ER Mapper Compressed Raster

The GDB library supports ER Mapper's Compressed Raster Formats (ECW) for live linking and import. ER Mapper's Compression and Decompression Software Development Kits (v.3.3) were used to introduce this functionality in PCI products. This format is supported on Windows and Linux 64-bit operating systems.

A compressed image is one that has been reduced in size, but still maintains an almost perfect version of the original. There are various compression techniques, but it is wavelet compression that is used by ER Mapper's software. This technique enables very high compression rates where a typical color image can be compressed to less than 2% to 5% of its original size (50:1 to 20:1 compression ratios). This means that, at 20:1 compression, it is possible to compress 500 MB of color imagery down to 25 MB.

Lossless compression, as the name implies, results in a compressed image that can be perfectly reconstructed to be identical to the original image. Its main advantage is this ability of perfect reconstruction, but its main disadvantage is that it is typically limited to a compression of 2:1. Wavelet compression, however, is a lossy compression, which means that some information is thrown away in order to achieve the much higher compression rates than are possible with lossless compression. Here, the main advantage is the higher compression ratios, but the disadvantage is that the compressed image can not be perfectly reconstructed to be identical to the original.

An ER Mapper Compressed Raster consists of a single file with an ECW extension. Sometimes an accompanying file with an ERS extension is included. This ".ERS" file will contain metadata about the ECW file, but is only used by ER Mapper software. It is not used while importing or viewing ECW files in PCI software. The relevant data is contained within the ".ECW" file itself.

PCI reads and applies the georeferencing information contained in the ECW file. Many of ER Mapper's supported spheroids and datums are translated to use the same in PCI products. Please see the subsection "Supported ER Mapper Datums/Spheroids" for a comprehensive list of the datums supported and the PCI datums used for them.

Additionally, ER Mapper's supported SPCS (State Plane Coordinate System), UTM Metre projections and their Lambert1 and Lambert2 projections are fully supported in PCI products. Please see the subsection "Supported ER Mapper Projections" for a comprehensive list of the projections supported.