Raster images: A guide to good practice

Images appear in many contexts and can be used for different purposes. An image is usually a two-dimensional representation of something and may form the empirical data material in a context or, for example, be used as an educational and visual aid for example. There are two main image file formats used to describe digital images. These are usually referred to as raster images (sometimes also called bitmaps) or vector images. Both image file formats have their advantages and disadvantages. For example, the vector format handles enlargement better than the raster format but is not able to represent colour nuances as accurately. The most common type of raster images are digital photographs, while vector images are mainly used in GIS and CAD. This guide aims to provide guidance on what to consider when preserving raster images. For guidance on handling vector images, see our guide in the same series: Vector images: a guide to good practice. 

The guide covers the most common file formats used for storing digital image files and which formats are suitable for archiving. It will also discuss the archival strategies that may be used to ensure that the quality of the image files is maintained. Although some basic technical descriptions will be given in the guide, it is not intended to provide a more in-depth account of the technical specifications of raster images.