So accurate has the modern chart plotter become that it’s tempting to ignore the transits that guided seamen through the rocks since the days of the Vikings. It doesn’t do to forget that no chart is any better than its survey. We know for sure that the transits work, because they were set up by pilots – often on pain of death – whose sole purpose was threading a specific gap. A plotter can’t think for itself and it tries to show the whole scene in detail. It may succeed or it may not, depending on the scale of the chart and the accuracy of the survey, while an unexpected datum shift could still shove the whole shooting match off-centre. If you had to trust your life to a man threatened with hanging for getting it wrong, or an electric box carrying some degree of inbuilt uncertainty, which would you choose?