Approach: In addition to active approaches, passive remote sensing in the solar wavelengths canprovide information on canopy structure (fractional cover, mean canopy height) via inversion ofcanopy reflectance models, such as geometric-optical (GO) models. Sensitivity to 3-D structure isgreatly enhanced when multi-angle reflectance data are used because geometric effects such asshadow-hiding can be exploited and the signal can be decomposed into canopy/background andcover/height components. High quality multi-angle reflectance data are provided by the NASA JetPropulsion Laboratory’s Multi-angle Imaging SpectroRadiometer (MISR) that is flying on the NASAEarth Observing System Terra satellite launched in December 1999. MISR has nine camerasaligned in the along-track direction, providing up to nine looks in the space of ~7 minutes.

Data from MISR can be interpreted through adjustment of the simple geometric-optical model(SGM) to retrieve estimates of crown cover, mean canopy height, and woody biomass. Thisapproach relies on the use of Bidirectional Reflectance Distribution Function (BRDF) model kernelweights and spectral measures from the MISR nadir camera to predict the contribution of the non-tree background a priori, dynamically for each location; then two of the GO model parameters canbe adjusted to retrieve fractional canopy cover and mean canopy height. In the research presentedhere, the study area comprises taiga and tundra on the Kola Peninsula, Russia (Figure 1). The KolaPeninsula is almost completely to the north of the Arctic Circle but has unusually warm wintertemperatures for these latitudes (66 – 69° N), as it receives warmth from the North Atlantic Drift.

IEEE International Geoscience and Remote Sensing Symposium 2012, July 22-27, 2012, Munich. This work was supportedby NASA Terrestrial Ecology Award NNX09AL03G and NASA/JPL subcontract #1365499. Thanks to David Diner, MISR PI.Data credits: NASA LARC/ASDC, Digital Globe/Google. Maps & code: http://csam.montclair.edu/~chopping/tundra

Method: MISR Level 1B2 Terrain radiance data from overpass Terra Path 185 (08/06/00), MISRBlocks 35-36, were converted to surface bidirectional reflectance factors (BRF) using MISR Toolkitroutines and MISR 1 km LAND product BRFs and mapped to a 250 m grid. The background angularresponse in the MISR viewing plane was estimated prior to model inversion using the isotropic,geometric, and volume scattering weights of the LiSparse-RossThick_Reciprocal (RTLSR) kernel-driven BRDF model, plus nadir camera green and near-infrared reflectance factors. Calibration ofthese relationships was effected using canopy information obtained from QuickBird/Google Earth250 x 250 m image chips interpreted with CANAPI (Fig. 2, Table I; Chopping 2010). Thesestatistics and the predicted background BRDF were used to drive the model in forward mode, withgood results: there is a good match in terms of both absolute error and shape (Figure 3; Table II).

The SGM is expressed as BRF = (RTLSR × kG) + (cr × kC) + (sh × kZ), where kG, kC, and kZ are theproportions of viewed and sunlit background, viewed and sunlit crown, and viewed shaded crownand background, respectively; and cr is a crown reflectance estimate, set at 0.06, assuming mostlyneedleleaf trees. The combined shaded component, sh, was assigned a signature of 0.02, based onthe high proportion of diffuse irradiance. The SGM’ s tree number density and mean radiusparameters were adjusted against red MISR BRFs using the Praxis algorithm to minimize the RMSEbetween model and data, producing maps of crown cover and mean canopy height. The startingpoint for the inversions was density = 0.001 and r = 0.5. The crown shape (b/r) and crown height(h/b) parameters were fixed at 1.0 and 2.0, respectively. Retrieval of mean canopy height ispossible by via h = h/b × b; b = b/r × r.

250 m Canopy Cover and Height Maps over the Kola Peninsula via GO Model Inversion with MISR Mark Chopping

Department of Earth & Environmental Studies, Montclair State University, Montclair, New Jersey 07043

Retrievals of fractional cover and mean canopy height were correlated with the reference data(R2 = 0.65 and 0.54, respectively), with low absolute RMS errors (0.04 and 1.9 m; Figure 5).However the retrieved mean height distribution is compressed into a very narrow range andrelative error is high (the canopy is relatively short in stature). Nevertheless, these results areencouraging because model inversion using solar wavelength reflectance data is challenging intaiga and other Northern High Latitude landscapes: there is low contrast between the treecanopy and the background and the solar path length is greater, resulting in a higherproportion of diffuse irradiance.

These results show that MISR data can be interpreted through a simple GO model to providemaps of canopy crown cover and mean canopy height in taiga landscapes. It is not knownwhether the retrieval precision is sufficient for the determination of changes in the tree-line;further work using ground inventory is required to establish this. Aboveground biomass canalso be estimated sing cover and height information, exploiting allometric or simple regressionrelationships. This approach could be used to construct a record spanning more than a decadeand thus provide historical context for the high resolution maps anticipated from the newgeneration of active lidar and radar instruments that will be launched in the 2020s.

Results: The 3,768,950 GO model inversions over the map area resulted in retrievals for bothtree fractional cover and mean canopy height that are spatially smooth, with distributions thatmatch patterns in high resolution imagery well (Figure 4). Model-fitting RMSE values greaterthan 0.01 are usually cloud/plumes, surface water bodies, or rocks/bare soil; together withMISR nadir camera near-infrared BRF data and RTLSR kernel weights, these can be used toconstruct a mask that reliably allows separation of these features from forested surfaces(Figure 4(b)-(c) and Figure 5(c).

Mean CanopyHeight Map

FractionalCanopy Cover Map

25 km

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MISR NRGFalse Color Composite

Fig. 4 MISR NIR/Red/Green False Color Composite; Canopy Cover and Height Maps over the Kola Peninsula. Data: MISR 08/06/00 Terra Path 185, Blocks 35-36.

Figure 1. Location of the Kola Peninsula (blue box)

Figure 2. Generation of calibration/validation data set using the Canopy Analysis with PanchromaticImagery (CANAPI) method (Chopping, 2010). Fifty-three 250 x 250 m image chips corresponding tomapped MISR cells were converted to grayscale and submitted to CANAPI; the figure shows oneexample. The algorithm detects crowns and provides measurements of crown radii, fractional cover,and tree/shrub heights (via shadow-following and knowledge of the satellite overpass time).

Original QuikcBird/Google Earthimagery (north is “up”). This isfirst converted to grayscale.

Rotated image showing CANAPIestimates of crown locations andsizes (solar direction is up).

Crown map rotated to the originalorientation.

Chopping, M. (2012), Geometric-optical modeling with MISR over the Kola Peninsula, 2012 IEEE InternationalGeoscience and Remote Sensing Symposium, Munich, Germany, July 22–27, 2012.

Chopping, M. (2010), CANAPI: Canopy Analysis with Panchromatic Imagery, Remote Sensing Letters 2(1): 21–29.

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Table II. RESULTS OF FORWARD-MODELINGMISR RED BAND BRFS (RMSE AND R2 W.R.T.

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TABLE I. CANAPI-DERIVED CANOPY STATISTICS

Figure 3. MISR red band BRFs (rhoMISR), modeled BRFs (SGM),and the predicted background contribution (kG.RTkLiSp).

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Figure 5 Detail over the Kola Peninsula (a) MISR NIR/Red/Green False Color Composite (b) MISR/GO canopy cover (c) MISR/GO meanheight map (d) fractional cover inversion results vs. reference data (e) mean canopy height inversion results vs. reference data

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