The Optical Mapping Instrument (OMI) is a space-based Earth observation system being developed to collect data from which medium-scale mapping of high quality can be produced. In this paper, consideration is first given to existing and planned optical Earth observation systems, and it is noted that the topographic mapping market is not well served by these instruments. The requirements of this topographic market are examined, and the specification of an instrument to meet this market demand are derived. A brief overview of the OMI system meeting these requirements is given. Finally, specific aspects of the design concerning optics and detectors are presented in more detail.