Spatial resolution influence on the identification of land cover classes in the Amazon environment

To evaluate the role played by the spatial resolution in distinguishing land cover classes in the Amazon region, different levels of spatial resolution (60, 100, 120, 200 and 250 meters) were simulated from a Landsat_5 Thematic Mapper (TM) image. Thematic maps were produced by visual interpretation from the original (30 x 30 meters) and simulated set of images. The map legend included primary forest, old and young woody secondary succession, and non-forest. The results indicated that for the discrimination between primary forest and non-forest, spatial resolution did not have great influence for pixel size equal or lower than 200 meters. The contrary was verified for the identification of old and young woody secondary vegetation due to their occurrence in small polygons. To avoid significant changes in the calculated area of these land cover types, a spatial resolution better than 100 meters is required. This result is an indication that the use of the future Brazilian remote sensing satellite (SSR-1) for secondary succession identification may be unreliable, especially for latitudes between S10degrees and S15degrees where critical areas of deforestation are located and pixel size is expected to vary within the same scene from 100 meters (S10degrees) to 200 meters (S15degrees)

Variation of MODIS reflectance and vegetation indices with viewing geometry and soybean development

Directional effects introduce a variability in reflectance and vegetation index determination, especially when large field-of-view sensors are used (e.g., Moderate Resolution Imaging Spectroradiometer - MODIS). In this study, we evaluated directional effects on MODIS reflectance and four vegetation indices (Normalized Difference Vegetation Index - NDVI; Enhanced Vegetation Index - EVI; Normalized Difference Water Index - NDWI1640 and NDWI2120) with the soybean development in two growing seasons (2004-2005 and 2005-2006). To keep the reproductive stage for a given cultivar as a constant factor while varying viewing geometry, pairs of images obtained in close dates and opposite view angles were analyzed. By using a non-parametric statistics with bootstrapping and by normalizing these indices for angular differences among viewing directions, their sensitivities to directional effects were studied. Results showed that the variation in MODIS reflectance between consecutive phenological stages was generally smaller than that resultant from viewing geometry for closed canopies. The contrary was observed for incomplete canopies. The reflectance of the first seven MODIS bands was higher in the backscattering. Except for the EVI, the other vegetation indices had larger values in the forward scattering direction. Directional effects decreased with canopy closure. The NDVI was lesser affected by directional effects than the other indices, presenting the smallest differences between viewing directions for fixed phenological stages.<br>Efeitos direcionais introduzem variabilidade na reflectância e na determinação de índices de vegetação, especialmente quando sensores de amplo campo de visada são usados (p.ex., Moderate Resolution Imaging Spectroradiometer - MODIS). Neste estudo, nós avaliamos os efeitos direcionais sobre a reflectância e quatro índices de vegetação (Normalized Difference Vegetation Index - NDVI; Enhanced Vegetation Index - EVI; Normalized Difference Water Index - NDWI1640 e NDWI2120), calculados a partir de dados MODIS, em função do crescimento da soja em duas estações agrícolas (2004-2005 e 2005-2006). Para manter o estádio reprodutivo de uma dada variedade como um fator constante, variando a geometria de visada, pares de imagens obtidas em datas próximas e com ângulos de visada opostos foram analisados. Usando uma abordagem estatística não-paramétrica com análise bootstrapping e normalizando os índices para suas diferenças angulares entre as direções de visada, sua sensibilidade para os efeitos direcionais foi estudada. Os resultados mostraram que a variação da reflectância do MODIS entre estádios fenológicos consecutivos foi geralmente menor do que aquelas resultantes da geometria de visada para dosséis fechados. O contrário foi observado para dosséis esparsos. A reflectância das primeiras sete bandas do MODIS foi maior na direção de retro-espalhamento. Exceto o EVI, os demais índices de vegetação tiveram maiores valores na direção de espalhamento frontal. Os efeitos direcionais diminuíram com o fechamento do dossel. O NDVI foi menos afetado pelos efeitos direcionais do que os demais índices, apresentando as menores diferenças entre as direções de visada para os mesmos estádios fenológicos