9 NAVIGATION Chapter 9 NAVIGATION


Ashore on our home turf, we rely on familiar landmarks to find our way around – left at the church will take us to the pub, and so on. This is done without thinking; our local knowledge is ingrained. Then, when we stray further afield, we switch our trust to road signs and maps and there’s often a stranger who can give us directions.


Not so at sea, where there’s a bewildering lack of road signs and no handy bloke to point the


way. But at least there are maps – or ‘charts’ as they are called. And these have been trusted guides to sailors for centuries. Traditional charts are printed on paper, but the trend today is more and more towards electronics – the modern chart plotter, with all its wonderful functions – and, of course, its potential unreliability.


Latitude and longitude


Any location can be pinpointed by its latitude and longitude. Both are angular measurements (in degrees, minutes, and seconds) taken from the Earth’s centre: vertically north and south from the equator; and horizontally in a westward direction from the prime (Greenwich) meridian. When these angles are extended to the Earth’s surface, they produce lines of latitude and longitude – a huge imaginary grid stretching from pole to pole.


west east


N S


The red dot on the chart indicates the position 45°N 30°W shown on the globe, right


30 W


This grid is reproduced on every chart but, of course, there’s an obvious snag. Charts are flat while the Earth is round. And, whereas the lines of latitude are equally spaced, the lines of longitude converge until they meet at the poles – rather like the segments of an orange. To overcome this problem, most charts use what’s known as a Mercator projection (left), where all the lines are shown parallel. Since lines of longitude can’t possibly be parallel, this can only be achieved by stretching horizontal distances – that’s to say, in the east-west direction. This necessary distortion has an important bearing on how we use charts.


45 N


60


RYA Seamanship for Sea Anglers


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