Fusion of broadband panchromatic data with narrow band multispectral data – pansharpening – is a common and often studied problem in remote sensing. Many methods exist to produce data fusion results with the best possible spatial and spectral characteristics, and a number have been commercially implemented. This study examines the output products of 4 commercial implementations with regard to their relative strengths and weaknesses for a set of defined image characteristics and analyst use-cases. Image characteristics used are spatial detail, spatial quality, spectral integrity, and composite color quality (hue and saturation), and analyst use-cases included a variety of object detection and identification tasks. The imagery comes courtesy of the RIT SHARE 2012 collect. Two approaches are used to evaluate the pansharpening methods, analyst evaluation or qualitative measure and image quality metrics or quantitative measures. Visual analyst evaluation results are compared with metric results to determine which metrics best measure the defined image characteristics and product use-cases and to support future rigorous characterization the metrics’ correlation with the analyst results. Because pansharpening represents a trade between adding spatial information from the panchromatic image, and retaining spectral information from the MSI channels, the metrics examined are grouped into spatial improvement metrics and spectral preservation metrics. A single metric to quantify the quality of a pansharpening method would necessarily be a combination of weighted spatial and spectral metrics based on the importance of various spatial and spectral characteristics for the primary task of interest. Appropriate metrics and weights for such a combined metric are proposed here, based on the conducted analyst evaluation. Additionally, during this work, a metric was developed specifically focused on assessment of spatial structure improvement relative to a reference image and independent of scene content. Using analysis of Fourier transform images, a measure of high-frequency content is computed in small sub-segments of the image. The average increase in high-frequency content across the image is used as the metric, where averaging across sub-segments combats the scene dependent nature of typical image sharpness techniques. This metric had an improved range of scores, better representing difference in the test set than other common spatial structure metrics.
Typical automatic clustering methods struggle to determine the correct number of clusters to properly characterize the
data. To estimate the number of clusters in a spectral image data cloud explicitly from the data structure, the pairwise
relationships between pixels in the n-dimensional spectral space are exploited. By plotting the average ith co-density
between pixels and neighbors, a monotonically increasing function will emerge that characterizes the clusters in the data.
Large upward steps in the average neighbor distance function represent the well-grouped clusters in the data. This
process can accurately identify the number of clusters in a wide variety of image data automatically.
Automatic clustering of spectral image data is a common problem with a diverse set of desired and potential solutions.
While typical clustering techniques use first order statistics and Gaussian models, the method described in this paper
utilizes the spectral data structure to generate a graph representation of the image and then clusters the data by applying
the method of optimal modularity for finding communities within the graph. After defining and identifying pixel
adjacencies to represent an image as an adjacency matrix, a recursive splitting is performed to group spectrally similar
pixels using the method of modularity maximization. The careful selection of pixel adjacencies determines the success of
this spectral clustering technique. The modularity maximization process uses the eigenvector of the modularity matrix
with the largest positive eigenvalue to split groups of pixels with non-linear decision surfaces and uses the modularity
measure to help estimate the optimal number of clusters to best characterize the data. Using information from each
recursion, the end result is a variable level of detail cluster map that is more visually useful than previous methods.
Additionally, this method outperforms many typical automatic clustering methods such k-means, especially in highly
cluttered urban scenes. The optimal modularity technique hierarchically clusters spectral image data and produces results
that more reliably characterize the number of clusters in the data than common automatic spectral image clustering
techniques.
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