Possibilities of discriminating tropical secondary succession in Amazônia using hyperspectral and multiangular CHRIS/PROBA data

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Abstract

CHRIS/PROBA data collected in the Brazilian Amazônia in 4 view angles (−36°, nadir, +36°, +55°) and 62 bands (410–1000 nm range) were evaluated for the discrimination between primary forest and 3 stages of secondary succession after deforestation: initial (SS1; <5 years), intermediate (SS2; 5–15 years) and advanced (SS3; >15 years). Single view angle and multiangular approaches (nadir plus anisotropic information derived from reflectance ratios between view angles) were tested for discrimination. Both approaches used principal components analysis (PCA) applied to pixel spectra representative of each class in order to reduce data dimensionality at each dataset, to enhance separability between the classes, and to provide input variables for multiple discriminant analysis (MDA). The results showed that the off-nadir viewing improved discrimination between the successional stages. Discrimination between SS2 and SS3 was enhanced with PCA at +36° view angle. Primary forest and SS3 presented a more anisotropic behavior than SS2 and SS1, especially in the backward scattering direction (positive view angles) in which great amounts of sunlit canopy components were viewed by the sensor. MDA classification results showed that the multiangular approach produced an overall improvement in the discrimination. From the single (nadir) to the multiangular approach, classification accuracy using a separate set of pixels increased from 83.3% to 98.3% for SS1, 53.3% to 70.0% for SS2, and 58.3% to 76.7% for SS3. The nadir and multiangular classifications were statistically different at a 0.05% level of significance. Kappa statistics increased from 0.63 to 0.82. The results showed that multiangular data can improve the differentiation between primary forest and old stages of natural vegetation regrowth, which have been reported in the literature as the most difficult classes to be mapped in the Amazonian environment.

Introduction

New satellite sensors acquiring quasi-simultaneous multiangular images facilitate reflectance anisotropy characterization of the land cover types (Mahtab et al., 2008). The anisotropic effects of solar illumination and sensor viewing geometry detected by multiangular instruments can provide improved and unique diagnostic information on the sensed surface (Diner et al., 2005, Schaepman et al., 2005). Changes produced in reflectance due to off-nadir viewing are dependent on the geometry of data acquisition, the wavelength and on the vegetation type under study (Strub et al., 2003, Goodin et al., 2004). Furthermore, modifications in vegetation structure produce different patterns of reflectance anisotropy. As a result, multiangular data can be used also to predict accurately the vertical structure of forest canopies measured by lidar systems (Kimes et al., 2006). When coupled to nadir data, the anisotropic information may be useful to improve classification accuracy of vegetation, as demonstrated by several researchers (Sandmeier and Deering, 1999, Liesenberg et al., 2007, Su et al., 2007, Mahtab et al., 2008; references therein).

Among the available multiangular satellite sensors, the Compact High Resolution Imaging Spectrometer (CHRIS), on board the European Space Agency's Project for On Board Autonomy (PROBA) satellite launched in October 2001, provides a new and unique opportunity to evaluate orbital hyperspectral and multiangular data simultaneously for different applications (Barnsley et al., 2004). This small-satellite mission is a technology-proving experiment that provides pointing in both along-track and across-track directions to produce multiangular data. The hyperspectral instrument is capable of acquiring data in five operational modes with different combinations of spatial resolution and spectral band sets. In Mode 1, the images are acquired with nominal spatial resolution of 36 m, in 62 bands (410–1000 nm) and in 5 view angles due to the pointability of the platform (Barnsley et al., 2004). However, in spite of being considered the first full spectrodirectional spaceborne instrument, only a small number of investigations have tested the potential of CHRIS/PROBA data for different applications. First results over land and water bodies were presented by Guanter et al. (2005).

In this study, the potential of hyperspectral and multiangular CHRIS/PROBA data (Mode 1) collected over the Amazonian forest was evaluated for the discrimination between primary forest and three stages of secondary succession (initial, intermediate and advanced) after deforestation. Several deforested areas in Amazônia have reverted to secondary succession, which increases the interest to map this class because of its importance to carbon cycle and ecosystem changes (Foody et al., 1996, Lucas et al., 2002, Lu et al., 2003, Vieira et al., 2003). Discrimination between successional stages is usually difficult because of the smooth transition between them, the heterogeneity of their vegetation structure, the land use history before the abandonment of productive activity and the frequency of cutting or burning events (Lu et al., 2003, Vieira et al., 2003). Despite the limited geographic coverage for mapping purposes (swath width of 14 km), the experimental CHRIS/PROBA provides an opportunity to evaluate the impact of the anisotropy on the discrimination of secondary succession.

Section snippets

Study area

The study area (14 km × 14 km) is located in the Tapajós National Forest (Pará state, Amazônia) and its surroundings between 03°05′S/54°54′W and 03°11′S/55°02′W. In the Köeppen classification, the climate of the study area is AmW. The rainy season occurs between December and May and the average values of annual precipitation and temperature are 2272 mm and 26 °C, respectively (Santos et al., 2003, Aragão et al., 2005). The area is relatively flat with minimum and maximum altitude values of 65 m and 200

Image acquisition and pre-processing

CHRIS/PROBA images were acquired on 23 June 2006 (dry season and clear sky conditions) close to the principal plane with a solar zenith angle of 33°. Data were collected in the visible and near infrared wavelengths (410–1000 nm) in 62 bands (Mode 1) with a variable nominal full-width-half-maximum between 5 nm and 10 nm. The ground instantaneous field of view (GIFOV) was 40 m at nadir and the swath width 14 km. By pointing the platform within a short time period (less than 2.5 min), data were acquired

Spectral-angular variations of primary and secondary succession

Variations in CHRIS-derived surface reflectance anisotropy between primary forest and the initial (SS1), intermediate (SS2) and advanced (SS3) secondary succession after deforestation are shown in Fig. 1 for two extreme view angles (−36° and +36°). Reflectance increased in the 410–1000 nm range from the forward (Fig. 1a) to the backward (Fig. 1b) scattering direction due to the predominance of sunlit canopy components at +36° view angle. For a given view angle, the near infrared reflectance

Conclusions

In comparison with the single view angle approach (nadir), the CHRIS/PROBA-derived multiangular approach produced an overall discrimination improvement of secondary succession in the study area, especially between primary forest and old stages of vegetation regrowth (SS3 and SS2). These classes have been reported in the literature as the most difficult to be discriminated. The largest anisotropic behavior was displayed by primary forest and SS3 in the backward scattering direction in which

Acknowledgements

The authors are grateful to European Space Agency (ESA) for providing CHRIS/PROBA data through selection of Category 1 PROBA Proposal 3774, and to Luiz Eduardo Aragão and Fernando Espírito Santo for providing useful field inventory data. Thanks are also due to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), to Waldir Paradella and Cleber de Oliveira for helpful assistance in geometric correction, and to the anonymous reviewers for very useful suggestions.

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