structure analysis and classification of boreal forests ... · structure analysis and...

Structure Analysis and Classification of BorealForests Using Airborne HyperspectralBRDF Data from ASAS

St. Sandmeier* and D. W. Deering†

A new approach is presented for deriving vegetation INTRODUCTIONcanopy structural characteristics from hyperspectral bidi- Several climate simulation models indicate that therectional reflectance distribution function (BRDF) data. northern latitudes will experience a significant tempera-The methodology is based on the relationship between ture increase as a result of increasing levels of atmo-spectral variability of BRDF effects and canopy geome- spheric CO2 (Schlesinger and Mitchell, 1987). Withtry. Tests with data acquired with the Advanced Solid- global warming, species composition, structure, and phe-State Array Spectroradiometer (ASAS) over Canadian nology of boreal forests could potentially change theirboreal forests during the BOREAS campaign show that carbon balance. Due to their large spatial extent and bio-vegetation structural characteristics can be derived from mass, such changes could ultimately impact the globalthe spectral variability of BRDF effects. In addition, the carbon cycle (Myneni et al., 1997). Thus, it is crucial thatincorporation of both BRDF effects and hyperspectral we improve our understanding of the ecological functionresolution data substantially improve the classification of the boreal biome and carefully monitor changes in theaccuracy. Best classification results are obtained when extent and structure of this forest. Considerable successhyperspectral resolution and BRDF data are combined, in mapping forest cover characteristics has been achievedbut the improvement is not consistent for all classes. For in recent years through the use of remote sensing tech-example, adding BRDF information to hyperspectral data niques (Meyer et al., 1993; Hall et al., 1997; Steyart etincreases the overall classification accuracy for a six-class al., 1997), but most of these studies relied on nadir-viewfen site from 37.8% to 44.7%. The addition, however, re- multispectral reflectance data only and have largely ne-duces the accuracy for the jack pine class from 43.6% to glected bidirectional reflectance effects. Field data of the28.8%. These new findings provide evidence for im- bidirectional reflectance distribution function (BRDF) ofproved capabilities for applications of MISR and MODIS boreal forests have shown that the structure of a forestdata. The spectral resolution of MODIS is expected to be canopy is related to its BRDF characteristics (Deering etsufficient to derive canopy structural information based al., 1999), and modeling studies have revealed a strong

relationship between vegetation canopy architecture andon the spectral variability of BRDF effects, and for MISRthe hot spot phenomenon (Qin and Xiang, 1994). Never-a significant improvement of classification accuracies cantheless, BRDF data have not been well utilized for deriv-be anticipated from the combination of nadir reflectanceing canopy structure parameters. Only very recently hasand off-nadir data. Elsevier Science Inc., 1999the information content of remotely sensed BRDF databeen examined (Barnsley et al., 1997; Russell et al. 1997;Abuelgasim et al., 1996). Hyperspectral remote sensing

*USRA/NASA Goddard Space Flight Center, Biospheric Sciences imagery and processing techniques have also been usedBranch, Greenbeltfor forest applications (e.g., Martin et al., 1998), but the†NASA Goddard Space Flight Center, Biospheric Sciences

Branch, Greenbelt potential for combining both high spectral resolution andAddress correspondence to St. Sandmeier, NASA Goddard Space BRDF information has not yet been explored, to our

Flight Center, Biospheric Sciences Br., Code 923, Greenbelt, MD 20771. knowledge, mainly due to a lack of adequate data.E-mail: [email protected] 23 November 1998; revised 10 March 1999. In this study, we use airborne hyperspectral BRDF

REMOTE SENS. ENVIRON. 69:281–295 (1999)Elsevier Science Inc., 1999 0034-4257/99/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(99)00032-2

282 Sandmeier and Deering

data from the Advanced Solid-State Array Spectroradi- tor and Rmin is the minimum bidirectional reflectanceometer (ASAS) (Irons et al., 1991) in an attempt to 1) factor.explore the potential of hyperspectral BRDF data for de- Both quantities, ANIF and ANIX, are naturally de-riving forest canopy structure parameters and 2) evaluate pendent on the instantaneous field-of-view of the sensor,the ability of hyperspectral BRDF data to be used for the sampling resolution of the BRDF data, and the qual-boreal forest classifications. The methodology applied is ity of the image registration. The angular sampling reso-based on laboratory and field goniometer results that lution defines the completeness of the BRDF character-have shown the distinct spectral variability of the BRDF ization. Broad sensor field-of-views tend to smooth theeffect for erectophile grass and planophile watercress BRDF effect, particularly in the hot spot where reflec-surfaces (Sandmeier et al., 1998; Sandmeier and Itten, tance intensity can change rapidly within a few degrees1999), and has proven useful for structure characteriza- of viewing angle. Geometrical misregistration and imagetion of prairie grasslands (Sandmeier et al., 1999). The distortions deteriorate ANIX data since they are a ratiodata analyzed were acquired in July 1994 in the southern of two different view-angle images. Thus, comparisons ofstudy area of the Boreal Ecosystem–Atmosphere Study ANIF and ANIX data derived from different sensors and(BOREAS) in Saskatchewan, Canada (Sellers et al., viewing-illumination geometries must be performed with1997) and include BRDF characteristics of various conif- care. Within a single multiangle scene, however, the twoerous and deciduous trees. quantities allow a direct comparison of the BRDF effect

and its spectral variability for various land cover classes,as long as the data are geometrically well registered.METHODOLOGY

