Charts carry a lot of information for navigational use. They represent a portion of the Earth’s surface with a suitable scale on a flat surface.

**MARCATOR CHART**

Projection is “Cylindrical Orthomorphic Projection”.

- Projection is cylindrical.
- Meridians are represented by equidistance, parallel straight lines.
- Equator and parallel of latitudes are horizontal parallel straight lines is selected distance from equator and from each. It increases towards the pole due to distortion.

Lat*Scale= Long*Scale*sec (lat)

**Meridional parts** – The meridional parts of latitude is the number of longitude units from the equator to the latitude along the meridian.

**Distance in meridional parts **– DMP between two latitudes is the length of the meridian between those latitudes on a Mercator chart expressed in units of longitude scale.

**Geographical Mile ( longitude unit )**– It is the length of an arc along the equator that subtends an angle of 1’ at the center of circle.

**Some important formulas:**

D’lat = dis*cos (course)

D’long/DMP = Tan(course)

D’long = DMP.tan(course)

**Advantages**

- The directions can be measured accurately.
- Easy to plot course on Mercator charts.
- Course lines are straight lines.
- Shapes stay true.
- The plotting lines can be moves anywhere on the chart as parallel lines.

**Disadvantages**

- Distortion increases as we go towards the poles.
- Land masses cannot be compared.
- Polar regions cannot be represented.
- Great circle courses cannot be laid off easily.

**GNOMONIC CHARTS**

- There charts are used for great circle sailing, as in these charts the great circles appear to be straight lines.
- The projection is tangential.
- These are small scale charts and therefore are not used for navigation.
- Meridians are straight line converging towards the pole.
- Meridians are not parallel to each other.
- Small circles and Rhumb lines are curved.
- After marking the shortest distance on Gnomonic chart, points are marked on the track at regular intervals of longitude and then transferred to the Mercator chart.

**Advantages**

- Great circle track appear as straight line.
- Polar regions can also be represented on Gnomonic charts.

**Disadvantages**

- As we go away from the centre the distortion for shape, size and distance increases.
- Rhumb line courses cannot be plotted as they appear to be curved lines.
- Plotting lines cannot be transferred from one portion of the chart to another.

**Difference between Mercator and Gnomonic charts**

MERCATOR | GNOMONIC |

Orthomorphism (Shape retention). | The shape is not retained. |

Rhumb lines are straight. | Rhumb lines are not straight. |

Great circles are curved. | Great circles are straight. |

The projection is cylindrical. | The projection is Zenithal. |

There is no tangent point. | There is a tangent point. |

Meridians are parallel. | Meridians are not parallel. |

**Difference between Mercator and Gnomonic charts**

**WHAT IS NATURAL SCALE?**

Natural scale is the ratio of the length of 1’ of latitude on the chart to the length of 1 nautical mile on the surface of the Earth.

Length of 1’ of latitude (l) on the chart = length of 1’ of long (L). sec (L)

Length of 1 nautical mile on the Earth = 1852.3 – 9.4 cos2L (in min)