Since the advent of radar, the need for correct interpretation has become only too clear from the record of so called radar-assisted collisions. The practical navigator has been shown various methods of noting PPI data and solving the problem of how best to avoid collision. We have the practical system of plotting, either with ship's head or true north upwards. We are now being offered some ingenious graphs and tables by Captain Wylie, Mr. Slater and others (Journal, 9, 161, 448). The navigator has an opportunity to study these methods and use them at sea, in clear weather, in conditions where he can assess the advantages of each method and also their accuracy.