# ECW -- ERMapper Compress Wavelets (.ecw)

GDAL supports .ecw format for read access and write. The current implementation reads any number of bands but returns only as eight bit image data. Coordinate system and georeferencing transformations are read, but in some cases coordinate systems may not translate.

Support for the ECW driver in GDAL is optional, and requires linking in external ECW SDK libraries provided by ERMapper.

By default the ECW SDK will use up to one quarter of physical RAM for internal caching and work memory during decoding. This is normally fine, but the amount of memory to use can be adjusted by setting the GDAL_ECW_CACHE_MAXMEM configuration variable with an amount of memory in bytes. Config variables can be set in the environment or using the --config commandline switch.

## Creation Issues

The free ECW compression kit only supports compression of images up to 500MB. To compress very large images it is necessary to license ECW technology from ERMapper. Files to be compressed into ECW format must also be at least 128x128. Non-eight bit source data will be rescaled by the ECW SDK in some manner that I don't understand ... the resulting image is effectively eight bit.

When writing coordinate system information to ECW files, many less common coordinate systems are not mapped properly. If you know the ERMapper name for the coordinate system you can force it to be set at creation time with the PROJ and DATUM creation options.

Creation Options:

- TARGET=percent: Set the target size reduction as a percentage of the original. If not provided defaults to 75 for an 75% reduction.

- PROJ=name: Name of the ERMapper projection string to use. Common examples are NUTM11, or GEODETIC.

- DATUM=name: Name of the ERMapper datum string to use. Common examples are WGS84 or NAD83.

- LARGE_OK=YES: Under some circumstances this option can be set to allow compressing files larger than 500MB. It is the users responsibility to ensure that ERMapper licensing requirments for large file compression are being adhered to.

ECW format does not support creation of overviews since the ECW format is already considered to be optimized for "arbitrary overviews".

See Also:

- Implemented as `gdal/frmts/ecw/ecwdataset.cpp`.

- ECW SDK available at www.ermapper.com.