It’s natural to think of GPS in terms of a full fixing system or a means of tracking to waypoints, but waypoints take time and effort to input and it isn’t always necessary to go to the trouble of plotting a fix. Often, a single lat or long coordinate is all that’s needed. This skipper is sailing along a strange coast looking for a harbour entrance in poor visibility. The tide is sluicing along and he has plenty else to think about. IN fact, al he need do is note the longitude of the entrance. When he’s on it, it must bear due south. It’s surprising how many other uses can be found for single position lines which, when you think about it, is all a latitude or a longitude really is.