Data compression

The 1:250 000 Scale Colour Raster product is compressed using the lossless Lempel–Ziv–Welch (LZW) data compression algorithm.

Data volumes for the TIFF file format are influenced by the level of data compression. The required storage volume for the Great Britain supply (56 tiles) is approximately 128 MB compressed and 130 MB uncompressed.

Image compression

When an image is compressed, duplicated data that has no value is removed or saved in a shorter form, reducing the file size. For example, if large areas of water are the same tone, the value for one pixel is saved, together with the locations of other pixels of the same tone. When the image is displayed, the compression process is reversed. Compressed raster data allows users to download, display, edit, and transfer images more quickly.

There are two types of image compression:

• Lossless compression: No information is lost during lossless compression. When the image is uncompressed the original quality is retained. File size remains large because the process does not provide much compression. Lossless compression is used mainly where detail is important, such as when planning to make large prints.

• Lossy compression: This process degrades the image to some degree. When the image is uncompressed the quality is not as good as the original. The more you compress the image, the more degraded it becomes. In many situations, such as web page images or small to medium-sized prints, the image degradation is an acceptable trade-off for the reduced file size. If a lossy compressed image is over-enlarged, the degradation becomes obvious.

TIFF format

Tag Image File Format (TIFF or TIF) is one of the most commonly used lossless image formats. TIFF is primarily designed for raster data interchange and is supported by many image-processing applications. This permits more efficient access to very large files that have been compressed.