CHAPTER -15

CHART PROJECTIONS

Learning Outcomes

1. Understand Basic Concepts of Chart Projections

Define what a chart projection is and explain its importance in maritime navigation.
Understand that the Earth’s surface is curved, and chart projections are used to represent this surface on flat charts.
Explain the difference between small-scale charts (for large areas) and large-scale charts (for detailed, smaller areas).

2. Identify Different Types of Chart Projections

Describe the main types of chart projections used in navigation:
- Mercator Projection
- Gnomonic Projection
- Polyconic Projection
Identify the characteristics and limitations of each projection.

3. Apply Mercator Projection in Navigation

Understand that the Mercator projection shows rhumb lines (lines of constant compass bearing) as straight lines.
Use Mercator charts to plot courses, bearings, and positions accurately.
Recognize the distortion of size and scale near the poles.

4. Apply Gnomonic Projection in Navigation

Understand that the Gnomonic projection displays great circles as straight lines.
Use Gnomonic charts for planning long-distance voyages along great circle routes.
Transfer plotted courses from Gnomonic to Mercator charts for practical navigation.

5. Compare and Choose the Right Chart Projection

Explain when to use Mercator projections (for short, constant-bearing courses) and Gnomonic projections (for long-distance, great-circle routes).
Understand the limitations of each projection for specific navigational tasks.

6. Interpret and Use Navigational Charts Correctly

Read and interpret information from different types of nautical charts.
Apply knowledge of projections to understand distances, bearings, and positions on a chart.

7. Solve Practical Problems Using Chart Projections

Calculate distances and courses using the appropriate chart projection.
Correctly apply variation and deviation when plotting courses on different chart types.

8. Communicate and Record Navigational Data

Accurately record courses and positions based on the chart projection used.
Communicate navigational plans clearly, specifying the projection type.

MERCATOR PROJECTIONS

It is quite evident that the surface of sphere or spheroid such as the earth cannot be correctly represented on a plane, therefore any extensive portion of the surface so represented must involve distortion of some of the following qualities:

shape
bearing
scale
area

It is possible to derive a projection which will eliminate one or more of these distortions while keeping the other within acceptable limits.

This is the most useful projection to the navigator. It allows him to draw a straight line from departure point to destination and to measure the steady course he has to maintain to arrive there.

The Mercator projection has straight meridians and parallels that intersect at right angles. Scale is true at the equator or at two standard parallels equidistant from the equator. The projection is often used for marine navigation because all straight lines on the map are lines of constant azimuth.

Features of the Mercator chart

The projection is classed as a cylindrical orthomorphic projection (describes a map that shows the correct shape and scale of a small area) in which:
rhumb lines appear as straight lines
the angles between rhumb lines are unaltered between chart and earth
the equator appears as a straight line
parallels of latitude appear as straight lines parallel to the equator
meridians appear as equally spaced parallel lines perpendicular to the equator
Distance can be measured with Reasonable ease

Disadvantages

Magnifies areas in higher latitudes
Cannot use for polar regions
G.C appears as curve (except meridians & equator)

An unmounted globe map over a Mercator world map. Greenland looks larger than South America, though it is smaller than Argentina. Antarctica (actually almost a circular continent) stretches endlessly across the bottom of the map.

Because of this latter property the polar regions cannot be reproduced on this projection and it is rarely used beyond latitude 70º north or south.

The projection preserves the shape of the land over small areas although

distortion in area occurs. On a map of the world on a Mercator projection

Greenland appears larger than South America although the latter is nearly nine times larger in area.

The latitude scales are always on the left and right hand margins with north latitude increasing toward the top and south latitude increasing toward the bottom of the chart. The longitude scales are always on the upper and lower margins with west longitude increasing to the right.

Scales of a Mercator Chart

Meridians on the earth are in fact converging.

Making a chart to represent the portion of the earth on the flat paper, the curved surface of the sphere to be spread out. Distortion will take place but for us the chart should hold some important properties.

Gnomonic Chart

A chart which shows all G.C.s. As straight lines is a gnomonic chart. It is useful as G.C to be laid off on it and positions of points along the G.C. can then be transferred to a Mercator chart.

Meridian radiate from the pole the appearance of the chart is like spokes of a wheel whilst the parallel are concentric circles.

If the tangent point as the equator is chosen,

The meridians parallel straight lines but their spacing increases from the tangent point outward.

To measure distance on a Mercator Chart

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