Gnomonic charts ship navigation advantages disadvantages


A great circle is a circle on the surface of the sphere, whose radius is the same as the radius of the sphere. Equator is a great circle. A great circle drawn between two points on earth is the shortest distance between points. Sailing through a great circle in a Mercator chart is hard, because great circle appear curved on the Mercator chart.
In Gnomonic charts, it is possible to represent great circles as straight lines, making great circle sailing easy. These charts are constructed on a tangential or gnomonic projection. All the points on the surface of the sphere are projected from the centre of sphere to a plane which is tangential to the sphere in gnomonic projection. The chosen tangent point will be usually at the centre of the area being represented. 
Distortions are present on gnomonic charts, like all other charts. Distortion is zero at the tangent point and will increase as the distance from the tangent increases. There are gnomonic charts with the pole being one of the poles. This chart is called a polar gnomonic chart. As great circles appear as straight lines, all the meridians will be appear as straight lines converging at the pole. But in gnomonic charts the small circles and Rhumb lines appear curved. Since meridians are convergent, compass roses are not shown on gnomonic charts as they are only valid for a particular location.
Gnomonic charts help in plotting straight line great circle courses between a departure and arrival position easily and conveniently. Along the track, positions are taken off at convenient intervals of longitude. These positions when transferred to a Mercator chart, Rhumb line courses are laid off between successive positions which are taken off. This is done because, if we are following a true great circle path, we should alter the course continuously to follow the track. But by sailing in various Rhumb line courses, ship can follow the great circle track in approximately in an easy manner. This does not require continuous course alteration.

Advantages of Gnomonic Charts:
. Area all around the world, including polar regions can be represented on a gnomonic chart.
. Great circle courses can easily be represented as straight lines on the chart.

Disadvantages of Gnomonic Charts:
. Difficultly in representing Rhumb line courses as they appear curved on the gnomonic chart.
. Difficulty in measuring distances and courses on the chart( No compass rose).
. Since the meridians are converging, the bearings and position lines cannot be transferred from one part of the chart to another as parallel lines.