We know we make mistakes from time to time inputting all those lat / long numbers into our GPS units when we’re plotting waypoints. The classic answer for checking a lat / long position plotted at sea is to note its range and bearing from a handy waypoint on the chart, then hit the ‘GoTo’ button with that waypoint activated. If all’s well, the two sets of numbers will coincide. Used ‘upside down’, the same technique provides confirmation that the lat / long of a plotted waypoint has been correctly transferred to your GPS. As likely as not, you’ll be entering this at the passage-planning stage in harbour. Measure the range and bearing on the paper chart, as shown in this electronic example, then hit ‘GoTo’ the waypoint and make sure the figures coincide.