The Mercator Chart Pojection Explained
Understanding The Mercator Projection
If the earth was flat like the chart or in the shape of a cube with equal dimensions then we could safely assume that the squares in this grid would be of equal dimensions as well as equidistant and of the same length Unfortunately, life is not that simple The earth is round (well almost) not square and meridians of longitude that appear as parallel lines on a chart or cube converge together at the poles on the earth’s surface
A chart portrays a portion of the earth’s curved surface on a flat piece of paper and as it is not possible to reproduce a portion of a sphere on the plane of a flat surface there will be distortion causing the East West scale to expand with increasing latitude To compensate for this distortion and to ensure that at any point on our chart that the North South scale of distance corresponds with East West scale the parallels of latitude are drawn at increasing distances apart This is known as a Mercator Projection
To be simplistic a Mercator Projection is a portion of the earth’s surface that we want to represent on the flat square surface of our chart This portion of the earth is in fact shaped like a piece of orange peel and to get it square we have to stretch it at the end nearest the Poles Therefore the scale on the Mercator chart varies from minimum at the equator to maximum at the poles On a large-scale map a feature at 60 degrees north A
appears twice as large as a feature of the same size on the Equator B
Chart projection, Mercator and Plane Sailing are complex and a subject for the module on Coastal Navigation so we need not concern ourselves to much with it here The important thing to remember is that when measuring distance on a chart using the scales
1 We always use the scales of longitude, that is the scale that runs down the left or right hand side of the chart never the scale of latitude that runs along the top or bottom
2 We always pick of the scale adjacent to the object we are measuring
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