SURVEILLANCE APPARATUS

In an infra-red surveillance system, a digital memory (1) has a storage location for each of a number of zones, defined by azimuth and elevation co-ordinates, the zones together covering 360.degree. in azimuth and 20.degree., for example, in elevation. In each location there is stored a data word built up from the responses on a zone in successive scans. Each word can be read into a decoder (5) which selects an appropriate video display level in the form of a two-bit word which is then stored in a buffer store (6). A buffer store data controls the display on a cathode ray tube (2). A manually controlled cursor circuit (8) generates a cursor address which results in a cursor display at the corresponding position on the cathode ray tube. The display is in the form of four strips, one above the other, each representing a quadrant of azimuth.