Prior to the introduction of three Ocean Surveying Ships in the years 1965–66, the Hydrographer's surveys were generally concerned with the recording of depth against a geographical position obtained from manually reading and plotting sextant angles, radar ranges or, more recently, electronic navigational aid data. The new ships were however the first Royal Naval ships to be designed with combined oceanographic, geophysical and hydrographic roles and to have no supplementary naval function. They contained instruments for measuring the Earth's gravitational and magnetic fields as well as depth; as a consequence the number of men required to read, record and plot this data increased. With the use of an electronic positioning system to provide the horizontal control for the surveys it became possible to carry out surveys throughout the full 24 hours each day.