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ECW (Enhanced Compression Wavelet) is a proprietary wavelet compression image format optimized for aerial and satellite imagery.

From Wikipedia:

ECW (Enhanced Compression Wavelet) is a proprietary wavelet compression image format optimized for aerial and satellite imagery. It was developed by Earth Resource Mapping, and is now owned by ERDAS, which is owned by Intergraph. The lossy compression format efficiently compresses very large images with fine alternating contrast.

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