The physical mechanisms underlying the relationship be-DATA SET AND TEST SITEStween hyperspectral BRDF data and vegetation canopy

structure parameters are described in detail by Sand- Test Sitesmeier et al. (1998). In short, they are due to multiple Three test sites are selected from the southern BOREASscattering effects which determine the contrast between study area in Saskatchewan, Canada. They are associatedshadowed and illuminated components of a vegetation

with flux tower locations at the Old Black Sprucecanopy and therefore strongly influence the BRDF char-(OBS) (538599N/1058079E), Old Aspen (OA) (538389N/acteristics. Since multiple scattering effects are depen-1068129E), and Fen (FEN) (538489N/1048379E) sites, lo-dent on the canopy absorbance characteristics, BRDF ef-cated in the general vicinity of Candle Lake and Princefects are spectrally variable. In erectophile vegetationAlbert. The OBS site is a mixture of treed muskeg, a wetcanopies, the spectral BRDF variability is most obviousspongy land saturated and sometimes partially covereddue to strong BRDF effects. In planophile canopies,with water, black spruce (Picea mariana Mill.), and jackhowever, BRDF effects are relatively small and the spec-pine (Pinus banksiana Lamb.) with mostly moss andtral BRDF variability is limited. Consequently, the extentsome lichens in the understory. Trees are between 7.5of spectral variability of BRDF effects is related to them and 12.5 m high and .100 years old. The crown clo-canopy architecture and thus might be expected to pro-sure is .55%, and tree density is 3700–5800 stems/havide a measure of this important structural parameter.(Lindenas, 1985; Chen et al., 1997a). Based on opticalTwo quantities are used to derive structure informa-measurements with an LAI-2000 plant canopy analyzer,tion from hyperspectral BRDF data: The first is the an-the leaf area index (LAI) is estimated at 4.261.0 for theisotropy factor (ANIF), which is simply a normalizationold black spruce stands (without moss) for midsummerwith nadir reflectance [Eq. (1)]:1994 (Chen et al., 1997a).

The OA site is a broadleaf deciduous forest withANIF(k,hi,ui,hr,ur)5R(k,hi,ui,hr,ur)

R0(k,hi,ui)[dimensionless], (1)

trembling aspen (Populus tremuloides Michx.) in the pri-mary stratum and hazelnut shrubs (Corylus americana)where R is the bidirectional reflectance factor, R0 is thein the understory. The aspen are between 13 m and 21mnadir reflectance factor, k is wavelength, h is zenithhigh, have a moderate density of 900 stems/ha, and rangeangle, u is azimuth angle, i is illumination direction, andin age between 50 years and 100 years (Padbury et al.,r is viewing direction.1978; Chen et al., 1997a). The LAI estimated for 21 JulyThe second is the anisotropy index (ANIX) defined as1994 (the date of measurements presented in this study)the ratio between the maximum and minimum reflec-determined with an LAI-2000 instrument was 2.30 fortance values in the principal plane (or defined azimuththe aspen and 3.23 for the hazelnut, resulting in a siteplane) per spectral band [Eq. (2)]:total LAI of 5.53 (Chen et al., 1997b).

ANIX(k,hi)5Rmax(k)Rmin(k)

[dimensionless], (2) The FEN site consists of a variety of coniferous anddeciduous species surrounding a muskeg area. The per-cent coverage for each dominant cover class in the ASASwhere Rmax is the maximum bidirectional reflectance fac-

Forest Structure Analysis 283

Figure 1. Percentage of A) crown closure (density) and B) tree height for four dominant species associations in the Fen testsite. The data are derived from the Saskatchewan Environment and Resource Management 1:12,500-scale forest cover mapsand are based on field reconnaissance and aerial photographs taken as recently as 1988.

test site according to Lindenas (1985) is: 13% black species in the three test sites. The tree heights areadapted to the conditions in the FEN and OA sites. Forspruce (Picea mariana Mill.), 14% jack pine (Pinus

banksiana Lamb.), 8% tamarack (Larix laricina), 25% further clarification of the boreal forest structure charac-teristics, we refer to the photographs of black spruce,spruce/pine (a mixture of black spruce and jack pine),

