COMPARING MODIS AND ETM+ IMAGE DATA FOR INLAND WATER STUDIES: SPATIAL RESOLUTION CONSTRAINTS

Resumo

The objective of this paper is to compare the performance of medium spatial resolution (250 m and 500 m) Terra MODIS images to finer resolution Landsat ETM+ images. MODIS Terra images have high frequency of acquisition (1 day revisit at high latitudes) and that makes them more useful for inland water studies. Assessing the performance of ETM and MODIS to map relevant features for the functioning of aquatic systems is very important for fostering water resource remote sensing. To carry out the comparison, concurrent ETM+ and MODIS images were acquired over the Lago Grande de Curaui Lake. The images were georeferenced and resample to the 100 x 100 m ground resolution using a next neighbor algorithm to make the data comparable. The resampling rationale was making the digital processing easier, without changing the effective resolution properties of the original data. It was assumed that the improved MODIS radiometric resolution as compared to the ETM+.would make both data set comparable at a 100m x 100m resolution. In the next step, a series of tests were carry out in order to define the best approach to map aquatic system features such as lakes, islands, and levees in the study area. The tests indicated that the best approach was the segmentation of the shadow fraction derived from the application of the linear unmixing model to both ETM+ and MODIS images. The segmentation was then followed by the application of a non-supervised region classifier. The final classes were mapped into two categories: lakes (water) and island (land). The polygon distribution generated by the classification procedure was then statistically analyzed to assess the polygon size frequency of features mapped by each data set. The results showed that ETM+ and MODIS were able to recover the water surface area in the studied region with a difference of 11 %. The analyses of polygon distribution, however, showed that there were many polygons which were detected in MODIS image data, but not in ETM+ image data. This result suggests that the spectral and radiometric resolution improvements presented by MODIS image tend to compensate for the losses in spatial resolution. That is to say that under the boundary conditions adopted in this study, MODIS images performed better than degraded ETM+ images of 100m x 100m. This explains the small difference in performance presented by comparing two sensors with such large differences in nominal spatial resolution.