1 - 3cw - 2 basics -part two
TRANSCRIPT
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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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