13% treed muskeg, and 27% clear muskeg. The crown jack pine, and trembling aspen stands in the BOREASsouthern sites included in Deering et al. (1999).closure and tree height of the forest stands in the FEN

site are heterogeneous (Fig. 1). Most of the black sprucestands are between 10 m and 15 m high and provide a Forest Cover Mapscrown closure of .80%. The relatively young jack pine The forest cover maps are a digital version of thestands are not as tall and dense as black spruce, but the 1:12,500-scale forest cover polygon maps generated andmixed category “spruce/pine” contains tree heights of maintained by the Saskatchewan Environment and Re-about 15 m and a coverage between 30% and 100% source Management, Forestry Branch—Inventory Unit.which results in a rather variable appearance for jack They were primarily produced for forest managers forpine stands. Tamaracks are the only deciduous species in silvicultural purposes (Lindenas, 1985). The maps arethe FEN site and are about 10 m tall and fairly dense given in Universal Transverse Mercator (UTM) projec-with a crown closure .80%. The clear muskeg area is a tion and are based on stereographic interpretation offen and exhibits marsh vegetation comprising various black-and-white 1:12,500-scale infrared aerial photogra-sedge and herbaceous species as well as mosses (Suyker phy and field reconnaissance notes taken as recently aset al., 1997). 1988. In addition to land cover types, they offer a wealth

Figure 2 illustrates the structures of the dominant of information about crown closure, tree height and age,and understory composition. The original vector cover-ages have been resampled with a nearest neighbor tech-

Figure 2. Tree structure of dominant species in the nique to a raster grid with a resolution of 4 m34 m,OA, OBS, and FEN test sites. The height of the treescorresponding to the spatial resolution of the ASAS data.are adapted to the conditions in the FEN site (source:Due to logging activity and other disturbances, such asParish, 1994).fires between 1988 and 1994, some areas in the ASASdata legitimately appear differently than the maps. In theFEN site, one obvious clear-cut area was manually up-dated here according to aerial photographs.

ASASASAS is an airborne pushbroom imaging spectroradiom-eter capable of acquiring off-nadir imagery from approxi-mately 708 forward to 558 backward viewing directionsalong the flight direction. Most BOREAS study siteshave been imaged in the principal, orthogonal, and


Table 1. Specifications of ASAS, POLDER, and PARABOLA Data Sets

Sun Rel. ViewSensor Type Test Site Date Zenith View Zenithsa Azimuthb

ASAS OBS 21 July 94 33.68 1458, 1268, 08, 2268, 2458, 2558 176.58

ASAS OA 21 July 94 34.58 1458, 1268, 08, 2268, 2458, 2558 177.38

ASAS FEN 21 July 94 39.98 1558, 1458, 1268, 08, 2268, 2458, 2608 172.18

POLDER OBS 7 June 94 348 1518 to 2518, continually 1808

PARABOLA OBS 7 June 94 35.18 1608 to 2608, every 158 1808

PARABOLA OBS 21 July 94 40.58 1608 to 2608, every 158 1808

a Positive angles are in the backward scatter direction; negative angles are in the forward scatter direction.b View azimuth angle relative to principal plane, with the sun at 1808.

oblique azimuth planes from eight different view zenith spatial resolution is gradually decreased. At 608 view ze-nith angle, the pixel size across-track was about 7 m. Theangles: 1708, 1608, 1458, 1268, nadir, 2268, 2458, and

2558 where positive numbers are usually in the back- along-track nonoverlapping pixel size was about 3 m forall view angles, varying with aircraft speed (Russell et al.,ward scatter direction, and negative numbers are in the

forward scatter direction. Data were acquired in 62 spec- 1997). All ASAS data were resampled to a pixel size of4 m34 m with a rubber sheet approach using polynomialtral bands ranging from 404 nm to 1023 nm. Each band

had a spectral resolution of approximately 10 nm full- functions of second to fifth degree. To preserve the orig-inal data values, a nearest-neighbor resampling techniquewidth–half-maximum. The analog signals were digitized

and stored on tapes with a resolution of 12 bits. was applied. A parametric geocoding approach could notbe applied since the required aircraft navigation dataDue to sensor uncertainties, only Bands 14–52 are

used in this study. These bands cover a spectral range of were not collected.For the OBS and FEN test sites, the nadir ASAS530–919 nm. Since BRDF effects are most pronounced

in the solar principal plane, only measurements from this images were first rectified to the forest cover map inUTM. In a second step, the off-nadir images were re-plane were analyzed. The viewing and illumination con-

ditions of the ASAS data sets used are summarized in sampled to the georectified nadir data. In the OA site,the forest cover map did not provide enough unique de-Table 1. Some images evidenced strong geometrical dis-

tortions and had to be excluded. tails to be used as a base map for geometric adjustment.Thus, off-nadir images were directly rectified to the geo-The instantaneous field-of-view of ASAS center de-

tectors is 0.66 mrad (0.0388) across-track and 0.44 mrad metrically uncorrected nadir imagery. Between 20 and40 ground control points were selected in each ASAS im-(0.0258) along-track. During BOREAS, ASAS was flown

at an altitude of about 5500 m above ground, resulting age, showing an overall RMS error between 0.5 and 1pixels in x- and y-directions. Prior to the geometrical cor-in an across-track ground pixel size of approximately

3.7m at nadir. With increasing view zenith angles, the rection, ASAS data were atmospherically corrected by

Figure 3. Anisotropy factors (i.e., nadir-normalized reflectance) versus view zenith angle for various spectral bands acquiredin the solar principal plane over A) spruce/pine stands in the Old Black Spruce (OBS) site and B) aspen in the Old Aspen(OA) test site. Solar zenith angles and dates of acquisition for the three data sets from ASAS (curves a–c), PARABOLA(curves d–e), and POLDER (curves f–h) are summarized in Table 1.


