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Chart Basics Part Two

A guide to how charts work and howto use them

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Introduction

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This presentation looks at chart projections

and why we need to be aware of them 

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Lines of Longitude, called ‘Meridians’, run from pole to pole

dividing the earth into segments rather like an orange

Meridian of longitude

North Pole

Greenwich Meridian

Latitude and Longitude

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Lines of Latitude called ‘Parallels’ are obtained by projecting

angles made from the centre of the earth to points on its surface

0° 

15° 

15° 

30° 

45° 

45° 

60° 

60° 

75° 

30° 

75° 

Latitude and Longitude

Equator

Parallel of Latitude

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Navigators use a combination of latitude and longitude to fix

their position on the earth’s surface 

Meridian of longitude

Latitude and Longitude

Parallel of latitude

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Chart projections are an attempt to represent the curvedsurface of the earth on a flat piece of paper

Great circle

The shortest distance between two points on the earth’s

surface is a Great circle line......

Chart Projections

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Great circle

Rhumb line

Sailing an unchanging course you will cut the lines ofLongitude at equal angles...

...this is known as a Rhumb line

Chart Projections

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Great circle

Rhumb line

Chart Projections

 You sail a slightly longer distance using a Rhumb line, but

this is only really significant on long ocean passages

The most useful chart for practical use is one where a Rhumb

line is shown as a straight line

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This stretches the land masses at the top of the chart

in an east-west direction

Mercator Projection

These are the charts you are likely to use for coastal navigation 

In order to represent a Rhumb line as a straight line themeridians have to be made parallel

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.....and to keep them the correct shape...

Mercator Projection

...they must also be stretched in a north-south direction

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The scale gradually increases towards the top of the chart...

...this is why you shouldmeasure distance on the

latitude scale opposite your

position

Mercator Projection

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Latitude and Longitude

Longitude scale

Latitude scale

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Plotting latitude and longitude on a chart

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46°27´.6N

First - mark off latitude

Plotting a Position

GPS position 46°27´.6N 006°20.´6W

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Next - mark off longitude

006°20´.6W

Latitude and Longitude

GPS position 46°27´.6N 006°20.´6W

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Mark off latitudeMark off longitude

12:00

(10M)

Latitude and Longitude

GPS position 46°27´.6N 006°20.´6W

.......and note the time and log reading

If the plotted position is a fix....encircle it

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Further Reading

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We highly recommend Tim Bartlett’s 

RYA Navigation Handbook (G6)

You can buy a copy of this book by visiting our on-line shop 

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