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 reconstruct to be identical to the original.
MrSID files may be stand-alone (.SID) or have an associated ".SDW" file. The differences inare similar to having a TIF vs. a GeoTIF; that is, if you only have the SID file, it is just a picture and the coordinates are pixel coordinates, but if you have an associated SDW file (associated meaning it has the same basename), PCI will use that information and assume a "Metre" coordinate system. An example of an SDW file and its geo-referencing 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)
upper-left pixel
2127219.90200000000000 y map unit co-ordinate of centre 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. Projection information (that is, the map unit co-ordinate system being used is not indicated) is not stored in SID or SDW files generated by MrSID Encoder version 1.2 or less. This has changed with MrSID Encoder v.1.3, but the Decompression Library API v.2.0 enabling access to this information was released too late to incorporate their support for this release (Winter/00). This means that PCI does not currently support the Metadata generated by MrSID Encoder 1.3 or better.