GENERAL CHART PROPERTIES
The transfer of information from a globe on to a flat piece of paper is achieved by
projection.
Chart that are obtained maybe:
 PERSPECTIVE/GEOMETRIC PROJECTION: Charts obtained directly from a
projection
 NON-PERSPECTIVE/MATHEMATICAL: CHARTS obtained by mathematical
calculations
Reduced Earth: Scale model of the earth on which the projection of the chart is
based
TYPES OF PROJECTION SURFACES:
 CYLINDRICAL
 CONICAL
 PLANE/AZIMUTHAL
CYLINDRICAL PROJECTION:
Obtained by cylinder of paper wrapped around the reduced Earth
The paper touches the Reduced Earth at the equator
Most of the Earth is covered except the higher latitudes(POLES)
Mercator Chart uses cylindrical projection
CONICAL PROJECTION:
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Obtained by wrapping conical paper around Reduced Earth.
The paper touches the reduced Earth at a parallel of lat.
This parallel of lat is known as Parallel of Origin.
This may be changed by changing the apex angle of the cone.
Can be used to plot places at higher latitudes which would have otherwise been not
possible with cylindrical projection.
PLANE/AZIMUTHAL PROJECTION:
The paper touches the Reduced Earth at one of the Poles
Useful in projection of Poles and other High Latitudes
PROPERTIES OF AN IDEAL CHART AND REPRESENTATION OF THE EARTH’S SURFACE
1.Angles on the Earth’s surface should be represented by the same angles on the
chart.
2.Scale should be constant and correct.
3.Areas should be represented with their true shape on the chart.
4.Equal areas on the Earth’s surface should be shown as equal areas on the chart.
NAVIGATION REQUIREMENTS
1.Rhumb Lines should be straight lines.
2.Great Circles should be straight lines.
3.Latitudes/Longitudes should be easy to plot
4.Adjacent sheets should fit correctly.
5.Coverage should be worldwide.
Two of these properties can never be obtained, except on the globe
1.Scale can never be constant and correct. We can modify charts mathematically to
give nearly constant scale in small areas, but not over large areas.
2.The shapes of large areas cannot be represented perfectly. However, it is possible
to represent the shapes of small areas reasonably accurately.
ORTHOMORPHISM / CONFORMALITY
1.Of all the ideal properties, the only essential one is that navigation bearings must
be“correct” and the critical property is that angles on the Earth must be
representedcorrectly on the chart.
2.Charts that do have this property are known as orthomorphic or conformal charts.
3.If you draw a line joining 2 points on the chart and measure the angle but then find
thatthis does not correspond to the true direction on the Earth, the chart is useless
fornavigation.
There are two fundamental conditions which must be met to achieve
orthomorphism/ conformality.
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Condition 1 The meridians and parallels on the chart must intersect at right angles
(as they do on the Earth).
Condition 2 The second condition relates to the scale at a point on the chart.
On an orthomorphic chart, scale at a point should be the same in all directions.
The condition 2 implies for Constant scale which we know is only true on a globe. On
a chart, scale will change.
The condition 2 is modified to be:
At any point on a chart, scale should be the same in all directions, or should change
at the same rate in all directions.
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