surface reflectance in visible and near-infrared bands. Itconsists of a wide-angle lens CCD camera which cap-tures BRDF effects of homogenous surfaces with aunique angular sampling resolution of about 0.48 (Breonet al., 1997). POLDER data acquired during BOREASfrom a stabilized helicopter platform at approximately300m altitude were used for exploring the spectral vari-ability of BRDF effects and for comparison with ASASdata. Due to the low flight altitude, the data were notatmospherically corrected. Unfortunately, measurementswith solar zenith angles close to the ASAS data acquisi-tion were only available for the OBS test site. A rigorouscomparison with the ASAS data was further hamperedby a time gap in data acquisition (Table 1).

PARABOLAData from the Portable Apparatus for Rapid Acquisitionof Bidirectional Observations of Land and Atmosphere(PARABOLA) are included in this study to allow com-parisons between airborne- and ground-based BRDFmeasurements. Mounted on a tram 13–14 m above the

Figure 4. ASAS imagery acquired over the Old Aspen test site canopy height, PARABOLA provided a unique series ofon 21 July 1994 under a solar zenith angle of 34.58 from 268 BRDF data for two of the BOREAS sites investigated inbackscatter view zenith angle (i.e., close to the hot spot

this study. The instrument is a three-band, motor-drivendirection) for four different spectral bands A) 560 nm, B)multidirectional radiometer that permits BRDF data ac-671 nm, C) 712 nm, and D) 794 nm. The flight line differs

from the solar principal plane by only 2.78. Note that the hot quisition with a sampling interval of 158 and 308 in viewspot effect which appears as a “white line” along the flight zenith and azimuth directions, respectively. Due to thedirection in the right part of the images is most pronounced rather broad field-of-view of 158, the hot spot feature isin the red band (B).

smoothed (Deering et al., 1999). In both the OA andOBS test site, a direct comparison between PARABOLA

ASAS staff personnel using the 6S code (Vermote et al., measurements and ASAS data is limited because of dif-1997). Clouds present in the FEN site were masked and ferent solar zenith angles and acquisition dates (Table 1).excluded from the analysis.

Aerial PhotographsPOLDER Aerial photographs taken with a Dual Zeiss Camera us-Polarization and Directionality of the Earth’s Reflectance ing color-IR film (Aerochrome IR 2443, 510–900 nm)

were collected from a C-130 aircraft. These photographs(POLDER) is an optical sensor designed to observe the

Figure 5. Anisotropy factors (i.e., nadir-normalized reflectance) of various view zenith angles versus wavelength, averaged forA) spruce/pine stands in the Old Black Spruce site and B) aspen in the Old Aspen test site. The data were acquired fromASAS on 21 July 94 under solar zenith angles of 33.68 and 34.58 for A) and B), respectively.


Figure 6. Anisotropy factors (i.e., nadir-normalized reflectance) of various view zenith angles plotted versus nadir reflectancefor A) spruce/pine stands in the Old Black Spruce site and B) aspen in the Old Aspen test site. Each line combines datafrom multiple spectral bands acquired from ASAS on 21 July 94 under solar zenith angles of 33.68 and 34.58 for A) and B),respectively. The anisotropy factor data correspond to Figure 5.

were obtained on 21 July 1994 at a scale of approxi- tively reduce the contrast between shadowed and illumi-mately 1:22,000 and provide high resolution spatial docu- nated canopies components which results in low BRDFmentation of the condition of the primary BOREAS effects (Sandmeier et al., 1998). In addition, BRDF ef-study sites at the time of ASAS data collection. To en- fects are most pronounced in erectophile canopies if soil/hance the analysis, the photographs were digitized and background influences are negligible, and are reduced ingeometrically rectified to the corresponding forest planophile surfaces. Figure 3 confirms both assumptionscover maps. through nadir-normalized reflectance values, that is, an-

isotropy factors, averaged for A) spruce/pine stands ac-cording to the forest cover map in the Old Black SpruceRESULTS AND DISCUSSIONsite (OBS) and B) aspen stands in the Old Aspen (OA)

Structure Analysis test site. For all three sensor types involved, BRDF ef-fects were most pronounced in the red band and showedAccording to the physical mechanisms in vegetation can-lowest dynamics in the near-infrared range. Differencesopies, BRDF effects are particularly strong in spectralin the amplitude and shape of the hot spot phenomenonranges of high absorbance such as the visible blue andin the OBS site (Fig. 3A) are mostly due to the variablered chlorophyll absorbance bands. In highly reflective

near-infrared bands, multiple scattering processes effec- angular sampling resolutions and field-of-views of the

Figure 7. (A) ASAS nadir reflectance and B) anisotropy index data versus wavelength for the three dominant cover types inthe OBS test site. Error bars show 60.5 standard deviation for black spruce, spruce/pine, and treed muskeg for representa-tive spectral bands.


