Catastrophic marine accidents are rare. Of those collisions and standings which do occur, many start as quite routine navigational situations; just another night, just another watch, just another target. Something then goes awry perhaps the radar fails to display an islet or a target's calculated closest point of approach (CPA) is erroneous jeopardising the crew, ship, cargo, passengers, other shipping or the environment, with losses running to a hundred million pounds. All significant targets have to be displayed to scale on the screen, often nowadays with chart features superimposed. We need to look in detail at the validity or accuracy of the plotted positions, for this accuracy, or lack of it, should always inform the navigators judgement of the situation and the prudent action to take. So far, we have concentrated on the process of detecting targets. Detection, although necessary, is not sufficient the navigator would give little thanks to be told that there was another target somewhere or other within busy Tokyo Bay (Chapter 5, Figure 5.14). We have looked at how plots of target current positions are laid down on the display, showing where targets and coastal features are relative to own ship at the present time. This basic information is a useful start and must contain little error, but the navigator needs more: are targets moving or manoeuvring? What have they been doing? Are they hazardously close? Will they approach to close quarters? Where? When? Most of these questions involve movement, which is the rate of change of position with time, necessitating data collection during two scans at the very least. Movement is represented as a track, which is the target velocity vector either relative to own ship (relative motion display) or to the ground or the slowly moving water mass.