CHAPTER 3 – CHARTS

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.

Image credits – https://www.britannica.com

MARCATOR CHART

Projection is “Cylindrical Orthomorphic Projection”.

  1. Projection is cylindrical.
  2. Meridians are represented by equidistance, parallel straight lines.
  3. 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

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

Disadvantages

  1. Distortion increases as we go towards the poles.
  2. Land masses cannot be compared.
  3. Polar regions cannot be represented.
  4. Great circle courses cannot be laid off easily.
GNOMONIC PROJECTION – image credits Wikipedia

GNOMONIC CHARTS

  1. There charts are used for great circle sailing, as in these charts the great circles appear to be straight lines.
  2. The projection is tangential.
  3. These are small scale charts and therefore are not used for navigation.
  4. Meridians are straight line converging towards the pole.
  5. Meridians are not parallel to each other.
  6. Small circles and Rhumb lines are curved.
  7. 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

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

Disadvantages

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

Difference between Mercator and Gnomonic charts

MERCATORGNOMONIC
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)

Article by Ritika Singh(Tolani Maritime(2020-2022),Sailing as Deck Cadet now as in 2022). Verified by Swapnil Singh.