This presentation came about as a result of my role as Editor of the Western Australian Cruising Guide (a free downloadable 600 page book, see xxx. Many readers of the Guide have asked me why the anchorage waypoints and the like are published to only one decimal point (i.e. +-100m accuracy), despite us being sent reports containing waypoints with up to 3 decimal points (i.e. +-1m). Why has this been done? The short answer is that positions are often not accurate to more than one decimal place. This may surprise some people, so a fuller explanation is given here.