The three principal tasks of the navigator on an aerial survey expedition are to get to the survey base, to recognize and define the area to be surveyed, and to fly across this area in parallel flight lines so as to photograph it adequately and economically. During the survey, the courses flown, height, speed and the time of day chosen are all influenced by the demands of the camera and of the photogrammetric machines which will produce the map from the air photographs. For this reason it is essential that the navigator, on whose competence during the survey the results will largely depend, should have a proper understanding of photogrammetry, photography and of the basic principles of ground survey. It is true that the crew will include a photographer and that the photogrammetric aspects of the operation will have been considered beforehand by those responsible for briefing the crew; nevertheless there are always unexpected problems which arise in the field and every aspect of the undertaking must be at the fingertips of the navigator who, in the final count, is responsible for the photographic coverage of the area.