When reading articles on radar-navigation in various nautical magazines, one is constantly struck by the difficulty of comparing the different targets on the screen and of interpreting the changes that take place in the right way. So plotting becomes still more necessary in order to get: (a) the nearest approach, and (b) the present course and speed of other vessels, before we can manœuvre to avoid collision, especially in thick weather. When plotting, comparisons are hampered by the fact that the echoes must be recorded on a piece of paper or on the reflection-plotter, and that the different distances, courses and speeds must be measured very accurately. And nearly all these tasks depend upon the skill and accuracy of one man.