Figure 8. (A) ASAS nadir reflectance and B) anisotropy index data versus wavelength for six dominant cover types in theFEN test site. Error bars show 60.5 standard deviation for black spruce, jack pine, and clear muskeg for representativespectral bands.

sensors (see the previous section). In the OA test site aspen trees (plate B). Furthermore, the relationship be-tween BRDF effect and nadir reflectance intensity is(Fig. 3B), the BRDF characteristics in the ASAS and

PARABOLA data are quite consistent, except for the hot shown in Figure 6, incorporating data from multiplespectral bands. Both figures are in good agreement withspot positions which vary due to different sun zenith

angles of 358 and 408 (see Table 1). laboratory experiments of small-scale samples of erecto-phile grass and planophile watercress (Sandmeier et al.,The spectral variability of the BRDF effect is also

captured in the ASAS imagery of the OA site (Fig. 4). 1998). As presented earlier, the old black spruce andspruce/pine canopies show a much stronger BRDF effectThese data were acquired with a deviation from the prin-

cipal plane of ,38 (Table 1). The hot spot effect, visible and a much higher spectral variability than the aspendata set, and in both test sites, BRDF effects are highlyas a bright stripe along the flight direction of the push-

broom scanner, is best observed in the red band (plate B, related to nadir reflectance intensity.The potential utility of ANIX data is clearly evident671 nm) and decreases rapidly in the transition to the

near-infrared range (plate C, 712 nm; plate D, 794 nm). in Figure 7, where they are compared with nadir reflec-tance signatures for the dominant species in the OBSIn Figure 5, the spectral variability of BRDF effects

are summarized for the various view angles and are com- test site. Note that the separability of vegetation typesspectrally and anisotropically differ for different spectralpared for the erectophile canopies of black spruce and

spruce/pine (plate A) and the more planophile deciduous regimes. In the reflectance data, the two rather differenttreed muskeg and black spruce categories are moreclosely related to each other than the forest categories

Figure 9. ASAS anisotropy index (ANIX) data versus typed as black spruce and spruce/pine. This is most likelynadir reflectance for six dominant cover types in the due to the strong influence of the soil/background signalFEN test site. Each line combines data from 19 spectral

on nadir imagery. The understories in the treed muskegbands ranging from 530 nm to 919 nm (Bands 14–52).and black spruce stands are moss-dominated whereas inthe spruce/pine stands the vegetative ground cover con-sists of lichen that is considerably less dense. Further-more, the soil in the spruce/pine stands tends to drainmore (Peck et al., 1997). In the ANIX data, the soil/back-ground signal is less important and the canopy reflec-tance characteristics are emphasized in the oblique viewdirections comprised in ANIX. As a result, the ANIX sig-natures for both spruce/pine and black spruce stands arevery similar and show strong spectral variability, indicat-ing a pronounced erectophile canopy structure (see thesecond section and Fig. 2). The treed muskeg, however,differs markedly from the forest categories and exposesa weaker spectral variability, which suggests a less erecto-phile canopy structure than found in the mature forest


stands. The same conclusion can be drawn from exami- cover classes is also demonstrated in the backward andnation of the corresponding ANIX imagery in Figure forwardscatter view directions of the FEN site (Fig. 11C/10B, where the muskeg area appears much darker, that D). The clear muskeg appears rather dark in the back-is, more planophile than the forest stands. ward scatter direction (plate C), but very bright in the

Considering the FEN site, the ability of ANIX data forwardscatter image (plate D) due to the high waterto determine canopy structure is even more apparent. In content in the fen. The forested areas, especially blackthe nadir reflectance signatures (Fig. 8A), the structure spruce, exhibit fairly strong backward scatter characteris-of the six land use covers can hardly be distinguished. In tics and are bright in the backward scatter view (platethe ANIX data (Fig. 8B), however, the shorter wave- C), but are dark in the forwardscatter direction (platelength canopy reflectance characteristics dominate in the D). Clear-cut areas and roads expose relatively isotropicanisotropy, and three distinct groups of canopy structures reflectance characteristics and are bright in all viewingcan be identified. The most erectophile canopy struc- directions. Since the backward scatter image in the FENtures are found in the black spruce (curve a) and spruce/ site (plate C) is taken close to the hot spot direction,pine stands (curve d); mediocre erectophile geometries shadows are almost invisible, and the image appearsare present in jack pine (curve b), tamarack (curve c), bright and relatively low in contrast. In the forward scat-and treed muskeg (curve e); and a generally planophile ter direction (plate D), between-crown and within-crownstructure exists for the clear muskeg (curve f). These shadows are emphasized, and structural informationfindings are in good agreement with the tree structures about the land cover is revealed.represented in Figure 2. The conical geometry of spruceleads to an erectophile canopy geometry, whereas jack Classificationpine and the deciduous tamarack have a lower canopy Three supervised classification techniques are deployedgap fraction due to their crown shapes. Even though the in this study to examine the potential of BRDF and highcrown dimensions of jack pine are generally smaller than spectral resolution data: the maximum likelihood classi-those of tamarack, both species seem to form a similar fier (MLC), the minimum distance algorithm (MINDIS),canopy structure and cannot be distinguished from each and the spectral angle mapper (SAM). Training samplesother based on ANIX data. Similarly, treed muskeg areas

for all classifiers were taken from the forest cover maps.in the FEN site are difficult to distinguish from tama-Except for areas affected by clouds, all ground referencerack, most likely because this species dominates thedata were included in the calculation of the class sta-treed muskeg (Suyker et al., 1997).tistics.In Figure 9, the ANIX data presented in Figure 8B

