CHARTS

                                                  NAVIGATIONAL CHARTS

A chart is a representation of the curved shape of the earth on a flat surface.Whenever this is attempted a certain amount of distortion is unavoidable.

The form of the distortion would take  depends on the type of projection used.As long as the distortion inherent in a particular projection has only a minimum effect upon the use to which the chart is put .then such a projection would be selected for the purpose.The projection selected for navigational chart is Mercator's projection.

MERCATOR'S  CHART 

Imagine a transparent globe with all latitudes and longititudes marked in dark ink and with a source of light placed in the center.

Imagine also paper rolled in cylinder fashion tangential to the wquatorial plance of the globe
Now observe the shadows cast by the parallels of latitudes and longitudes on the paper.
If the paper was photograph this would print these shadows on the  paper and when the paper is unrolled.we have the  Mercator's charts.


GNOMONIC CHART

Imagine a transparent globe with all latitudes and longitudes marked in dark ink and with a source of light placed in the center.

Hold a sheet of paper flat and tangential to the surface of the globe at one of the poles and observe the shadows of parallel of latitudes and meridians on this sheet of paper.
If the paper was photograph this would print these shadows on the paper as Gnomonic charts.
On polar  Gnomonic charts all meridian would be appear as a straight lines crossing each other at the pole which is the tangent point.