variability. Stronger backscattering in sparse canopies may be attributed to the appearance of the soil and understory. These effects were minimal due to the dark absorptive characteristics of the marsh mud. In early hours with low Sun zenith, sparse canopy responds much like full canopy, but as the Sun rises, vegetation shadows in the forward scattering direction. As the off-nadir viewing angle increases, more mud is viewed. As the Sun rises, the minimum reflectance area moves away from nadir. This area moves less away from nadir in fuller canopies. The leaf orientation distributions are a primary factor in determining the probability of gap through the canopy as a function of view angle, thus lowering the reflectance due to the gap effect (Kimes, 1983). In addition, the Clam Bank site had fuller, hence more opaque, leaves. The more opaque the leaf, the more azimuthal variation can be observed. This is because the front facet can appear very bright, but the backside of
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Fig. 8. Comparison of Sandmeier Field Goniometer derived hourly maximum, mean, and variance of BRDF anisotropy factors of the aggregated 8 bands (400–2430 nm) for the Boardwalk site. A. March 21–22, 2000. B. October 12, 2000. C. March and October hourly maximum BRDF anisotropy factors.
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Fig. 9. Comparison of hourly three dimensional plots of BRDF data collected at 8 a.m., 9 a.m., 12 p.m., and 4 p.m. at the Boardwalk Site on March 21–22, 2000 for band 624.20 nm.
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Fig. 10. Comparison of three-dimensional plots of BRDF data showing the reflection variation that exists between wavelengths 450 nm, 760 nm, 1250 nm, and 2253 nm collected at the Clam Bank Site on March 24, 2000 at 1:00 p.m.
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the leaf is very dark which leads to a more overall darker appearance of the canopy. Peak reflections were in the SPP and reflectance decreased linearly from this point. Interpreting the Three-Dimensional Graphs The three-dimensional graphs in Figures 10–11 are designed to show the prominence and steepness of the hotspot as well as the distribution and magnitude of forward and backward scattering. When interpreting these graphs, it is important to remember that the curvature and steepness of the bowl is a function of the scattering in the canopy structure and the anisotropic distribution of solar radiance. The effects of BRDF vary throughout the day. Figure 9 compares hourly BRDF data collected at 8 a.m., 9 a.m., 12 p.m., and 4 p.m. at the Boardwalk Site on March 21–22, 2000 for band 624.20 nm. As expected, BRDF variation is greatest in the early and late hours of the day (8 a.m. and 4 p.m.) and minimal near solar noon (12 p.m.). The flattening of the curves as the Sun approaches zenith suggests that the optimal time to avoid the effect of BRDF is during the mid-day hours. The three-dimensional graphs are also useful for visualizing the changing forward and backward scattering that occurs throughout the day. Forward scattering occurred to a greater degree at the Boardwalk site where the canopy was dense, short, and level. Backward scattering was more pronounced at the Clam Bank site where the canopy was tall, leafy, and structured irregularly. In addition, asymmetrical forward and backward scattering at the Clam Bank site can be attributed to the opaqueness of the leaves and the sensor viewing the understory and mud through the sparse canopy. BRDF also varies by wavelength. Figure 10 compares the spectral reflection variation that exists between wavelengths 450 nm, 760 nm, 1250 nm, and 2253 nm collected at the Clam Bank Site on March 24, 2000 at 1:00 p.m. In general for vegetation canopies, BRDF variation is greater in the visible and short-wave infrared portions of the electromagnetic spectrum. Lower effects were measured in the near-infrared region. Leafs in general reflect high amounts of IR and lower amounts of visible light. This leads to higher scattering in the IR, which makes the distribution more azimuthally symmetric. Lower scattering in the visible makes vegetative canopies prone to impacts associated with the BRDF. In contrast, soils reflect lower amounts of IR because of high leaf reflectance and transmittance, and higher amounts of visible light. Sparser canopies tend to have more pronounced hotspots, particularly for relatively small solar zenith angles. In general, soils have a much lower hotspot and the diffuse radiation tends to decrease azimuth variations. While 1100–1300 nm appears to be least affected by the impacts of BRDF, the range between 2080 and 2430 nm, although noisy and less predictable, tended to be the most impacted. Consistent with the literature, the lowest variances throughout the hemispherical sampling corresponded to the mid-day hours when the Sun was at zenith (Kribel, 1978; Kimes, 1984, Deering et al., 1999; Solheim et al., 2000). CONCLUSIONS This research establishes the first hyperspectral and multiangular resolution BRDF database of Spartina alterniflora and provides remote sensing scientists with insight into maintaining estuarine health that is threatened by impending urban