MLC and MINDIS are purely statistical approachesare plotted versus nadir reflectance intensity. Each lineand have been widely used in multispectral remote sens-combines data from 19 spectral bands ranging from 530ing applications. The MLC calculates the probability thatnm (Band 14) to 919 nm (Band 52). For clarity, the orig-a given pixel belongs to a specific class based on the as-inal data are substituted by sixth degree polynomial func-sumption that the multispectral data associated with eachtions with r2.0.92, except for clear muskeg, whereland cover class are Gaussian-distributed. The MINDISr250.76. As in the OBS and OA test sites (Fig. 6), theclassifier uses the shortest Euclidean distance to classANIX characteristics of the various land cover classes aremeans rather than the maximum likelihood and does notcomparable to the corresponding laboratory measure-rely on Gaussian distribution.ments and demonstrate an impressive relationship be-

In the OBS site, results from Kolmoroff–Smirnofftween the spectral variability of BRDF effects and nadir(KS) tests indicate that nadir reflectance and ANIX datareflectance (Sandmeier et al., 1998).for all three land cover types (black spruce, spruce/pine,Figures 10 and 11 illustrate the different informationand treed muskeg) exhibit Gaussian distributions incontained in nadir and oblique view directions. In com-ASAS Bands 17, 28, and 50 (significance level of 0.01).parison to off-nadir views, nadir images (Figs. 10A andLikewise, in the FEN site, all nadir reflectance and most11A) generally appear sharper due to the higher geomet-of the ANIX data of ASAS Bands 17, 28, and 50 show arical resolution. Similar to nadir-view aerial photographsGaussian function (significance level of 0.001), but the(Figs. 10C and 11B), ASAS nadir images allow insightANIX data for tamarack and clear muskeg in Bands 17into the canopy understory, but the derivation of struc-and 28 cannot be considered a Gaussian distribution. Wetural information would be difficult without stereoscopicused the MLC knowing that some limitations may occur.images. However, in the ANIX imagery (Figs. 10B and

Compared to MLC and MINDIS, SAM is a rather11E), pixel brightness can be directly related to the can-new classification technique. It has specifically been de-opy structure: bright areas indicate erectophile canopysigned to take advantage of high spectral and hyperspec-structures, such as black spruce and spruce/pine areas,tral resolution data (Kruse et al., 1993). The similarityand dark areas are associated with planophile structuresbetween a test and a reference spectrum is measured bysuch as clear muskeg.

The distinct BRDF characteristics of boreal land the angle between the two spectra, treating them as vec-


Figure 10. False color images acquired over the Old Black Spruce site on 21 July 1994: A) ASAS Bands 17 (560 nm), 28(671 nm), and 50 (898 nm) obtained from nadir view under a solar zenith angle of 33.68; B) ANIX data (560 nm, 671nm, and 898 nm) derived from 458 backscatter and 458 forward scatter directions; and C) nadir view photograph of a DualZeiss Camera using a color-infrared film (510–900 nm). Plates D) and E) show results from a minimum distance classifi-cation using Bands 17, 28, and 50 of nadir reflectance (D) and a combination of nadir reflectance and ANIX data (E),respectively. Plate F) depicts the Saskatchewan Environment and Resource Management 1:12,500-scale forest cover map,which is based on field reconnaissance and aerial photographs taken as recently as 1988. The OBS flux tower is located inthe spruce/pine stand in the center of the image.

tors in a space with dimensionality equal to the number cept is that SAM is not able to distinguish between thespectra of two land cover types if they differ by a con-of bands. SAM focuses on the “shape” of the spectra and

neglects the overall intensity in order to minimize influ- stant reflectance value for all wavelength bands. Two dif-ferent sets of spectral data were selected to investigateences of different illumination. A drawback of this con-


Figure 11. Comparison of A) nadir image from ASAS, B) nadir view aerial photograph, C) ASAS 458 backscatter view,D) ASAS 458 forward scatter view, and E) anisotropy index (ANIX) data derived from a ratio of ASAS images C) and D).F) shows the corresponding forest cover map provided by the Saskatchewan Environment and Resource Management. PlatesA), C), D), and E) are false color composites based on ASAS Bands 17 (560 nm, shown in blue), 28 (671 nm, shown in green),and 50 (898 nm, shown in red). The aerial photograph in plate B) was taken with a Dual Zeiss Camera using an infrared film(510–900 nm). Clouds and cloud shadows visible in the right part of the images have been excluded from the image analysis.The flux tower is located in the clear muskeg area near the center of the image.


