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 colour 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 for 10GB (10,000MB) of colour imagery to be compressed down to 500MB, small enough to fit on a single CD-ROM disk.
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 is possible with lossless compression. Here, the main advantage is the higher compression ratios, but the disadvantage is that the compressed image can not perfectly reconstruct 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 and a ".ERS" file is not created during export. 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), their 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.
When compressing imagery during Export, the following "Target Compression Ratios" are used by default:
Greyscale 15:1 RGB 25:1The user has the ability to change the Target Compression Ratio by entering an integer value in the Options field of the export dialogue. ER Mapper suggests the following guidelines when selecting the best compression ratio to use:
Colour Image for high quality printed maps 25:1 Colour Image for Internet or email distribution 40:1 Greyscale Image for high quality printed maps 10:1 to 15:1 Greyscale Image for Internet or email distribution 15:1 to 30:1Depending on the imagery, your final compression ratio may actually be higher than the target compression. The compression engine uses the target as a measure of how much information content to preserve. If your image, however, has large, similar areas that are well suited to compression, a greater rate of compression may be achieved while still maintaining the desired information content. It is important to note that this does not affect the quality of the resulting compressed image.