What is an ECW File?
An ECW file stores large geospatial raster imagery in Enhanced Compression Wavelet format, a proprietary wavelet format whose SDK is developed by Hexagon. It offers high compression and multiresolution access, so an application can decode only the level of detail and area it needs.
How ECW Files work
ECW applies a discrete wavelet transform, quantization, and encoding, which leaves the image as a set of multiresolution levels. A reader requests a region at a chosen resolution, and only the levels needed for that view are decompressed. Georeferencing travels in the file: projection, datum, and units values in the header for version 2, and GeoTIFF-style tags for version 3. Two file versions are in use: version 2, limited to 8 bits per channel, and version 3, which adds unsigned 16-bit integer (UInt16) data, statistics, and further metadata. Either version can hold RGB, grayscale, or multiband imagery, so the extension alone does not say which bands a file contains. Reading and writing depend on Hexagon’s ERDAS ECW/JP2 SDK; GDAL’s ECW driver is optional and must be built against it.
Key facts
- 1ECW needs no external pyramids or overviews. The wavelet transform itself yields multiresolution levels, a reader requests a region at a resolution, and only the levels needed are decompressed; GDAL’s driver does not create overviews for ECW for that reason.
- 2ECW support is absent from a GDAL build unless it was linked against Hexagon’s ERDAS ECW/JP2 SDK. QGIS reads ECW through GDAL, so it offers the format only when its GDAL build has the driver;
gdalinfo --formatslists the raster drivers a build includes. The SDK’s terms differ by use: GDAL’s documentation for SDK v5.x says desktop decoding is free of charge, while compressing, deploying on a server platform, or decoding unlimited-size files on mobile requires a purchased license, and writing needs an OEM key at runtime. Confirm the driver and the current license terms on every deployment target. - 3ECW compression is lossy, including in version 3 files. The SDK guide says ECW supports only an irreversible wavelet filter, suitable only for visually lossless requirements, and that numerically lossless compression is usually required where analysis such as remote sensing or photogrammetry needs exact pixel values; it presents JPEG 2000, handled by the same SDK, as offering both lossy and lossless coding. GDAL’s default targets reduce RGB imagery by 95 percent and grayscale by 90 percent. When analysis needs exact pixel values, request the original imagery or a numerically lossless copy of it, and check which bands the delivered file holds.
- 4ECW version 3 files are not backward compatible. GDAL notes that they fail to decode with the v3.x and v4.x SDKs, and its driver writes version 2 by default to retain the widest compatibility. Version 2 is limited to 8 bits per channel, so unsigned 16-bit imagery requires version 3 and therefore SDK 5.x readers everywhere downstream.
When ECW Files matter
Use ECW when GIS workflows must inspect or serve large aerial and satellite imagery efficiently. Confirm decoder, licensing, coordinate, and quality requirements because not every geospatial tool supports the format.
Sources
- GDAL documentation: ECW, Enhanced Compressed Wavelets (.ecw) raster driver
- ERDAS ECW JP2 SDK User Guide, version 5.3.0 (copy distributed with the OSGeo4W gdal-ecw package)
- The National Archives, PRONOM fmt/371: Enhanced Compression Wavelet
- QGIS User Guide: Opening Data (format support depends on the GDAL build)