Table 4. Accuracy of a Minimum Distance Classification inTable 2. Accuracy of a Maximum Likelihood Classificationin the Old Black Spruce Sitea the Old Black Spruce Sitea

Nadir Reflect.b ANIX b CombinationcNadir Reflect.b ANIX b Combinationc

Black spruce 30.3% 46.2% 52.9%Black spruce 68.9% 43.5% 63.8%Spruce/pine 73.4% 36.5% 63.6% Spruce/pine 80.5% 23.4% 59.3%

Treed muskeg 55.0% 74.1% 74.5%Treed muskeg 30.0% 72.6% 55.4%Overall accuracy 48.3% 57.9% 59.0% Overall accuracy 58.8% 55.3% 67.0%

Kappa coefficient 0.350 0.244 0.450Kappa coefficient 0.289 0.299 0.398

a Percent pixels correctly classified according to forest cover map; fora Percent pixels correctly classified according to forest cover map; num-ber of valid pixels: 18,393 for black spruce, 37,049 for spruce/pine, and number of valid pixels see Table 2.

b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 5071,601 for treed muskeg.b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 50 (898 nm).

c Combination of Bands 17, 28, and 50 from both ANIX and nadir(898 nm).c Combination of Bands 17, 28, and 50 from both ANIX and nadir reflectance data.

reflectance data.

10E) is probably even more accurate than the forestcover map, which is generalized (Fig. 10F). Clear-cuts inthe influence of spectral resolution on the classificationthe spruce/pine stands visible in the aerial photographresults: a multispectral set with three bands: 17 (560(Fig. 10C) are captured in the MINDIS classification butnm), 28 (671 nm), and 50 (898 nm), and a hyperspectralare missing in the forest map due to generalization. Inset consisting of 39 bands: 14 (530 nm) to 52 (919 nm).addition, the change between black spruce and spruce/All three classifiers, MLC, MINDIS, and SAM, con-pine stands in the northeast part of the test site is pic-sistently showed best classification results for the combi-tured as a sharp transition in the forest cover map,nation of nadir reflectance and ANIX data (Tables 2–7,whereas the classification result shows a more realisticFig. 12). Compared to nadir reflectance classification,gradual shift.the combination of nadir reflectance and ANIX data im-

The potential of ANIX data for classification im-proved the overall classification accuracies in the OBSprovements is also evident in the results of the SAMsite by 8–25% (kappa coefficient 0.09–0.31) and by 6–classifications for both the OBS (Table 6) and the FEN18% (kappa coefficient 0.07–0.18) in the FEN site. Insite (Table 7, Fig. 13). In the multispectral test case, themost cases, the ANIX data alone also resulted in higherincreases in classification accuracy through a combinationclassification accuracies than nadir reflectance classifi-of nadir and ANIX data in the OBS and FEN sites werecation.25.5% and 18%, respectively. With hyperspectral data,The results from the minimum distance classifier inthe advantage of combining nadir and ANIX data for thethe OBS site impressively demonstrate the potential ofclassification process was also evident, but the improve-BRDF information to increase classification accuracyments were not as significant as in the multispectral case.(Fig. 10D/E). Both the discrimination of treed muskegWhile a substantial increase in classification accuracyand the separation of black spruce from spruce/pinecould be obtained by adding either hyperspectral or mul-stands were substantially improved by the incorporationtidirectional information to nadir reflectance data, theof ANIX data. The MINDIS classification result from thecombination did not result in substantive improvementscombination of nadir reflectance and ANIX data (Fig.in the numerical classification result (Tables 6 and 7,Fig. 12).

Table 3. Accuracy of a Maximum Likelihood Classificationin the Fen Sitea

Table 5. Accuracy of a Minimum Distance Classification inNadir Reflect.b ANIX b Combinationc the Fen Sitea

Black spruce 54.3% 28.0% 55.6% Nadir Reflect.b ANIX b Combinationc

Jack pine 50.2% 12.0% 53.6%Black spruce 66.8% 47.0% 50.3%Tamarack 74.7% 58.8% 73.5%Jack pine 37.8% 22.5% 22.9%Spruce/pine 10.9% 17.8% 12.8%Tamarack 33.9% 6.8% 32.7%Treed muskeg 55.9% 47.5% 57.5%Spruce/pine 10.2% 17.8% 18.1%Clear muskeg 49.5% 74.8% 67.7%Treed muskeg 59.7% 17.7% 30.0%Overall accuracy 43.3% 40.5% 49.3%Clear muskeg 15.9% 87.8% 87.0%Kappa coefficient 0.329 0.289 0.402Overall accuracy 31.4% 40.1% 44.1%

a Percent pixels correctly classified according to forest cover map; num- Kappa coefficient 0.190 0.258 0.314ber of valid pixels: 19,580 for black spruce, 20,588 for jack pine, 11,362for tamarack, 37,080 for spruce/pine, 19,046 for treed muskeg, and 39,068 a Percent pixels correctly classified according to forest cover map; for

number of valid pixels see Table 3.for clear muskeg.b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 50 b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 50

(898 nm).(898 nm).c Combination of Bands 17, 28, and 50 from both ANIX and nadir c Combination of Bands 17, 28, and 50 from both ANIX and nadir

reflectance data.reflectance data.


