MrSID compressed rasters (SID)

Long name MrSID Raster
Short name SID
File extension(s) .sid

The GDB library supports LizardTech's Multiresolution Seamless Image Database format (MrSID) for live linking and import. LizardTech's Decompression Library API (DSDK 4.0) was used to introduce this functionality in PCI products. However it is only available on Windows operating systems. MrSID files up to version 3.0.26 are supported. Locked files are supported by prompting for a password.

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 LizardTech's software. 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 reconstructed to be identical to the original.

MrSID files may be a stand-alone file ".SID" or have an associated ".SDW" file. A SDW file is similiar to a TIFF World file. See TIFF (TIF) for the specifics on how georeferencing is assigned using such a file.

Georeferencing will be assigned from a MapInfo TAB file if one is present. The TAB file will have the base name as the SID file with a TAB extension.

If no SDW or TAB file is present, the image will be assigned a PIXEL projection.

An example of an SDW file and its georeferencing is as follows:

0.20000000000000           x-scale; dimension of a pixel in map units
                           in x-direction
0.00000000000000           rotation term (not used by PCI)
0.00000000000000           rotation term (not used by PCI)                             
0.20000000000000           negative of y-scale; dimension of a pixel
                           in map units in y-direction
3615780.10000000000000     x map unit co-ordinate of center of
                           upper-left pixel
2127219.90200000000000     y map unit co-ordinate of center of
                           upper-left pixel

The y-scale (E) is negative because the origins of an image and a geographic coordinate system are different. The origin of an image is located in the upper-left corner, whereas the origin of the map coordinate system is located in the lower-left corner. Row values in the image increase from the origin downward, while y-coordinate values in the map increase from the origin upward.

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