A sailor must know two things before he can chart a course: his current location and the location of where he wants to go. Picking a destination may be easy – the Bahamas perhaps – but determining the precise distance and direction the ship must travel to get to the tropical paradise requires a skipper know his exact location at the time of his departure.
During his journey a navigator also must constantly measure his progress. Unexpected weather conditions, changes in ocean currents and breakdowns in equipment all conspire to throw him off course. To reach his destination safely and successfully requires an accurate assessment of the ship’s progress compared to where it started from and where it needs to go.

Kevin Craine. Where are you now? Where are you going?