Table 6. Accuracy of a Spectral Angle Mapper Classification in the Old Black Spruce Sitea

Multispectral (Three Bandsb) Hyperspectral (39 Bandsc)

Nadir Refl. ANIX Comb.d Nadir Refl. ANIX Comb.d

Black spruce 59.3% 37.7% 32.8% 69.0% 42.2% 34.7%Spruce/pine 16.4% 30.2% 65.3% 64.8% 58.1% 62.5%Treed muskeg 43.2% 69.0% 69.9% 44.3% 71.8% 72.2%Overall accuracy 37.7% 53.2% 63.2% 53.9% 63.5% 64.0%Kappa coefficient 0.086 0.213 0.399 0.317 0.384 0.405

a Percent pixels correctly classified according to forest cover map; for number of valid pixels see Table 2.b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 50 (898 nm).c Data derived from ASAS Bands 14–52 (530–919 nm).dCombination of ANIX and nadir reflectance data.

Figure 13 shows a different perspective by visually CONCLUSIONScomparing the various classification results in the FEN Based on the structure analysis and classification results,site (for the corresponding ground reference data see the following conclusions can be drawn from this study:Fig. 11F). SAM revealed excellent classification results

• Hyperspectral BRDF characteristics of erecto-for the ANIX data even when only three bands werephile and planophile canopies acquired withused (Fig. 13C). A combination of ANIX data with nadirASAS are in good agreement with results ob-reflectance and hyperspectral resolution further im-tained from corresponding laboratory data.proved the classification (Fig. 13F). The actual classifica-

• The spectral variability of hyperspectral BRDFtion results are probably even better than specified indata is related to spectral and structural charac-Table 7. Examining the aerial photograph in Figure 11B,teristics of vegetation canopies and allows theit appears that many of the areas specified as spruce/pinederivation of their overall structure from remotein the forest cover map are indeed black spruce standssensing data on a pixel-per-pixel basis.as suggested by the SAM classifier in Figures 13C–F.

• Maximum likelihood, minimum distance, andThe classification results for nadir reflectance data,spectral angle mapper classifier show substantialon the other hand, are rather poor in spite of the higherimprovement in classification accuracy when na-geometrical resolution for this viewing direction. Surpris-

ingly, the inclusion of hyperspectral resolution hardly im- dir reflectance and ANIX data are combined.ANIX data are mainly dependent on the crownproved the visual appearance of the classification, even

though the quantitative analysis suggests a classification cover characteristics of a canopy rather than onthe understory. They bring structure informationquality similar to the results of the ANIX data (Table 7).

The poor performance of SAM with nadir data is proba- into the classification process, while nadir reflec-tance data improve the overall spatial resolutionbly due to the similarity of the nadir reflectance signa-

tures in the FEN site (Fig. 8A) which differ mostly by and add soil/background characteristics.• With the spectral angle mapper, a combinationan offset rather than by shape. The corresponding ANIX

data, however, expose distinct signatures (Fig. 8B). of nadir reflectance data with either hyperspec-

Table 7. Accuracy of a Spectral Angle Mapper Classification in the Fen Sitea

Multispectral (Three Bandsb) Hyperspectral (39 Bandsc)

Nadir Refl. ANIX Comb.d Nadir Refl. ANIX Comb.d

Black spruce 31.5% 45.4% 55.7% 22.7% 50.3% 52.9%Jack pine 10.1% 33.5% 20.7% 43.6% 37.7% 28.8%Tamarack 67.9% 30.2% 24.9% 53.5% 24.0% 22.9%Spruce/pine 24.7% 13.0% 20.7% 14.3% 17.6% 21.5%Treed muskeg 30.1% 10.8% 25.5% 47.0% 19.4% 26.1%Clear muskeg 16.9% 78.7% 85.0% 55.4% 82.2% 86.4%Overall accuracy 25.5% 38.7% 43.5% 37.8% 42.7% 44.7%Kappa coefficient 0.124 0.246 0.303 0.260 0.291 0.319

a Percent pixels correctly classified according to forest cover map; for number of valid pixels see Table 3.b Data derived from ASAS Bands 17 (560 nm), 28 (671 nm), and 50 (898 nm).c Data derived from ASAS Bands 14–52 (530–919 nm).d Combination of ANIX and nadir reflectance data.


Figure 12. Overall accuracy for various classification algorithms for nadir reflectance, ANIX, and a combination ofnadir reflectance and ANIX data for A) the OBS and B) the FEN sites. The data are further described inTables 2–7.

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Figure 13. Results from the SAM classification in the FEN test site based on: A, C, E) multispectral data from Bands 17(560 nm), 28 (671 nm), and 50 (898 nm) and B, D, F) hyperspectral data from Bands 14–52 (530–919 nm). Classificationresults are derived from nadir reflectance in plates A, B), from anisotropy index (ANIX) data in plates C, D), and from acombination of nadir reflectance and anisotropy index data in plates E, F). Unclassified areas appear in white. Note: Comparecorresponding ground reference information from the Saskatchewan Environment and Resource Management forest covermap in Figure 11F.


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