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sprawl. It is hoped that this database will lead to the development of new image processing techniques to be used in identifying indicators of stress to mitigate damage in wetlands and other environments (EPA, 1994). The need to develop better baseline maps for future environmental investigations requires quantitative biophysical information about the species or habitats through time (Jensen et al., 1998). The spectral data derived from BRDF modeling provides unique information about the structure of a canopy. By modeling the spectral reflectance of a canopy at varying look and Sun angles, scientists will better understand how light responds to certain biophysical characteristics and have a foundation that will enhance classification accuracy. In addition, this research fills a need among the BRDF community to increase the in situ validation studies. The quantitative assessment of canopy characteristics that was performed for both study sites provides insight into biophysical mechanisms that may be altering the BRDF response. Further examination of these relationships may lead to the discovery of previously unknown underlying physiological processes that contribute to the BRDF. For example, the BRDF hotspot, bowl-shape, etc. may have a unique pattern that can routinely and unambiguously be retrieved from directional reflectance measurements and may relate directly to certain key biophysical properties that cannot be derived by other means. Another gap this researcresearch fulfills is the increased the knowledge of vegetation BRDF variation in two contrasting periods of phenological change. While the greater percentage of BRDF research has been focused on characterizing and modeling BRDF patterns between plant species, this study has clearly quantified the differences using a hyperspectral approach. Knowledge gained in this research also builds on the few studies that exist on the BRDF in vegetation canopies with erectophile leaf orientation. The majority of the BRDF studies that have been conducted have been with vegetation canopy with planophile leaf orientation. Few studies have documented reflection patterns in erectophile canopy. In summary, the BRDF measurements acquired for various canopy structures of Spartina during periods of seasonal biomass flux are the first of its kind in many aspects and a foundation for future investigations. The immediate future need in BRDF research is how best to apply the BRDF information for radiometric image normalization. The accurate computation of surface albedo will make possible corrections to reflectance measurements of features, whether measured from nadir or off-nadir pointing remote sensing systems. Empirical modeling of the BRDF data collected in this research will enable remote sensing scientists to identify bands that are least impacted by BRDF, recognize optimal Sun/ sensor angle-of-views, and provide insight into radiometrically adjusting remotely sensed data to minimize BRDF effects. Additional suggestions for increasing the diffusion of BRDF research include: • The development of user-friendly software tools that permit users to analyze and investigate BRDF data of any coastal surface under a variety of illumination and biomass conditions—for example, tools that permit angle-induced error correction, the achievement of higher levels of land cover classification, and identification of canopy changes in coastal vegetation in remotely sensed imagery. The end result will be better, more accurate mosaics and land cover classifications.
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• More BRDF concepts and case studies need to be published in remote sensing textbooks so those lacking mathematical backgrounds can better understand the concepts. In addition, web-based resources that tutor and educate users on BRDF concepts and procedures need to be developed. These resources could be used for image radiometric correction as well as providing access to an archive database of BRDF surfaces for analysis and evaluation. By improving the process of communicating BRDF research results, those outside the bidirectional reflectance research community will clearly understand its benefits. • Faster and more efficient algorithms are needed to allow useful parameters to be obtained from directional data, including improved calibration and atmospheric correction algorithms. ACKNOWLEDGMENTS This research was conducted from funding provided by the NASA EPSCoR and Space Grants from NASA Stennis Space Center. Technical assistance was provided by Stefan Sandmeier and NASA Stennis Space Center’s Ground Reference Information Team (GRIT). Additional field support was provided by Ben Jones, Laura Schmidt, Gunnar Olson, and Jason Tullis. REFERENCES Cranford, P. J., Gordon, D. C., and C. M. Jarvis, 1989, “Measurement of Cordgrass, Spartina alterniflora, Production in a Macrotidal Estuary, Bay of Fundy,” Estuaries, 12(1):27-34. Dame, R. F. and P. D. Kenny, 1986, “Variability of Spartina alterniflora Primary Production in the Euhaline North Inlet Estuary,” Marine Ecology, 32:71-80. Decagon Devices, Inc., 1994, AccuPAR Operator’s Manual, Version 2.1, Pullman, WA: Decagon Devices, Inc. Deering, D. W., 1999, “Introduction to BRDF and PARABOLA” [http://parabola-web.gsfc.nasa.gov/parabola/]. Deering, D. W., Eck, T. F., and B. Banerjee, 1999, “Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer,” Remote Sensing of Environment, 67:205-229. Demircan, A., Schuster, R., Radke, M., Schonermark, M., and H. Roser, 2000, “Use of a Wide Angle CCD Line Camera for BRDF Measurements,” Infrared Physics & Technology, 41:11-19. EPA, 1994, Landscape Monitoring and Assessment Research Plan, Washington, DC: EPA, EPA/620/R-94/009, 53 p. Gemmell, F. and A. McDonald, 2000, “View Zenith Angle Effects on the Forest Information Content of Three Spectral Indices,” Remote Sensing of Environment, 72:139-158. GER, 1997, GER-3700 Spectroradiometer User’s Manual, Version 2.0, Millbrook, NY: Geophysical Environment Research Corp., 59 p. Golden Software Inc., 2000, SURFER Version 7.02 On-Line Help, Golden, CO: Golden Software Inc.
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